Showing papers in "BioScience in 1983"
TL;DR: Brown-headed cowbird populations and their rate of brood parasitism on forest songbirds in eastern North America have increased since 1900 and may be responsible for their recent declines.
Abstract: Brown-headed cowbird populations and their rate of brood parasitism on forest songbirds in eastern North America have increased since 1900. Brood parasitism of forest songbirds is highest near open habitat. High brood parasitism rates within isolated fragments of forest habitat reduce reproductive success of certain forest songbirds and may be responsible for their recent declines. (Accepted for publication 10 July 1982)
595 citations
TL;DR: A regional network of preserves, with sensitive habitats insulated from human disturbance, might best perpetuate ecosystem integrity in the long term.
Abstract: Land managers have traditionally assumed that achieving maximum local habitat diversity will favor diversity of wildlife. Recent trends in species composition in fragmented landscapes suggest, however, that a more comprehensive view is required for perpetuation of regional diversity. A regional network of preserves, with sensitive habitats insulated from human disturbance, might best perpetuate ecosystem integrity in the long term. (Accepted for publication 5 May 1983)
575 citations
TL;DR: A conservative approach to the maintenance of services through minimizing anthropogenic extinctions is recommended in this paper, where a conservative approach is proposed to the minimization of anthropogenic extinction, which is based on minimizing the number of elements deleted and the degree of control each exerted in the system.
Abstract: The loss of services to humanity following extinctions ranges from trivial to catastrophic, depending on the number of elements (populations, species, guilds) deleted and the degree of control each exerted in the system. Most attempts to substitute other organisms for those lost have been unsuccessful, to one degree or another, and prospects for increasing the success rate in the foreseeable future are not great. Attempts to supply the lost services by other means tend to be expensive failures in the long run. A conservative approach to the maintenance of services through minimizing anthropogenic extinctions is recommended. (Accepted for publication 20 October 1982)
417 citations
TL;DR: Polyamines and their biosynthetic enzymes are probably ubiquitous in plant cells and their distribution, their changes in response to such stimuli as light, hormones, pollination, stress and senescence, suggest a regulatory role in plants.
Abstract: Polyamines and their biosynthetic enzymes are probably ubiquitous in plant cells. Their distribution, their changes in response to such stimuli as light, hormones, pollination, stress and senescence, and their effects when applied exogenously suggest a regulatory role in plants. Polyamine effects probably involve interaction with nucleic acids and membranes. (Accepted for publication 12 January 1983)
348 citations
TL;DR: In the marine euphotic zone, the bacterial biomass in the coastal and open oceans is large and productive, suggesting that bacteria are important as producers of organic matter as discussed by the authors, and it appears likely that the bacterial production is removed quantitatively by small predators.
Abstract: Recent research suggests that aerobic, heterotrophic bacteria in the marine euphotic zone are not merely decomposers of detritus. The bacterial biomass in the coastal and open oceans is large and productive, suggesting that bacteria are important as producers of organic matter. It appears likely that the bacterial production is removed quantitatively by small predators. (Accepted for publication 16 March 1983)
264 citations
TL;DR: For several years the University of Alaska and the Institute of Northern Forestry (USDA Forest Service) have conducted a multidisciplinary study of interior-Alaska forest ecosystems, especially the black spruce type as mentioned in this paper.
Abstract: For several years the University of Alaska and the Institute of Northern Forestry (USDA Forest Service) have conducted a multidisciplinary study of interior-Alaska forest ecosystems, especially the black spruce type. Black spruce forests are widespread in interior Alaska and are the most fire-prone forest type. They are also the most nutrient-limited and least productive forest type, especially in the late stages of succession. Ecosystem differences in productivity and degree of nutrient limitation are controlled mainly by soil and forest-floor temperatures. (Accepted for publication 3 August 1982)
220 citations
186 citations
TL;DR: This discussion of the structure, function, and value of freshwater tidal wetlands is based on studies of three Delaware River wetlands: the Hamilton Marsh near Trenton, NJ, Woodbury Creek Marsh south of Camden,NJ, and Tinicum Marsh near Philadelphia, PA.
Abstract: ties of less than one percent, but insufficient flow to dampen upstream tidal movement. Odum et al. (1979) conservatively estimate that there are 500,0001,000,000 ha of freshwater tidal wetlands along the Atlantic and Gulf Coasts, of which 100,000-140,000 ha are in New Jersey. Almost all of the major East Coast cities from Trenton, NJ, to Richmond, VA, are near freshwater tidal wetlands. Consequently, these wetlands are greatly affected by human activities. Our discussion of the structure, function, and value of freshwater tidal wetlands is based on studies of three Delaware River wetlands: the Hamilton Marsh near Trenton, NJ, Woodbury Creek Marsh south of Camden, NJ, and Tinicum Marsh near Philadelphia, PA. In freshwater tidal wetlands the major system components-producers, consumers, detritus, sediment, and nutrients-are coupled by biological and physical processes that transfer materials and energy (Figure 1). Materials, such as organic matter, nutrients, heavy metals, and sediment, enter freshwater tidal wetlands from sources including the atmosphere, tides, point-source effluents, non-point-source runoff, groundwater, and consumer immigration. Outputs are via the atmosphere, tides, and consumer emigration. Along the urbanized upper Delaware River estuary, tidal waters provide the most important inputs, although point-source effluent and non-point-source runoff may locally contribute significant quantities of nutrients and heavy metals (Walton and Patrick 1973). Wetland function is ultimately controlled by climate, but hydrologic parameters such as duration and frequency of inundation, and the velocity and source of the water determine the physical and chemical properties of wetland substrates (Gosselink and Turner 1978). In turn, substrate characteristics dictate specific ecosystem responses, including primary production, species diversity, decomposition, and uptake and release of nutrients.
157 citations
126 citations
TL;DR: In agriculture, the main objectives are to achieve maximum yields, operate with a maximum profit, minimize year-to-year instability in production, and prevent long-term degradation of the productive capacity of the agricultural system as mentioned in this paper.
Abstract: Since its beginning, agriculture has tested the resiliency of nature. Natural communities have been replaced with artificially supported, productive communities. Agriculture's objectives generally have been to achieve maximum yields, operate with a maximum profit, minimize year-to-year instability in production, and prevent long-term degradation of the productive capacity of the agricultural system (Watt 1973). Theoretically, these objectives should be compatible and mutually reinforcing. Unfortunately, developments in agriculture have removed the crop ecosystems from their parent nonagricultural ecosystems (Potts and Vickerman 1974) to the extent that agroecosystems and natural ecosystems have become strikingly different in structure and function (Table 1). The maintenance of an imposed order of simplified agricultural systems against the natural tendency toward entropy, diversity, and stability demands energy and resources (Turnbull 1969). The depletion of nutrients, loss of soil fertility, and the alteration of soil structure must
110 citations
TL;DR: Pocosins-nutrient-poor, freshwater, evergreen shrub bogs-were developed by peat accumulation and blocked drainage over the last 10,000 years have resulted in a shift of hydrologic output from evapotranspiration to runoff; significant increases in carbon flux and P, K, and Ca output; and a reduction in habitat for rare and endangered biota while dramatically increasing the economic value of these lands.
Abstract: Pocosins-nutrient-poor, freshwater, evergreen shrub bogs-were developed by peat accumulation and blocked drainage over the last 10,000 years. They once covered nearly 1 million hectares on the North Carolina coastal plain. Extensive drainage for agriculture, forestry, and peat mining reduced these natural wetland areas to 281,000 hectares by 1980. This development has resulted in a shift of hydrologic output from evapotranspiration to runoff; significant increases in carbon flux and P, K, and Ca output; and a reduction in habitat for rare and endangered biota, while dramatically increasing the economic value of these lands. (Accepted for publication 25 March 1983)
TL;DR: Plasmodesmata traverse the wall between adjacent plant cells and interconnect them by way of concentric cylinders of plasma membrane and endoplasmic reticulum, analogous to the gap junctions of animal tissues with respect to continuity with the cytosol compartment and molecular size cut-off for transport.
Abstract: Plasmodesmata traverse the wall between adjacent plant cells and interconnect them by way of concentric cylinders of plasma membrane and endoplasmic reticulum. The endoplasmic reticulum component probably has no lumen, but the annulus between the two membranes is analogous to the gap junctions of animal tissues with respect to continuity with the cytosol compartment and molecular size cut-off for transport. (Accepted for publication 4 November 1982)
TL;DR: A recent analysis of the relation between animal coat color and solar heat gain stated "there still is a question as to whether dark or light plumage is more effective in utilization of direct solar radiation" (Lustick et al. as mentioned in this paper ).
Abstract: A recent analysis of the relation between animal coat color and solar heat gain stated "there still is a question as to whether dark or light plumage is more effective in utilization of direct solar radiation" (Lustick et al. 1980). This may surprise some, since common experience indicates that dark surfaces acquire greater heat loads when exposed to solar radiation than do light surfaces. For animals such as birds or mammals that possess insulating coats of fur or feathers, however, recent studies have shown that the thermal consequences of coloration present the potential for widely varying effects that have not been generally appreciated by biologists. It is clear that an animal's surface coloration might importantly affect heat exchange with its environment and thus its thermoregulatory demands. Surface coloration arises from differential reflection or transmission of short-wave radiation (the longest wavelength visually perceived by animals typically is near 700 nm). That radiation not reflected or transmitted by the surface is absorbed, resulting in sensible heat generation. With the exception of bioluminescence, the only important natural sources of short-wave radiation are very hot objects, notably stars such as the sun. The intensity of solar radiation reaching the earth's surface varies, but under clear skies often reaches values of about 1000 W/m2 on a plane perpendicular to the solar beam. Roughly one-half of this energy lies in visible wavelengths and hence is of sufficient magnitude that coat color might significantly modify an animal's heat balance.
TL;DR: In this paper, natural isotopes of strontium (87Sr/86Sr) were used to quantify instantaneous atmospheric inputs and their net accumulation over successional time, identify the distribution and movement of atmospheric inputs within the ecosystem, and quantify weathering inputs.
Abstract: Forest canopies can collect nutrients from the atmosphere, but the magnitude is difficult to quantify. Natural isotopes of strontium (87Sr/86Sr) can be used to quantify instantaneous atmospheric inputs and their net accumulation over successional time, identify the distribution and movement of atmospheric inputs within the ecosystem, and quantify weathering inputs. (Accepted for publication 23 June 1982)
TL;DR: A computer-operated spectrograph was recently built at Okazaki, Japan as discussed by the authors, where different specimens can be placed on a horseshoe-shaped focal curve (10 m long) covering a wavelength range of 250 to 1000 nm.
Abstract: A computer-operated spectrograph was recently built at Okazaki, Japan. Different specimens can be placed on a horseshoe-shaped focal curve (10 m long) covering a wavelength range of 250 to 1000 nm so they can be irradiated simultaneously. The linear dispersion is about 0.8 nm/cm. The photon fluence rate on the focal curve is 5 x 1015. photons x cm-2 x s-1 at 300nm and 1 x 1016 photons x cm-2 x s-1 at 600 and at 900 nm. The spectral half width is 5.5 nm or less on the focal curve. The stray light content is about 10-5 of the main peak at the peak wavelength ± 100 nm. Specimens are set in microcomputer-controlled threshold boxes so that wavelengths, photon fluence rates, photon fluences and timing of irradiations are controlled automatically according to a pre-programmed schedule. An optical fiber system is also provided for remote irradiations.
TL;DR: Berenbaum et al. as mentioned in this paper provided a case study with either direct or circumstantial evidence for each part of the coevolutionary process of the Rhopalocera family.
Abstract: Ehrlich and Raven (1964) were among the first to focus on coevolution as a distinct evolutionary process. In their formulation, insect-plant coevolution is a five-step sequence: 1. by mutation and recombination, angiosperms produce novel secondary substances; 2. by chance, these new secondary substances alter the suitability of the plant as food for insects; 3. the plants, released from the restraints imposed by herbivory, undergo evolutionary radiation in a new adaptive zone; 4. by mutation or recombination, insects evolve mechanisms of resistance to the secondary substances; 5. able to exploit a plant resource hitherto excluded from herbivores, the adapted insects enter a new adaptive zone and undergo their own evolutionary radiation. This scenario was inspired by broad patterns of hostplant utilization among families of butterflies (Rhopalocera). Although the schema gained widespread acceptance, to date no specific example demonstrates most or all of the steps in the sequence. This lack of empirical evidence has been the subject of considerable criticism (e.g., Jermy, 1976; Janzen, 1980). Recent experimental work on associations between various insects and plants containing furanocoumarins and related compounds (Berenbaum, 1978, 1980, 1981a, 1981b, 1981c), however, provides a case study with either direct or circumstantial evidence for each part of the coevolutionary process.
TL;DR: Comparative studies of these particular organelle genes and the RNAs they encode now provide the most compelling evidence available that mitochondrial and plastid genomes are descended from the genomes of bacteria-like organisms that entered into an endosymbiotic relationship with a primitive host cell bearing the nuclear genome.
Abstract: The mitochondria and plastids of eukaryotic cells contain distinctive DNAs that encode the ribosomal RNA and transfer RNA components of translation systems that are specific for these organelles. Comparative studies of these particular organelle genes and the RNAs they encode now provide the most compelling evidence available that mitochondrial and plastid genomes are descended from the genomes of bacteria-like organisms ("protomitochondria," "protoplastids") that entered into an endosymbiotic relationship with a primitive host cell bearing the nuclear genome. (Accepted for publication 3 May 1983)
TL;DR: In male amphibians, reproductive behaviors are influenced by both steroid hormones and neuropeptides and testosterone and dihydrotestosterone are necessary for maintaining reproductive behaviors.
Abstract: In male amphibians, reproductive behaviors are influenced by both steroid hormones and neuropeptides. Testosterone and dihydrotestosterone are necessary for maintaining reproductive behaviors. Vasotocin, prolactin, and luteinizing hormonereleasing hormone (LHRH) each are implicated in activating amphibian behaviors. When amphibians are in harsh environments, corticosterone levels rise and rapidly inhibit reproductive behaviors. (Accepted for publication 11 March 1983)
TL;DR: Interactions between plants and animals that lived during the Carboniferous will be discussed in the context of animal morphology, plant morphology, and coprolite analysis.
Abstract: Perhaps the best-known plant and animal fossils are those that occur in rocks of Carboniferous (Mississippian and Pennsylvanian) age. The organisms that these fossils represent lived in the great coal-forming swamps that spanned a period of approximately 65 million years, beginning about 345 million years ago. There are two primary reasons why these organisms are known in such detail. First, the vegetation that contributed the plant material to the formation of coal was luxuriant and diverse. Second, the extensive mining of coal has provided paleontologists with a unique opportunity to discover organisms of this age that would certainly have not been unearthed were it not for the economic importance of coal. Thus, today it is possible to accurately characterize the Carboniferous fauna and flora from many geographic regions of the world. Although paleontology has traditionally been a rather descriptive science, in recent years the study of fossil organisms has become more biological in scope. For example, the life history of many fossil plants and animals is now known in great detail because of extensive collections that have provided an opportunity to investigate changes during the development of an organism. The study of pollination syndromes and reproductive biology (e.g., Taylor 1977) in certain plants and the spatial distribution (paleoecology) of plants and animals within the coal swamps (e.g., Phillips 1981, Scott 1979) are also rewarding avenues of paleontological research. One additional area that now t:an be seriously considered concerns the complex interrelationships of plants and animals that inhabited the coal swamps of the Carboniferous. In modern ecosystems a wide range of plant-animal interrelationships exist. Some of these are related to feeding, and many of the features associated with herbivory may have been the result of coevolution. Moreover, many plants have evolved complex defense mechanisms that may be both morphological and chemical. Clearly, an analysis of modern interactions can assist in better understanding the interrelationships deduced from the fossil record. In this paper interactions between plants and animals that lived during the Carboniferous will be discussed in the context of animal morphology, plant morphology, and coprolite analysis.
TL;DR: Repertoire sizes and song development among birds must be understood as facets of life history strategies that appear related to population densities, with more frequent interactions in dense populations leading to an escalation of signal diversity.
Abstract: Repertoire sizes and song development among birds must be understood as facets of life history strategies. In some groups repertoires appear related to population densities, with more frequent interactions in dense populations leading to an escalation of signal diversity. The extent and timing of vocal learning also differs among taxonomic groups and depends to some extent on habitat stability, site fidelity, and dispersal strategies of young. (Accepted for publication 1 October 1982)
TL;DR: Teleosts provide abundant opportunities to examine the physiological regulation of sexual behavior in fish and studies of fish with ancestral reproductive patterns should provide insight into the origins or vertebrate hormone-sex behavior mechanisms.
Abstract: Teleosts provide abundant opportunities to examine the physiological regulation of sexual behavior. As in other vertebrates, hormones synchronize behavior with gonadal development in the individual, while pheromones synchronize reproductive activities between individuals. Studies of fish with ancestral reproductive patterns should provide insight into the origins or vertebrate hormone-sex behavior mechanisms. (Accepted for publication 11 March 1983)
TL;DR: Experimental information on recombinationalrepair in the various organisms that have been studied, on the range of organisms that use recombinational repair, onThe variety of lesions against which it acts, and on its efficiency in overcoming DNA damage are reviewed to support the hypothesis that recombinational Repair is a major function of sexual reproduction.
Abstract: Many different organisms are known to repair their DNA. implying that this ability is widespread in nature and suggesting that DNA damage is a general biological problem. In higher organisms selective pressures to minimize DNA damage are probably stronger in the germ line than in the somatic line, since damage in DNA that is passed on to descendants will tend to have more serious consequences for genetic survival than damage in somatic-cell DNA. It has recently been proposed that an important function of sexual reproduction is to provide a special opportunity for recombinational repair of germ-line DNA through the pairing of homologous chromosomes (see Bernstein et al. 1981 and references therein). This pairing may allow a damaged DNA sequence in one chromosome to be restored through physical exchange of DNA (recombination) with another homologous chromosome. In addition to its probable role in sexual reproduction, recombinational repair seems to be an important mechanism for dealing with DNA damage in vegetatively growing bacteria and fungi. In experiments investigators usually infer that recombinational repair has occurred if, after treatment of an organism with a DNA-damaging agent, the viability of the treated organism and/or the restoration of the integrity of its damaged DNA can be shown to depend on the presence of more than one copy of a chromosome and gene functions needed for genetic recombination. This paper reviews experimental information on recombinational repair in the various organisms that have been studied, on the range of organisms that use recombinational repair, on the variety of lesions against which it acts, and on its efficiency in overcoming DNA damage. This evidence supports the hypothesis that recombinational repair is a major function of sexual reproduction.
TL;DR: Mutations affecting the circadian clocks of Neurospora crassa and Drosophila melanogaster have identified genes that play important roles in clock organization and have introduced recombinant-DNA technology to the study of circadian clocks.
Abstract: Mutations affecting the circadian clocks of Neurospora crassa and Drosophila melanogaster have identified genes that play important roles in clock organization and have introduced recombinant-DNA technology to the study of circadian clocks. In Drosophila they have also identified neurosecretory cells in the brain as candidates for a "master control" element in the circadian organization of the fly. (Accepted for publication 28 February 1983)
TL;DR: Individual higher plants may be male or female to a varying degree and patterns of parental sex allocation reflect the evolutionary fitness that may be gained by assuming various gender roles.
Abstract: Individual higher plants may be male or female to a varying degree. The division of sexual labor leaves the maternal parent in any successful mating bearing the additional cost of nurturing embryos within developing seeds. Patterns of parental sex allocation reflect the evolutionary fitness that may be gained by assuming various gender roles. The evolution of gender separation is discussed. (Accepted for publication 15 August 1982)
TL;DR: Evidence is reviewed suggesting that ovarian steriods modulate the expression by female primates of behavior that increases the likelihood of copulation (proceptivity), in addition to affecting females' sexual attractivity.
Abstract: Evidence is reviewed suggesting that ovarian steriods modulate the expression by female primates of behavior that increases the likelihood of copulation (proceptivity), in addition to affecting females' sexual attractivity. Data often cited as showing that circulating androgens regulate sexual proceptivity and receptivity in female primates are critically reviewed. The essential interaction of social and endocrine factors in the control of female primates' sexual activity is emphasized. Accepted for publication 19 March 1983)
TL;DR: It is suggested that the so-called "rhabdites" of acoel turbellarians are not homologous to the true rhabdite found in the higher turbellarian species, indicative of phylogenetic distance between the acoeel-nemertodermatid line and the higher turellarians.
Abstract: The use of turbellarian rhabdites for phylogeny has been extended by an investigation of the ultrastructure and histochemistry of mature rhabdites and some ultrastructural details of rhabdite formation in 14 turbellarian species representing the orders Acoela, Macrostomida, Rhabdocoela, and Polycladida. The results of this investigation are compared with other published descriptions; a new definition of a rhabdite, based on comparative ultrastructure and histochemistry, is proposed. Three morphological variants of a rhabdite are recognized: (1) the Macrostomum-type, found only in the genus Macrostomum; (2) the lamellated type, found in members of the orders Macrostomida, Polycladida, Rhabdocoela, Temnocephalida, and in at least one member of the order Proseriata; and (3) the triclad type, found only in the order Tricladida. There is a reasonably high probability of these three types being homologous. Our research, and that of others, suggests that the so-called "rhabdites" of acoel turbellarians are not homologous to the true rhabdites found in the higher turbellarians. This conclusion is indicative of phylogenetic distance between the acoel-nemertodermatid line and the higher turbellarians. Within the last decade, the electron microscope has become established as a tool of phylogenetic research, especially in those areas where the light microscope has failed to provide a sufficient number of complex characters ' The authors dedicate this paper to the late Professor Dr. Erich Reisinger, a pioneer in the use of ultrastructure for systematics. This study was supported in part by NSF Grant GB42211 (R. M. Rieger, P. I.). Partial support for the research on Convoluta was obtained from Sigma Xi. The senior author gratefully acknowledges Dr. David A. Doe, of Westfield State College, who generously provided both sectioned and unsectioned material of Macrostomum spp. 1, 2, and 3, and of Paromalostomum sp. for this study. The authors thank Ms. Sandra F. Zane for preparing the plates. 2 Present address: Department of Zoology, University of Maine at Orono, Orono, Maine 04469. 3 Electron Microscopy Center, Washington State University, Pullman, Washington and Department of Zoology, University of Maine at Orono, Orono, Maine 04469. 4 Department of Biology, University of North Carolina at Charlotte, Charlotte, North Carolina 28223. TRANS. AM. MICROSC. Soc., 101(3): 209-228. 1982. c Copyright, 1982, by the American Microscopical Society, Inc. This content downloaded from 207.46.13.142 on Fri, 10 Jun 2016 05:46:42 UTC All use subject to http://about.jstor.org/terms TRANS. AM. MICROSC. SOC. for reliable analysis. Comparative ultrastructural studies of the lower Metazoa generally have dealt with micro-organs or with single cell types (cf. Storch, 1979). Fewer studies have attempted to reach phylogenetic conclusions based on the ultrastructure of subcellular structures. Studies of this nature include comparative morphology of turbellarian rhabdites (Reisinger & Kelbetz, 1964; Reisinger, 1969), comparisons of microtubular termination patterns in ciliary tips (Tyler, 1979), and comparative morphology of eggshell globules (Gremigni, 1979). The methodology of systematics is applicable to subcellular structures when electron microscopy provides sufficient substructural information for comparison, and when this information is viewed in the context of other phylogenetic data (see Rieger & Tyler, 1979). One subcellular structure that offers promise for clarification of systematic relationships among the turbellarian orders is the rhabdite. Although these rod-shaped, epidermal, mucous-secretion bodies are found in most turbellarians, and frequently have an unusual and highly characteristic ultrastructure (cf. Reisinger & Kelbetz, 1964; Lentz, 1967; Martin, 1978a), the pioneering work of Erich Reisinger in the use of these structures for systematics has not been elaborated (see Reisinger, 1969). The term "rhabdite" was applied by Hyman (1951) specifically to rhabdoid secretions of turbellarians that are shorter than the height of the epidermis and that are secreted by glands located either in the epidermis (epidermal rhabdites) or below the epidermis in the mesenchyme (adenal rhabdites). Others have used the term more loosely to include all rod-shaped bodies in turbellarians or even in other groups of invertebrates, with little consideration given to whether or not they are homologous (Martin, 1978a). In an attempt to resolve the semantic confusion by more clearly defining the nature of a "rhabdite" in the Turbellaria, rhabdoids of 14 species representing eight different families among four turbellarian orders (see Table I) were examined morphologically. The rhabdoids of four of these species were characterized histochemically. In order that the terminology be clear from the outset, the term "rhabdite" will be applied as originally proposed by Hyman; other rod-shaped epidermal secretions will be referred to by the more general term "rhabdoid." MATERIALS AND METHODS Specimens of Microstomum sp., Myozona sp., Macrostomum spp. 1, 2, and 3, Psammomacrostomum sp., Bradynectes sp., Paramyozonaria sp., Paromalostomum sp., Messoplana falcata, and Theama sp. were collected and processed for transmission electron microscopy (TEM) as described by Tyler (1975). Specimens of Stylochus zebra were obtained from the Marine Biological Laboratory, Woods Hole, and processed for TEM as described by Fritz & Thomas (1976). Specimens of Convoluta sp., obtained from mid-tide-level surface sediment on the south bank of the White Oak River inlet at Swansboro, North Carolina, and specimens of Kuma sp. (emend. Faubel, 1976), obtained from low-tide-level sediment on Bogue Sound, North Carolina, near the Institute for Marine Sciences, were processed for TEM as described by 210 This content downloaded from 207.46.13.142 on Fri, 10 Jun 2016 05:46:42 UTC All use subject to http://about.jstor.org/terms VOL. 101, NO. 3, JULY 1982
TL;DR: The gonadotropic hormones act on the gonad where they stimulate both gametogenesis and steroidogenesis, resulting in gonadal growth and spermatogenesis, which are maximal at the time of mating.
Abstract: * The gonadotropic hormones in turn act on the gonad where they stimulate both gametogenesis and steroidogenesis. * Gonadal growth is associated with sexual activity; spermatogenesis and steroidogenesis are maximal at the time of mating. * The sex steroid hormones produced by the growing gonad are concentrated in specific areas of the brain and "activate" reproductive behavior. * The secretion of hypothalamic releasing hormones and brain neurotransmitters is also influenced by the sex steroid hormones.