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This chapter discusses the physiological energetics of marine molluscs. The measurement of the components of the energy balance equation for individual organisms—namely, ingestion, absorption, excretion, and respiration—allows the derivation of the energy available to the animal for growth and for reproduction. In this way, it is possible to analyze the relationships between growth and various endogenous and exogenous variables in terms of underlying physiological processes. The chapter discusses various physiological components of growth in marine molluscs (excluding the cephalopods) in relation to the major variables of body size, seasonal effects, temperature, oxygen concentration, and ration level. Studies on energy flow at the population level indicate the average rates and efficiencies that have been reported in the major trophic categories. Three efficiencies of energy transfer are commonly considered: (1) absorption efficiency (A/C); (2) gross growth efficiency (P/C); and (3) net growth efficiency (P/A). The concept of scope for growth, which was regarded as the difference between the energy of the food an animal consumes and all other utilizations and losses, has descriptive validity for a wide range of environmental variables and has been widely used in the study of physiological adaptation in molluscs. Recent advances in knowledge of the population genetics of marine molluscs pose a challenge to the whole-animal physiologist, and future research in either discipline must clearly take account of recent understanding that derives from the other.

9 – Physiological Energetics of Marine Molluscs

It has been speculated that marine microplastics may cause negative effects on benthic marine organisms and increase bioaccumulation of persistent organic pollutants (POPs). Here, we provide the first controlled study of plastic effects on benthic organisms including transfer of POPs. The effects of polystyrene (PS) microplastic on survival, activity, and bodyweight, as well as the transfer of 19 polychlorinated biphenyls (PCBs), were assessed in bioassays with Arenicola marina (L.). PS was pre-equilibrated in natively contaminated sediment. A positive relation was observed between microplastic concentration in the sediment and both uptake of plastic particles and weight loss by A. marina. Furthermore, a reduction in feeding activity was observed at a PS dose of 7.4% dry weight. A low PS dose of 0.074% increased bioaccumulation of PCBs by a factor of 1.1–3.6, an effect that was significant for ΣPCBs and several individual congeners. At higher doses, bioaccumulation decreased compared to the low dose, whic...

Effects of Microplastic on Fitness and PCB Bioaccumulation by the Lugworm Arenicola marina (L.)

Deposit-feeding animals acquire food by swallowing large volumes of sediment. Possible food sources include organic debris and sediment-associated microbes. The relative importance of these classes of food is currelty an area of active research. The idea that microbes attached to sediment and detritus particles constitute the major food source for deposit feeders is being replaced by more complex models that incorporate interactions between animals and the food sources in the sedimentary matrix. Many deposit feeders appear to require both microbial and detrital foods. Deposit feeders display many adaptations appear to be cosistent with the energy maximization principle of optimal foraging theory, but rigorous testing of foraging models has proven to be difficult. Elucidation of deposit-feeding strategies may develop as optimal foraging theory is integrated with physiological energetics.

Ecology of Deposit-Feeding Animals in Marine Sediments

Body size as an ecologically important characteristic has received significant attention in recent years (15, 93, 143, 148). These reviews however, deal primarily with higher vertebrates where determination of adult size has a large genetic component and where little chance for size alteration exists once full adult size has been achieved. Growth rates and final sizes of marine, freshwa­ ter, and certain terrestrial invertebrates are far less constrained. The genetic component is often less important than environmental conditions , and size is subject to radical alteration as these conditions change. Among the terrestrial vertebrates , significant differences in adult size may occur between pop­ ulations of a single species (e.g . small size in island populations, 20), and there are even greater habitat-related differences in adult size among fishes, (10 , 36, 155). These size differences are caused by some amount of genetic differentiation between populations and by plastic ontogenetic responses to local conditions . Therefore , even among vertebrates, a gradient exists from species with very fixed to quite variable adult size. The staggering array of morphologies displayed by invertebrates provides examples of widely divergent patterns of growth . Certain crustaceans and other arthropods are probably as constrained as the higher vertebrates in growth pattern, while the more soft-bodied forms vary greatly in rate of growth, final size, and response to changing conditions . Many such species have the ability to shrink or 'degrow' (143) to a small percent of their former mass as environmental conditions deteriorate and to grow again when con­ ditions improve (80, 89, 115). Coelenterates , echinoderms, annelids, mol­ luscs, and urochordates are examples of invertebrate groups with this set of growth characteristics. In many ways, the least constrained growth patterns

The Ecology of Indeterminate Growth in Animals

It has been hypothesized that persistent organic pollutants (POPs) in microplastic may pose a risk to aquatic organisms. Here we develop and analyze a conceptual model that simulates the effects of plastic on bioaccumulation of POPs. The model accounts for dilution of exposure concentration by sorption of POPs to plastic (POP “dilution”), increased bioaccumulation by ingestion of plastic-containing POPs (“carrier”), and decreased bioaccumulation by ingestion of clean plastic (“cleaning”). The model is parametrized for the lugworm Arenicola marina and evaluated against recently published bioaccumulation data for this species from laboratory bioassays with polystyrene microplastic. Further scenarios include polyethylene microplastic, nanosized plastic, and open marine systems. Model analysis shows that plastic with low affinity for POPs such as polystyrene will have a marginal decreasing effect on bioaccumulation, governed by dilution. For stronger sorbents such as polyethylene, the dilution, carrier, and c...

Plastic as a carrier of POPs to aquatic organisms: a model analysis

Published ingestion rates of total dry material (inorganic and organic) by benthic invertebrate deposit feeders and detritivores feeding at 15°C could be explained almost entirely by organic content of the ingested material and body size; the relation was consistent for 19 species from 3 phyla. Since ingestion rate of total dry material varied inversely with the organic content of the food, organic matter ingestion (C) was essentially a function of body size (W): [Formula: see text] where C is mg day-1 and W is mg dry weight. These animals may maintain a rate of intake of organic matter which is independent of the organic content of the food source by: (1) Actively adjusting their feeding rates according to some perception of food "quality", and/or (2) Adapting their feeding rates to different environments on an evolutionary time scale.

Ingestion rate: An empirical model for aquatic deposit feeders and detritivores.

Spartina alterniflora salt-marsh has been established from seed and transplants on dredged materials and sandy shorelines along the North Carolina coast. Transplants were more successful than seeding over a greater portion of the intertidal range and under more rigorous environmental conditions, but seeding was successful in the upper half of protected sites. Seeding at the rate of 100 viable seeds per square metre from April through May can result in complete vegetational coverage by the end of the first growing-season. At the end of the second growing-season, above-ground biomass accumulation from seeding approached that produced by transplants which had originally been planted on a 0.9-metre centre. Both above-ground and below-ground production of planted marsh compared well with values for these components in natural marshes. There were no differences in production by epiphytes between planted and natural S. alterniflora marsh at two different locations. Faunal production in the upper 13 cm of sediment was significantly less in planted than in natural marsh, and where marsh plants accumulated sediments, faunal numbers and biomass were less in planted than in unplanted areas. Sediment carbon content indicated that 4 to 25 years might be required for a newly-planted marsh to resemble a natural marsh. Based on our studies, the techniques developed to plant S. alterniflora on dredged material and along sandy shorelines can be employed to initiate new and functional salt-marsh where none existed previously. The length of time required for man-initiated marsh to resemble natural marsh depends upon how closely the new substrate resembles natural marsh in the type of sediment accumulated, in the elevation of the new substrate, in the natural sedimentation rate in the area, and in the relative maturity of the natural marsh system.

Establishing Spartina alterniflora Marsh in North Carolina

Energy budgets were developed for populations of the fiddler crabs Uca pugnax and U. minax and the periwinkle Littorina irrorata in a Spartina alterniflora marsh in the Cape Fear River Estuary, North Carolina. Production, respiration and assimilation (kcal mr-2 year-), respectively, were: 51, 55 and 106 for U. pugnax; 13, 44 and 58 for U. minax, and 8, 94, and 101 for L. irrorata. Assimilation efficiencies were relatively low: 9% for U. pugnax, 8% for U. minax and 14% for L. irrorata. Net growth efficiencies (production/assimilation) were 48% for U. pugnax, 23% for U. minax and 7% for L. irrorata. Together, the three species consumed the equivalent of about one-third of the net production of the emergent marsh and assimilated about onetenth of that amount. INTRODUCTION Fiddler crabs of the genus Uca and the periwinkle Littorina irrorata are among the most conspicuous invertebrates inhabiting salt marshes along the Atlantic and Gulf coasts (Smalley, 1958; Teal, 1958; Odum and Smalley, 1959; Shanholtzer, 1973; Wolf et al., 1975; Alexander, 1976; Krebs, 1976; Stiven and Hunter, 1976). Their abundance alone suggests that they are important in the flow of energy through the marsh ecosystem. As part of an attempt to formulate an energy budget for the entire salt marsh ecosystem at Walden Creek, North Carolina, an energy budget was constructed for the dominant invertebrate residents, U. pugnax, U. minax and L. irrorata. Ecological energy transformations are described by

Energy Flow Through the Fiddler Crabs Uca pugnax and U. minax and the Marsh Periwinkle Littorina irrorata in a North Carolina Salt Marsh

In situ ingestion rates of some infaunal deposit feeders can be determined without collecting feces by labeling the sediment with fluorescent particles and using these to trace ingested material through the gut of the animals. This technique was applied to the polychaeteNereis succinea and showed that ingestion rate, expressed as material ingested per body weight, increased with temperature and decreased with body size. Total annual ingestion of sediment and detritus for aN. succinea population in a salt marsh near Beaufort, N.C., was estimated to be 5 kg of dry material m−2, more than 4 times that reported for salt marsh epifaunal deposit feeders. *** DIRECT SUPPORT *** A01BY009 00005

A method for measuring ingestion rate of deposit feeders and its use with the polychaeteNereis succinea

Calow and Fletcher (1972) calculated assimilation efficiency from the ratio of an assimilated radiotracer (14C) to a non-assimilated tracer (51Cr) in food and feces. Wightman (1975) improved the efficiency of their technique by using liquid scintillation to count both isotopes simultaneously, but stated incorrectly that it was not necessary to convert counts per minute (CPM) to disintegrations per minute (DPM). Unless the CPM data are corrected for quenching and converted to DPM prior to calculation of assimilation efficiency, a significant error may be introduced.

Leon M. Cammen

Papers

Ingestion rate: An empirical model for aquatic deposit feeders and detritivores.

Establishing Spartina alterniflora Marsh in North Carolina

Energy Flow Through the Fiddler Crabs Uca pugnax and U. minax and the Marsh Periwinkle Littorina irrorata in a North Carolina Salt Marsh

A method for measuring ingestion rate of deposit feeders and its use with the polychaeteNereis succinea

On the use of liquid scintillation counting of51Cr and14C in the twin tracer method of measuring assimilation efficiency.