Showing papers in "Integrative and Comparative Biology in 1984"

Body Form, Locomotion and Foraging in Aquatic Vertebrates

Abstract: Four functional categories are denned to embrace the range of locomotor diversity of aquatic vertebrates; (1) body/caudal fin (BCF) periodic propulsion where locomotor movements repeat, as occurs in cruising and sprint swimming; (2) BCF transient propulsion where kinematics are brief and non-cylic, as occurs in fast-starts and powered turns; (3) median and paired fin (MPF) propulsion, with very diverse fin kinematics, used in slow swimming and precise maneuver; (4) occasional propulsion or “non-swimming.” Specialization in any one of these categories compromises performance in one or more of the others, thereby reducing locomotor diversity and hence behavioral options. Food characteristics influencing the role of locomotion in search and capture are; (1) distribution in space and/or time and (2) evasive capabilities. BCF periodic swimmers take food that is widely dispersed in space/time; BCF transient swimmers consume locally abundant evasive items and MPF swimmers consume non-evasive food in structurally complex habitats. Locomotor specialists under-utilize smaller food items in exposed habitats. This resource is exploited by smaller fish, which are locomotor generalists because of predation pressures. For such locomotor generalists, locomotor adaptations for food capture are of diminished importance and other adaptations such as suction and protrusible jaws in fish are common.

Alternative Mating Tactics and Evolutionarily Stable Strategies

Abstract: Difficulties with applying Evolutionarily. Stable Strategy (ESS) methodology and terminology to alternative mating behaviors (in which some males in local populations adopt strikingly different, often non-competitive, behavioral patterns) are reviewed. Definitions for “tactic” (behavioral phenotype) and “strategy” (evolved set of rules for expressing tactics) are given. Inconsistent and incorrect applications of “mixed,” “pure,” and “conditional” ESSs are discussed. Cases of condition-dependent alternative mating tactics are reviewed. Because most alternative behaviors are condition dependent, neither their population-wide nor individual fitness contributions are expected to equal the fitness contributions of “primary” tactics. Individuals should, however, switch tactics at “equal fitness points.” A particular conditional tactic will persist when its maintenance cost (genetic or physiological) is less than its fitness contribution. In only exceptional cases are the fitness contributions of tactics expected to be equal: 1) genetic polymorphisms, 2) stochastic “mixed” ESSs, 3) frequency-dependent choice and, 4) arbitrary assessment. Although alternative tactics may occur in cases of genetic polymorphism or genetic equipotence, most mating tactics probably occur when continuous heritable variation in underlying conditional strategy exists. Selection for genetically influenced “roles” (genetic background) may also uncover apparent heritability.

The Structure and Calcification of the Crustacean Cuticle

Abstract: The integument of decapod crustaceans consists of an outer epicuticle, an exocuticle, an endocuticle and an inner membranous layer underlain by the hypodermis. The outer three layers of the cuticle are calcified. The mineral is in the form of calcite crystals and amorphous calcium carbonate. In the epicuticle, mineral is in the form of spherulitic calcite islands surrounded by the lipid-protein matrix. In the exo- and endocuticles the calcite crystal aggregates are interspersed with chitin-protein fibers which are organized in lamellae. In some species, the organization of the mineral mirrors that of the organic fibers, but such is not the case in certain cuticular regions in the xanthid crabs. Thus, control of crystal organization is a complex phenomenon unrelated to the gross morphology of the matrix. Since the cuticle is periodically molted to allow for growth, this necessitates a bidirectional movement of calcium into the cuticle during postmolt and out during premolt resorption of the cuticle. In two species of crabs studied to date, these movements are accomplished by active transport effected by a Ca-ATPase and Na/Ca exchange mechanism. The epi- and exocuticular layers of the new cuticle are elaborated during premolt but do not calcify until the old cuticle is shed. This phenomenon also occurs in vitro in cuticle devoid of living tissue and implies an alteration of the nucleating sites of the cuticle in the course of the molt.

Feeding Currents and Particle Capture Mechanisms in Suspension Feeding Animals

Abstract: The apparent diversity of suspension feeding animals is, in one sense, more apparent than real. Virtually all suspension feeders capture particles from the water at low Reynolds numbers with cylindrical filtering elements, so, at the level of the filtering elements, flow patterns are identical and viscous forces dominate the situation. Six particle capture mechanisms are likely to be operating alone or in combination: (1) scan and trap (isolation of a parcel of fluid containing the particle), (2) sieving, (3) direct interception, (4) inertial impaction, (5) gravitational deposition, and (6) diffusive deposition. To insure that all variables relevant to the suspension feeding process are recorded, future work on suspension feeding should report the diameter and spacing of the filtering elements, flow speeds, diameter of particles available and captured, particle settling velocities, particle mobility (active or passive), and particle surface properties.

Elastic Energy Stores in Running Vertebrates

Abstract: Large mammals save much of the energy they would otherwise need for running by means of elastic structures in their legs. Kinetic and potential energy, lost at one stage of a stride, is stored temporarily as elastic strain energy and returned later in an elastic recoil. At high speeds, men and kangaroos seem to save in this way more than half the metabolic energy they would otherwise need for locomotion. It is shown by means of a generalized model that muscles and tendons could both be important as elastic energy stores. Analysis of films and force records of kangaroos hopping shows that strain energy stored while the feet are on the ground must be stored mainly distal to the knee. The principal muscles there have short fibres, and most of the storage must be in tendons. Investigation of camels shows that tendons in the feet, distal to the ankle and wrist, are especially important. The scope for elastic storage while the feet are off the groundis also considered. Though the evidence presented in this paper comes mainly from a few species, the conclusions presumably apply to large mammals in general.

How Do Benthic Organisms Withstand Moving Water

Abstract: Many aquatic plants and animals spend part of their lives anchored to the substratum as water flows by There are a number of mechanisms by which such sessile organisms can affect the magnitude of the flow-induced forces they encounter, as well as the distribution and magnitude of the mechanical stresses in their bodies produced by those forces Furthermore, the mechanical properties of the skeletal tissues of such organisms affect how much they deform and whether or not they will break in response to flow-induced stresses There are different mechanisms by which organisms can withstand the water flow characterizing a particular type of habitat Biomechanics is a useful tool for studying how the performance of organisms depends on their structure Biomechanical studies should be accompanied by knowledge of the natural history and ecology of the organisms in question if they are to lead to insights about how organisms work

Unsteady Aspects of Aquatic Locomotion

Abstract: Virtually all animals swim unsteadily. They oscillate appendages, undulate, and produce periodic propulsive forces so that the velocity of some part of their bodies changes in time. Because of their unsteady motion, animals experience a fluid force in addition to drag—the acceleration reaction. The acceleration reaction dominates the forces resisting rapid accelerations of animals and may be responsible for generating thrust in oscillating appendages and undulating bodies. The ever-present unsteady nature of animal swimming implies diverse applications of the acceleration reaction.

Drag and Flexibility in Sessile Organisms

Abstract: Most large, sessile organisms when exposed to rapid flows of air or water are markedly deformed as a consequence of their structural flexibility. Responses to air and water movement are similar, although both extreme and typical forces generated by water flows are greater, and erect organisms are commonly shorter in water than in air. A useful way of viewing data on the scaling of drag with flow speed is with a graph of speed-specific drag (drag divided by the square of speed) against speed. Since an ordinary solid body usually gives a horizontal line on such a plot, deviations from the ordinary are immediately evident. The slopes of the double logarithmic version of these graphs provide useful numerical comparisons. All of the cases considered here—trees, macroalgae, sea pens, etc.—give negative slopes at high flow rates, indicating that speed-specific drag drops with increasing flow. Such results may be taken as evidence that the flexible response commonly constitutes an adaptively useful reconfiguration as opposed to a mere incidental consequence of the material economy afforded by flexibility.

Evolution of the Vertebrate Central Nervous System: Patterns and Processes

Abstract: AS brains do not fossilize, most proposed phylogenetic sequences for central nervous system characters must be based on the patterns of variation of those characters in living organisms. Similarly, hypotheses regarding how brains change through time, and the evolutionary processes that produce these changes, are ultimately based on the character patterns recognized. It is critical in these analyses to distinguish between homologous and homoplasous characters if errors in the reconstruction and interpretation of phylogenies are to be minimized. Definitions of homology and homoplasy are reviewed, as are the concepts that bear on their application. Cladistic definitions are adopted, and criteria for distinguishing homologous from homoplasous characters are discussed. Analysis of a number of CNS characters that are usually assumed to be homologous reveals that homoplasous characters appear among them. As in other organ systems, homoplasous characters are actually common. A number of previous hypotheses regarding CNS evolution are reviewed in the context of new data on neural connections and their cladistic analysis. Some of these hypotheses may be falsified by a cladistic treatment of CNS characters, whereas sufficient data do not exist to evaluate others.

Fluid and Sediment Dynamic Effects on Marine Benthic Community Structure

Abstract: Fluid and sediment dynamics affect benthic community dynamics and structure in manifold ways. We single out three community-structuring processes that are both strongly affected and amenable to controlled manipulation: microbial population growth, faunal recruitment, and particle feeding. Attachment and colony growth rates of microbes depend on the details of near-bed fluid exchange. Their emigration and colony growth rates are affected by erosion of microbial filmsand by abrasion during sediment transport. Recruitment and successional patterns of metazoans, especially those resulting from the settlement of weakly swimming, small larvae and juveniles, also are very sensitive to local variations in boundary layer flow pattern and strength. While the importance of particle fluxes to suspension feeders has long been apparent, the foraging patterns of a growing number of surface deposit feeders are being found to reflect a dependence upon sediment transport. Although these three processes have spatial and temporal scales amenable to both laboratory and field experimentation, proper dynamic scaling of laboratory model flows may not always be easy. Even the simplest two-phase (particles plus liquids, where particles can be bacteria, floes, larvae, or sediments) flows must match appropriate laboratory and field Reynolds number, Froude number, particle-fluid density (weight per unit volume) ratio, and the ratio of boundary layer thickness to particle size, if the laboratory flow model is to provide accurate results.

A Classification of Alternative Reproductive Behaviors and Methods for Field-testing ESS Models

Abstract: A number of superficially similar evolutionary phenomena are often lumped together as “alternative reproductive behaviors (ARBs).” Several authors have previously organized the broad array of reproductive alternatives by their relation to ESS theory. Because such an organizing scheme begs the question of how much ESS models can contribute to our understanding of ARBs, I suggest a different scheme—devoid of ESS connotations—which classifies alternatives according to whether they represent genetic differences between individuals (genotypic versus phenotypic alternatives), whether the alternatives can be expected to manifest equal or unequal fitnesses (isogignous versus allogignous alternatives), and whether individuals may switch back and forth between alternatives (reversible versus irreversible alternatives). I point out that plausible selective regimes other than frequency-dependence can maintain genotypic alternatives, and that explanations for the maintenance of phenotypic alternatives usually can be examined only theoretically. I also urge more rigor in the field-testing of ESS models, review criteria which putative ESSs should meet, and suggest a statistical approach for evaluating evidence with regard to equality of fitnesses of reproductive alternatives.

Ionotropic Nucleation of Calcium Carbonate by Molluscan Matrix

Abstract: The hydrophilic, sulfated fraction of the organic matrix found in molluscan shells appears to be involved in crystal nucleation. It is located primarily at the sites of initial nucleation. The hydrophilic fraction favors in vitro formation of calcified deposits, when it is fixed in place on the hydrophobic fraction. Calcium is bound by the hydrophillic fraction with high affinity and selectivity. Enzymatic desulfation reduces the calcium binding. However, the binding stoichiometry of one calcium for every two ester sulfates is not altered. The calcium binding induces local anion binding, which induces secondary calcium binding. This coordinated ion binding is known as ionotropy. The resultant local high concentration of ions is thought to bring about nucleation.

Regulation of Carbonate Calcification by Organic Matrix

Abstract: The organic matrix of calcium carbonate structures can be involved in the formation of the biomineral by regulating various stages of crystal growth. The preponderance of evidence suggests that matrix can act as an initiator of crystal growth at the mineralizing front. The mechanism for this initiation is not known. However, the matrix may provide a template for the crystal lattice, or more likely, a surface that can simply stabilize the critical nucleus of calcium and carbonate. The degree of specialization of the nucleation sites is uncertain; however, they are hypothesized to be a class of soluble, acidic, calcium-binding polymers which are present in matrix extracted from biomineral. Recent studies have revealed that similar soluble components of extracted matrix have the capability of inhibiting nucleation and crystal growth in vitro and biomineralization in organisms exposed to the extracts. Further, crystals grown in vitro in the presence of extracted matrix have a morphology distinct from those of crystals grown without matrix. These findings generally support the hypotheses generated either a priori or from morphological observations which suggest that matrix must limit crystal growth and may well influence structure of biomineral. Hypotheses which explain a dual function of the soluble matrix have these components periodically released to the mineralization front where they can associate with more insoluble forms of matrix and sequentially initiate or inhibit growth. There also exists evidence to suggest that soluble regulatory components are intra-crystalline in orientation and thus may be released continuously

Organization of Organic Matrix Components in Mineralized Tissues

Abstract: The organic matrix is thought to play an important role in controlling crystal growth during the formation of skeletal hard parts. The structural organization of the matrix macromolecular constituents can provide a key to understanding the nature of the control processes. Although the data are limited, both vertebrate and invertebrate organic matrices appear to be organized according to the same “basic motif,” namely a core of relatively hydrophobic structural macromolecules (usually proteins) and surface layers of acidic proteins and polysaccharides. Analyses of the latter from different invertebrate phyla using reversed phase high performance liquid chromatography, reveal that the same two classes of macromolecules are present in each of the three cases studied, emphasizing the fundamental importance of these components in crystal growth. Substantial information, at the molecular level, on the conformations and orientations of matrix constituents in relation to the mineral crystal lattice, is available only for mollusk shells, and to some extent on vertebrate tooth enamel. In these cases the major matrix constituents are aligned with one or more mineral crystallographic axes. These observations suggest that the matrix performs active, specific roles in crystal growth. Although it is still premature to assess the importance of various basic crystal growth mechanisms, the data available do not preclude the possibility that epitaxial crystal growth is an important factor

Resource Acquisition and Alternative Mating Strategies in Water Striders

Abstract: Behavioral polymorphisms occur among male and female water striders, Gems remigis , when competing for food and mates. Individuals of both sexes vie for positions in the fastest flowing portions of streams. Here prey capture rates are highest, as are those of swimming and aggression. Only the largest females, and males with the largest first appendages, can regularly maintain positions in these areas. The remaining females are arranged along the flow gradient according to their size with the smallest holding positions in pools of slow moving water. For the remaining males neither overall size, nor the size of the first appendages, appears to determine which males swim near the edge of streams, or which males swim as satellites behind those occupying the fast flowing productive areas. Preliminary data show that mating success of edge and satellite males are about equal, but significantly less than that of the centrally positioned males with the largest first appendages. Thus although it appears that morphological phenotype influences male competitive behavior, when the absolute size of the critical trait is small males adopt behavior after assessing the actions of others. For these “subordinate” males, behavioral assessment appears to produce an “ideal free” spatial distribution.

The Evolution of Neuroanatomical Substrates of Reproductive Behavior: Sex Steroid and LHRH-Specific Pathways Including the Terminal Nerve

Abstract: Fairly recent anatomical methods have made possible the mapping of neurobehavioral systems involving two types of reproductive hormones, gonadal steroids and the peptide luteinizing hormone releasing hormone (LHRH). Brain sites of steroid uptake are detected using autoradiography; LHRH is localized in cells and fibers using immunocytochemical procedures. Both hormone types are known to strongly influence sex behavior and it can reasonably be assumed that these effects are mediated in large part via systems identified using the anatomical procedures. Analysis of the comparative anatomy of these systems should therefore provide information useful in the construction of models concerning the evolution of neurohormonal control of reproductive behavior. The results of such a study are reported. Sex steroid and LHRH systems in cyclostomes, teleosts, amphibians, reptiles, birds and mammals are considered in detail. A synthesis of this information has led to the following ideas. Androgenic control of male reproductive systems has evolved in a number of nonhomologous motor systems controlling male reproductive behavior. Sex steroid and LHRH systems may interact at several different levels of the neuraxis but the most obvious overlap of the systems occurs in the septal and POA areas. The latter especially is a fairly constant and perhaps primitive feature. LHRH secretion into the systemic circulation was most likely the earliest means for LHRH modulation of both pituitary function and neural systems controlling reproductive behavior.Pathways for more direct delivery of LHRH to pituitary cells and brain nuclei probably developed in the early gnathostomes. The terminal nerve appears to be a rather conservative LHRH-containing pathway connecting olfactory systems with septal-preoptic nuclei. A function in pheromonal control of sex behavior is suggested. The general distribution of steroid concentrating cells and LHRH pathways in tetrapods seems to be rather constant. Absence of the systems in neocortical areas and their homologs is conspicuous.

The Role of Carbonic Anhydrase in Blood Ion and Acid-Base Regulation

Abstract: The role of carbonic anhydrase (CA) in ion transport processes of aquatic and terrestrial arthropod species is reviewed In both insects and crustaceans CA is found in a variety of ion transporting tissues The bulk of CA activity in crustaceans is concentrated in the posterior gills, which are morphologically and biochemically adapted for ion transport The enzyme can be specifically localized to gill lamellae which contain large populations of salt transporting chloride cells Enzyme activity in the posterior gills of species having the ability to regulate blood ion concentrations increases when these organisms are acclimated to environmental salinities in which they ion regulate In stenohaline, ion conforming species branchial CA activity is uniformly low, being only 5–10% that in regulating species Studies on the blue crab, Callinectes sapidus , using the specific CA inhibitor acetazolamide have shown that the enzyme is indeed important in blood ion regulation Blood Na$ and Cl− concentrations are both severely lowered in drug-treated animals acclimated to low salinity, while they remain virtually unaffected in animals acclimated to high salinity, in which the animal is an ion conformer High salinity acclimated crabs treated with acetazolamide do not survive transfer to low salinity, and mortality is related to a breakdown in the ion regulatory mechanism Branchial CA most likely functions in the hydration of respiratory CO2 to H$ and HCO3−, which serve as counterions for the active uptake of Na$ and Cl−, respectively In terrestrial species the role of CA is unclear and merits further investigation

Mechanism of Fundulus Epiboly—A Current View

Abstract: This paper summarizes evidence for the following picture of Fundulus epiboly, with an eye toward laying groundwork for future investigation. The major force in epiboly is the yolk syncytial layer (YSL). Prior to epiboly, it spreads well beyond the border of the blastoderm to form the wide external YSL (E-YSL). This has contractile properties, which, however, are restrained prior to epiboly by the attached enveloping layer (EVL) of the blastoderm. Epiboly begins when the E-YSL contracts and narrows, throwing its surface into folds and pulling the internal YSL (I-YSL) and the attached EVL vegetally. When the narrowing of the E-YSL has ceased, it is postulated that its contractility continues as a circumferential wave of vegetally directed contraction that moves over the yolk toward the vegetal pole, dragging the I-YSL and the attached EVL (and blastoderm) with it. The most obvious visible manifestation of this wave is a marked marginal constriction, where the YSL joins the yolk cytoplasmic layer (YCL). As this contractile wave passes over the yolk, cytoplasm from the YCL mingles with that of the advancing E-YSL, and YCL surface adds to the already highly convoluted surface of the E-YSL. This folded surface is the site of a thin, highly localized band of rapid endocytosis that encircles the egg and passes over it with the E-YSL in a wave throughout epiboly. This internalization, which is receptor independent and therefore somehow programmed, accompanies the putative contractile wave, and accounts for the disappearance of the surface of the YCL. Since the YCL surface stands in the way of the advancing YSL, its internalization is part of the mechanism of epiboly. As the I-YSL expands in response to this marginal pull, its abundant microvilli gradually disappear, providing surface for its epiboly. The firmly attached EVL likewise expands toward the vegetal pole in response to the pull of the autonomously expanding YSL. As epiboly of the EVL progresses, it adjusts to the geometric problems posed by a sheet expanding over a sphere by active cell rearrangement within the cell monolayer. Thus, epiboly of the EVL has an active as well as a passive component. Deep cells are not causally involved in epiboly, but move about in coordinated ways in the constantly increasing space between the I-YSL and the EVL provided by epiboly and form the germ ring and the embryonic shield and eventually the embryo proper. An attempt is made to pull all of this together, and more, in order to achieve as comprehensive an understanding of epiboly as present evidence will allow.

Membrane-Bound ATPases in Arthropod Ion-Transporting Tissues

Abstract: Since Na$$K$-activated ATPase was first described, using arthropod tissues, it has become well-recognized as the enzymatic equivalent of the sodium pump. Occurring in the basolateral plasma membranes of epithelial ion-transporting cells, it is responsible for the transport of Na$ out of cells in exchange for the cytosol-directed movement of a counterion (K$ or NH4$). Its kinetic and dynamic properties suggest that it serves as a major limiting factor in whole-body Na$ regulation by aquatic arthropods. Its contribution to NH4$ excretion awaits isolation of Na$ $ K$-ATPase-enriched membrane fractions and determination of their transport properties. The role of Na$$K$-ATPase in insect epitheha is made uncertain by the apparent inaccessibility of the ATPase to the inhibitor ouabain. Two other membrane-bound ATPases, K$-stimulated ATPase and anion-dependent ATPase, have been described in arthropod tissues, but their physiological roles are not clear.

Alternative Mating Behaviors of Young Male Bullfrogs

Abstract: Yearling male bullfrogs often attempt to intercept and mate with females attracted to older, larger males (male parasitism) instead of defending a territory and attracting their own mates. Mating behaviors like male parasitism are thought to occur only when individuals suffer some strong disadvantage in using the behaviors that are more typical of their species. In this paper, I consider the relative benefits and costs of territoriality vs male parasitism for yearling males. Yearlings employing male parasitism are only occasionally successful at mating, and in years of reduced male-male competition, they produce significantly fewer young than territorial males of their age. Yearling males that vary in the degree to which they employ territorial behaviors instead of male parasitism do not differ measurably in growth rate or chances of mortality; however, indirect information suggests that territoriality should entail a greater chance of mortality. A computer simulation based solely on observed demographic parameters predicts that yearling males should be territorial whenever they can compete as effectively as two-year-old males. This condition appears to provide little restriction on the breeding tactics of yearling males; however, data on male-male aggressive encounters show that the size differences between one-year-old and two-year-old males are usually sufficient to preclude yearling males from competing successfully. Thus, the observed behavioral plasticity of yearling males may be best interpreted as a means of adjusting their behavioral tactics to levels of male-male competition that can vary within and between breeding seasons.

Alternative Female Choice Tactics in the Scorpionfly Hylobittacus apicalis(Mecoptera) and Their Implications

Abstract: Females of the scorpionfly Hylobittacus apicalis choose mates on the basis of material benefits (nuptial arthropod prey size) and probably on the basis of genetic benefits males deliver at mating. Females feed on the male's prey throughout copulation. They prefer males with large prey as mates and often refuse males who present small prey. That females may value male genetic quality is suggested by differences in ability of males to obtain large prey, which if inherited would influence offspring fitness, and by females often terminating mating with males with small prey before they transfer any sperm or a complete ejaculate. Females hunt only when males with prey are not available because hunting exposes individuals to predators. Female Hylobittacus apicalis exhibit alternative mate choice tactics, which are condition-dependent in expression and probably comprise a conditional strategy. Body size, recent feeding history, and male availability determine how discriminating an individual female actually is, and these conditions may determine the value of material and genetic benefits in mate choice decisions. The results suggest that female choice controls male behavior. When females become choosy, males are forced to obtain rare large prey despite the increased risks to males associated with this behavior. The implications of the findings on H. apicalis are discussed in relation to condition dependent female choice patterns in other species and the evolutionary maintenance of female choice.

Comparative Ultrastructure of Arthropod Transporting Epithelia

Abstract: The general organization of arthropod epithelia is compared to that of vertebrates. It is suggested that although ciliated epithelia, stratified epithelia and in some cases continuous muscle sheaths do not occur in arthropods, they have certain analogous structures which carry out the same functions. For example, the arthropod cuticle is compared to the squamous layer of vertebrate stratified epithelia, and complex arthropod basement membranes are compared to the muscle and connective tissue sheaths of certain vertebrate epithelia. The cellular organization of transporting epithelial cells is then discussed, with particular reference to elaboration of plasma membranes, and similarities and differences between vertebrates and arthropods, and between insects and crustaceans are pointed out. Specializations peculiar to insect cells are described, including the insertion of mitochondria into apical membrane microvilli, and the presence along this membrane of small particles called portasomes believed to be involved in active transport. Finally, it is shown that in the midgut of theinsect Manduca sexta , distinct ultrastructural changes accompany loss of potassium transport activity during a larval molt and in the prepupal stage. The ultrastructural changes which occur include a proliferation of the basement membrane and muscle tissue underlying the epithelium, and a change in the morphology of the potassium transporting goblet cells. Possible correlations between ultrastructural changes and loss of transport activity are discussed.

Primary Invagination of the Vegetal Plate During Sea Urchin Gastrulation

Abstract: The initial phase of echinoid gastrulation, primary invagination, involves an inpocketing of a monolayered epithelium. To gain information about the nature of the mechanical forces that are responsible for primary invagination, several experimental approaches have been taken, using the transparent embryos of the sea urchin, Lytechinus pictus , as the principal material. Vegetal plates isolated microsurgically well before the onset of gastrulation will invaginate normally, demonstrating that the forces responsible for primary invagination are generated by the cells in the vegetal ⅓ to ½ of the embryo. As shown by serial reconstructions of L. pictus embryos, relatively few cells (about 100) take part in primary invagination. Both the number of cells and the total volume of tissue in the wall of the archenteron increase with time. Even so, it can be shown that very little movement of cells over the lip of the blastopore takes place during primary invagination, and this process is best viewed as a simple inpocketing of the vegetal epithelium. The cells in the wall of the archenteron have a distinctive shape; they are elongated along their apico-basal axes and frequently have enlarged, rounded, basal ends. However, they do not undergo any dramatic changes in shape during primary invagination. In particular, there is only a slight decrease in the height of the cells (length along the apico-basal axis), a result that is inconsistent with the hypothesis that invagination is due to cell rounding (Gustafson and Wolpert, 1967). Examination of L. pictus and Strongylocentrotus purpuratus gastrulae by transmission electron microscopy reveals that cells in the wall of the archenteron continue to be joined by typical junctional complexes during primary invagination. In addition, the morphology of the junctional complex at the gastrula stage is more elaborate than previously described. Sparse bands of micronlaments are associated with the plasma membrane at the level of the junctional complexes in both endodermal and ectodermal cells. These and other relevant data on early echinoid gastrulation are discussed in relation to several possible mechanisms of epithelial morphogenesis.

The Cellular Basis of Gastrulation in Xenopus laevis: Active, Postinvolution Convergence and Extension by Mediolateral Interdigitation

Abstract: Time-lapse videomicrographic and SEM analyses of normal and microsurgically altered gastrulation show that the morphogenetic movements of the dorsal marginal zone (DMZ)—extension, convergence, and involution—all result from behavior that occurs after the marginal zone has involuted. Before its involution, the DMZ shows no detectable capacity for autonomous convergence or extension. If its involution is prevented, the DMZ will show convergence and extension but only at developmental stages at or beyond the stage at which it normally would have involuted. Thus autonomous convergence and extension, which have been ascribed to the DMZ are, in fact, properties of the dorsal mesodermal mantle (DMM) and the archenteron roof. SEM analysis of cell shape and packing patterns, suggest that cells of the DMM merge (interdigitate) mediolaterally, between one another, beginning just beyond the point of involution. This behavior is thought to reduce the width and increase the length (postinvolution convergence and extension) of the DMM. The decrease in circumference (width) at the vegetal-most part of the newly involuted DMM forms a constriction ring just inside the blastopore. Constriction and concurrent elongation of the DMM act in concert to move the blastoporal lip vegetally. The DMZ is passively pulled vegetally and over the blastoporal lip as deep cells are recruited for participation in mediolateral interdigitation at the vegetal end of the DMM.

Spatial Organization and Mineralization of the Basal Plate of Elasmoid Scales in Osteichthyans

Abstract: In Sarcopterygii ( Latimeria, Neoceratodus, Protopterus, Leptdosiren ) and Amiidae (Amia) collagen fibrils of the basal plate are packed in bundles whereas they remain isolated in Teleostei. The basal plate looks like plywood, a system of superimposed layers of parallel fibers or fibrils the directions of which rotate with a regular angle in two successive layers. The double twisted plywood is constituted of two imbricate systems, the odd and the even, where the rotation of the fibrillar directions is right-handed in Sarcopterygii and lefthanded in Amiidae and numerous primitive Teleostei. The orthogonal plywood, with its two main orthogonal fibrillar directions, characterizes the evolved Teleostei and some more primitive ones. In most teleostean species, as in Amia and Protopterus , mineralization of the basal plate in elasmoid scales involves Mandl's corpuscles that mineralize without contact with a pre-existing calcified tissue; they grow and coalesce with the neighbouring ones and fuse to the mineralizing front. Their shape is directly influenced by the local arrangement of the collagenous fibrils. In two teleostean families (Osteoglossidae and Mormyridae) Mandl's corpuscles are completely lacking but spreading of mineralization in the basal plate has a peculiar aspect. Whatever that may be, the various characteristic organizations of the skeletal tissues or isopedine that constitute the basal plate of osteichthyan elasmoid scales, all are varieties of bone tissue that have undergone more or less important specialization linked to the general regression of dermal ossifications and to functional adaptations.

Structure and Stress-Strain Relationship of Soft Tissues

Abstract: The mechanical properties of a soft tissue are related to its structure. Weshall illustrate this by the properties of the arteries and the lung. Viscoelasticity, strain rate effects, pseudo-elasticity, and constitutive equations ar discussed. The mecahnical properties of an organ is, however, not only based onthe tissues of the organ, but also on its geometry and relationship to the neighboring organs. A typical example is the blood vessel. The capillary blood vessels of the mesentery are “rigid”; those in the bat's wing are “distensible”; whereas the capillaries of the lung are “sheet” like: rigid in one plane, and compliant in another. The stress-strain relationship of the systemic arteries is highly nonlinear, stiffening exponentially with increasing strains; yet that of the pulmonary arteries in the lung is linear. The systemic veins are easily collapsible; yet the pulmonary veins in the lung are not: they remain patent when the blood pressure falls below the alveolargas pressure. The explanation of these differences lies in the varied interactions between the blood vessels and the surrounding tissues in different organs. The implications of these differences on blood circulation are pointed out. Therole of ultrastructure is discussed.

Mechanical Properties of Pedal Mucus and Their Consequences for Gastropod Structure and Performance

Abstract: This study examines the possiblility that the physical properties of pedal mucus limit the size and speed of gastropods. At small deformations hydrated pedal mucus is a viscoelastic solid. At large deformations the mucus yields and becomes a viscous liquid, the yield strength increasing as the rate of deformation increases. The mucus can ‘heal’ if left undeformed, its strength increasing with time. When dehydrated the mucus strength and stiffness increase substantially. These properties can be used to calculate the maximum speed of crawling and the maximum size for gastropods. In all the cases examined the predicted maxima are near those observed in nature, and it seems likely that pedal mucus indeed places limits on gastropod structure and performance.

By Jove!! Why Do Alternative Mating Tactics Assume So Many Different Forms?

Abstract: Tremendous diversity exists in the form of alternative mating tactics (AMTs) employed by males of many species. We develop a general framework in which to view alternatives that vary in (i) their frequency in the population, (ii) their fitness value with respect to the primary tactics, (iii) the extent to which the alternative tactic is site-fixed and (iv) the intrinsic ability of males to change tactics. The frequency and fitness value of alternatives should be influenced by Resource Holding Potential (RHP), which generally varies with age, size, and perhaps energy reserves. AMTs should be unequal in fitness value when RHP increases at an accelerating rate with age. “Subordinate AMTs” can result when various factors favor males attempting to reproduce before reaching the age with maximum RHP. An asymptotic relationship between age and RHP should result in most males in a population having essentially equal RHP. Several ways exist for males to partition the set of mating opportunities between two or more “equal AMTs.” Transient tactics may occur if (i) resources for females and territorial males differ and do not covary positively in their distribution, or (ii) local areas are so attractive to females that males effectively cannot defend them. We suggest Levins' (1968) “fitness set” analysis as a useful model predicting whether a male should specialize on a single tactic, or partition its effort between the two tactics.

Calcium Dynamics in Land Gastropods

Abstract: In land gastropods, calcium is precipitated in the shell, in connective calcium cells which are largely distributed through the whole connective tissue, in epithelial calcium cells of the digestive gland, and in the calcium gland cells of the skin and the mantle collar. Calcium is taken up from the external medium by food and by absorption through the sole skin. To adapt to terrestrial life, these animals have to eliminate appreciable amounts of calcium for their protection and their reproduction. During the egg laying period, a calcium flux occurs through the epithelium of the reproductive tract in order to supply the egg shell and the egg fluids. This egg calcium is taken up by the embryo. The maintenance of a positive calcium balance between its uptake and the loss is due to an important reservoir of easily mobilizable calcium in the form of calcium carbonate. This reservoir consists of the connective calcium cells which are constantly able to accumulate or release calcium as long as calcium is locally available or required. The epithelial calcium cells of the digestive gland are loaded with calcium phosphate; they are not a major calcium storage compartment, but have an essential function in detoxification. All of the calcium movement occurring across cell membranes and through epithelia concerns only calcium ions. All calcium movement can be regarded either as on-off systems or as reversible systems, both of which are certainly controlled by complex processes

Genetic Variation Underlying Sexual Behavior and Reproduction

Abstract: Selection depletes additive genetic variation underlying traits important in fitness. Intense mating competition and female choice may result in negligible heritability in males. Females often appear to choose mates, however, suggesting genetic variation in males which is important to females. Evidence is reviewed on allelic substitutions, karyotypic variation, and especially the heritable variation of continuous traits involved in sexual behavior and reproduction. Phenotypic variation in male mating speed and courtship intensity, female mating and oviposition behavior, egg size and number, body size, parthenogenesis, and the sex ratio generally have heritable variation. The maintenance of genetic variation, and the meaning of heritability estimates for natural populations is considered.