It seems particularly opportune to discuss food webs and evolving views on their structure here for both their genesis and first modern treatment (Elton 1927) and much of their later development (May 1973; Pimm & Lawton 1978) has a decidedly British accent to it. The central significance of webs is derived from the fact that the links between species are often easily identified and the resultant trophic scaffolding provides a tempting descriptor of community structure. If this structure is in any fashion related to the persistence of natural communities or their stability, however defined, then we are dealing with issues of vital ecological importance. Elton's views have admirably withstood the tests of time. They were especially useful to field biologists, and encouraged the assembly and organization of feeding data into networks of trophically bonded species or higher taxa. The early emphasis was on connectedness per se. Perhaps the first significant deviation from this theme was the development of the trophic dynamic viewpoint of Lindeman (1942) and all subsequent efforts to describe energy transfer and material flow through communities. A second departure, and one I believe to be conceptually richer, was the formalization of the view that web structure and community stability were related (MacArthur 1955). May (1973) in another landmark publication questioned this relationship and called attention to four primary web features: the number of species involved, the nature of their interconnections, the number of connections per species, and the intensity of interaction between web members. This focus has stimulated application to agroecosystems (Southwood & Way 1970), new interpretations of the number of trophic levels (Pimm & Lawton 1977), and a resurgence of interest in the significance of mutualism (Vance 1978). It has not been characterized by stunning breakthroughs, ecological stability remains a frustrating issue, and to a field ecologist, the ties between model and reality at times appear remote; All but ignored in these recent developments is an insightful recognition that trophic pathways might contribute little to ecosystem stability, and that the answers lie in the spatial patterning of the environment (Smith 1972). I wish to return to the basic observations on food webs as a naturalist and experimentalist, and employing an approach advocated by Sir Arthur Tansley (Godwin 1977), ask whether we are modelling their correct properties, and if not, what modifications might be made.

http://www.britishecologicalsociety.org/100papers/100_Ecological_Papers/100_Influential_Papers_057.pdf

Food webs : linkage, interaction strength and community infrastructure

Recent evidence shows that some amino acids and their metabolites are important regulators of key metabolic pathways that are necessary for maintenance, growth, feed intake, nutrient utilization, immunity, behavior, larval metamorphosis, reproduction, as well as resistance to environmental stressors and pathogenic organisms in various fishes. Therefore, conventional definitions on essential and nonessential amino acids for fish are challenged by numerous discoveries that taurine, glutamine, glycine, proline and hydroxyproline promote growth, development, and health of aquatic animals. On the basis of their crucial roles in cell metabolism and physiology, we anticipate that dietary supplementation with specific amino acids may be beneficial for: (1) increasing the chemo-attractive property and nutritional value of aquafeeds with low fishmeal inclusion; (2) optimizing efficiency of metabolic transformation in juvenile and sub-adult fishes; (3) surpressing aggressive behaviors and cannibalism; (4) increasing larval performance and survival; (5) mediating timing and efficiency of spawning; (6) improving fillet taste and texture; and (7) enhancing immunity and tolerance to environmental stresses. Functional amino acids hold great promise for development of balanced aquafeeds to enhance the efficiency and profitability of global aquaculture production.

New developments in fish amino acid nutrition: towards functional and environmentally oriented aquafeeds

Models of larval dispersal rarely incorporate the behavior of larvae, yet many potential settlers of marine invertebrates and fishes may navigate toward suitable settlement sites by responding to gradients of environmental stimuli. Accordingly, a variety of stimuli may be used for navigation (directed movements to the source of stimuli) and partial navigation (e.g., migration to a current that may favor transport to a settlement site) in the pelagic environment. A broad diversity of taxa have senses that allow them to detect variation in: water chemistry (biotic sources, e.g., amino acids and abiotic sources, e.g., salinity), sound and vibration (biotic sources, e.g., grunting fishes, abiotic sources, e.g., waves breaking), white light gradients and images, polarized light, current direction, magnetism and water pressure. Some organisms can detect multiple stimuli (e.g., decapods and fishes) and integrated sensory responses are likely to be common; many potential settlers of these taxa are good swimmers. Demonstrations of strong orientation to stimuli and navigation over short (centimeters to meters) and broad spatial scales (tens of meters to tens of kilometers) are most common for these groups. Partial navigation, involving vertical migration, is common for invertebrate larvae. A consequence of vertical migration can be transportation that favors movement to suitable settlement habitat. Navigation over a range of spatial scales may use stimuli that are very predictable regardless of location (e.g., water pressure, gravity). The gradients of other stimuli may be more useful for environment-specific signals and even the location of natal habitats, locations and conspecifics (e.g., using sound or smell of specific taxa). We argue that some larvae may use a hierarchy of sensory cues to find suitable settlement sites and that some of the same types of stimuli may be used at more than one spatial scale (as demonstrated for adult salmonid fishes). There are good demonstrations of the use of cues for orientation and navigation at small spatial scales (less than a few meters). More information, however, is required at spatial scales that are relevant to navigation over kilometers before behavior can be incorporated more accurately into models of larval dispersal.

/pdf/sensory-environments-larval-abilities-and-local-self-4p7nr6md73.pdf

Sensory environments, larval abilities and local self-recruitment

Settlement and recruitment of benthic marine invertebrates are complex processes, determined by the interaction of biotic and abiotic factors which operate at different temporal and spatial scales. This review analyses the settlement process, attempting to integrate aspects related to different levels of organization (i.e. ecological-physiological-molecular). This is important because many factors that act at any of these levels and at different times can explain by themselves the patterns of settlement andlor recruitment of a large number of species. From an ecological perspective, progress has been made in the identification of causal factors of vanations in larval availability for settlement. Many physical and ethological factors that act during settlement have, however, not received much attention. Likewise, since the great majority of settlement studies have been carried out at restricted spatial scales, fewer works consider different biological and physical factors acting at different scales simultaneously. Settlement patterns are frequently inferred from recruitment. In this sense, a density-independent action of post-settlement mortality has been cons~dered as prerequisite for thls type of inference. This has, however, recently been challenged on the basis that settler-recruit and mortality-settler density relationships change in time. At the physiological-molecular level, different settlement-inducing chemical cues have been identified. Those cues have, however, not yet been characterized to understand better the signal transduction mechanisms involved in larval responses. It is likely that the nervous system is involved. The use of artificial inducers would be useful in studying settlement induction, until more effective natural inducers are isolated and characterized Although few studies have analysed the acquisition of competence, stages of larval development have been related to changes in protein patterns or enzymatic levels of the nervous system. An inopportune exposure of larvae to inducers may delay settlement and may even have a negative impact on growth and subsequent survival of juveniles.

/pdf/settlement-of-benthic-marine-invertebrates-m4uwo3c4fs.pdf

Settlement of benthic marine invertebrates

The models presented here predict that surf-zone turbulence can have important consequences for wave-swept organisms. The rapid mixing of the water column ensures that larvae and spores can be rapidly transported to the substratum in the absence of sinking velocities or directed swimming. Thus, turbulent mixing removes one potential bottleneck in the recruitment of planktonic organisms to the shore. In contrast, the rapid dilution of gametes by turbulent mixing in the surf zone may drastically limit the efficiency of external fertilization, with a variety of consequences for animals using this form of reproduction. The "educational" models presented here are simplifications of an exceedingly complex flow regime, but they appear to be sufficiently valid to provide a useful perspective on life in the surf zone. We hope that they will stimulate discussion of and research into the many important biological consequences of nearshore turbulence.

Consequences of surf-zone turbulence for settlement and external fertilization

gamma-Aminobutyric acid (a simple amino acid and potent neurotransmitter in human brain and other tissues of higher animals) and certain of its congeners rapidly and synchronously induce planktonic larvae of the red abalone, Haliotis rufescens, to settle and commence behavioral and developmental metamorphosis. These naturally occurring inducers of algal origin apparently are responsible, in part, for the substrate-specific recruitment, induction of settling, and the onset of metamorphosis of abalone and other planktonic larvae upon specific algae which provide naturally favorable habitats for the young of these species in coastal waters. These observations provide a convenient experimental model for further analysis of the basic molecular mechanisms by which environmental and endogenous factors control the recruitment and development of planktonic larvae. Halogenated organic pesticides significantly interfere with larval settling, as quantified in a new bioassay based upon these findings.

https://science.sciencemag.org/content/sci/204/4391/407.full.pdf

γ-Aminobutyric Acid, a Neurotransmitter, Induces Planktonic Abalone Larvae to Settle and Begin Metamorphosis

Addition of hydrogen peroxide to seawater causes synchronous spawning in gravid male and female abalones, and certain other mollusks as well. This effect is blocked by exposure of the animals to aspirin, an inhibitor of the enzyme catalyzing oxidative synthesis of prostaglandin endoperoxide. Hydrogen peroxide activates this enzymatic reaction in cell-free extracts prepared from abalone eggs (a very rich source of the prostaglandin endoperoxide synthetase); this effect appears to reveal a fundamental property of prostaglandin endoperoxide synthesis. Applicability of these findings to both mariculture and medical purposes is suggested.

https://science.sciencemag.org/content/sci/196/4287/298.full.pdf

Hydrogen peroxide induces spawning in mollusks, with activation of prostaglandin endoperoxide synthetase

Costly and extensive early post-larval mortality of Haliotis species in cultivation results, in part, from a delay in normal development. This suggests that naturally required morphogenetic inducing substances may be lacking from certain intensive cultivation systems currently in use in the emerging mariculture industry.



We have found recently that chemical substances in certain crustose red algae (including species of Lithothamnium, Lithophyllum and Hildenbrandia) provide exogenous physiological triggers which induce normal substrate-specific “recruitment,” settling and the onset of metamorphosis of planktonic abalone veliger larvae. Of these, γ-aminobutyric acid (“GABA”), a simple amino acid readily and inexpensively available in pure form, conveniently can be used to induce rapid and synchronous settling and metamorphosis of competent abalone veligers, on virtually any substrate or food source, with 100% efficiency under physiological conditions.



Simple procedures for the use of GABA-induced settling are described for the economically efficient cultivation of abalone, the production of abalone seed, and for use in genetic breeding and engineering with these species. Procedures for use in a rapid, reliable, and highly sensitive GABA-dependent bioassay also are described, for further analysis of physiological and water-quality requirements of, and effects of various pollutants upon, the natural and artificial propagation of these species. Results obtained with this bioassay in quantitating the sensitivity of larval settling (and hence, reproductive success) to bacterial contamination and to trace levels of the common agricultural and industrial pollutants DDT and copper are reported. Significant predation on newly settled abalone by annelids cryptically associated with the natural red algal (recruiting) substrate also is documented.

INDUCTION OF LARVAL ABALONE SETTLING AND METAMORPHOSIS BY γ-AMINOBUTYRIC ACID AND ITS CONGENERS FROM CRUSTOSE RED ALGAE: II: APPLICATIONS TO CULTIVATION, SEED-PRODUCTION AND BIOASSAYS; PRINCIPAL CAUSES OF MORTALITY AND INTERFERENCE

Helen Duncan

Papers

γ-Aminobutyric Acid, a Neurotransmitter, Induces Planktonic Abalone Larvae to Settle and Begin Metamorphosis

Hydrogen peroxide induces spawning in mollusks, with activation of prostaglandin endoperoxide synthetase

INDUCTION OF LARVAL ABALONE SETTLING AND METAMORPHOSIS BY γ-AMINOBUTYRIC ACID AND ITS CONGENERS FROM CRUSTOSE RED ALGAE: II: APPLICATIONS TO CULTIVATION, SEED-PRODUCTION AND BIOASSAYS; PRINCIPAL CAUSES OF MORTALITY AND INTERFERENCE

GABA induces metamorphosis in Haliotis, V: Stereochemical specificity