Roger N. Hughes

An Energy Budget for a Tidal-Flat Population of the Bivalve Scrobicularia plana (Da Costa)

Ecological energetics can be a useful approach with which to assess the importance of different species populations in their contribution to the structure, productivity and functioning of communities (Macfadyen 1963; Odum 1963; Slobodkin 1962). Theoretical considerations of ecological energetics have been treated fully by Lindemann (1942), Phillipson (1966), Odum (1963) and Slobodkin (1962); however, the paucity of field data emphasizes the need for more research (Engelmann 1966). Following I.B.P. terminology (Petrusewicz 1967; Ricker 1968), the energy budget of a population may be summarized by the equation:

A study of feeding in Scrobicularia plana

Deposit-feeding by Scrobicularia plana Scrobiculariaplana (Da Costa) along the mud surface occurs throughout the period of low tide if the water table remains at the surface. The rate of deposit-feeding is unaffected by the time after emersion at low tide except where excessive drainage prevents feeding along the mud surface, in which case it is possible that animals feed from the sides of their burrows at or below the level of the water table.

Responses of benthic scavengers to fishing disturbance by towed gears in different habitats

The aggregation and feeding behaviour of invertebrate scavengers in areas disturbed by trawling was investigated at three different localities. At each site a fishing disturbance was created using a commercial 4 m beam trawl and scavenger density was quantified using a light beam trawl. At one site two diver surveys were also carried out; along a line fished with a scallop dredge or a beam trawl on two separate occasions. For all experiments the fished and adjacent unfished control areas were sampled before, and at intervals after, the initial fishing disturbance. Sampling with the light beam trawl revealed that hermit crabs Pagurus bernhardus moved into areas which had been fished with a 4 m beam trawl at an experimental site near Anglesey. The density of these hermit crabs increased significantly in the fished area after fishing had taken place, but no change in density occurred in the adjacent control (unfished) area. At two other sites (Red Wharf Bay, Anglesey and a site offshore from Walney Island) there were no detectable increases in scavenger numbers in the fished areas. Furthermore, at the site near Walney Island, numbers of hermit crabs P. bernhardus, swimming crabs Liocarcinus depurator and starfish Asterias rubens actually decreased after fishing. Thus the responses of scavengers to towed fishing gears varied considerably between different communities. At Red Wharf Bay, divers observed similar responses of scavengers to both beam trawl and scallop dredge disturbance. Four predatory species were observed feeding in the fished area; starfish A. rubens, hermit crabs P. bernhardus, brittlestars Ophiura ophiura and whelks Buccinum undatum. These predators fed on damaged bivalves, echinoderms, crustaceans, whelks and polychaetes. The proportion of starfish feeding in the fished area was significantly higher after fishing had taken place. Demersal fishing activities provide food for scavengers in the form of damaged animals which are left in the tracks of the trawl or dredge. The responses of scavengers to fishing disturbance are not always manifested as a large increase in their abundance. It is clear that the magnitude of response varies between species and between habitat types.

CRITERIA FOR PREY SIZE-SELECTION IN MOLLUSCIVOROUS CRABS WITH CONTRASTING CLAW MORPHOLOGIES

Abstract This paper briefly reviews our current understanding of prey size-selection in several families of tropical and temperate water molluscivorous crabs. Differences in the morphological and mechanical features of crab claws reflect their function and account for many of the observed differences in prey handling techniques and foraging behaviour. Claws, however, have functions other than feeding and these can confound simple correlations between form, strength and feeding habits. Species feeding predominantly on rapidly moving prey typically have fast, weaker acting claws whereas those specialising on heavily-armoured molluscan prey have claws that are slower but more powerful. Complex shell opening behaviour or specialised claw morphology can sometimes compensate for limited crushing power. Claw characteristics and foraging behaviour also vary geographically; thus, compared with their temperate water counterparts, tropical crabs generally are considered to be more specialised for attacking hard-shelled molluscan prey, possibly as a result of prolonged and intense coevolution between these predators and their prey. Most crabs forage selectively on small-sized molluscan prey well below the critical size that can be opened. The reasons for such size-selection are not entirely clear but energy maximisation, time minimisation, or the risk of claw damage associated with attacks on larger more resistant prey have been suggested as causal factors. The structural and ecological features evolved by molluscan prey that effectively reduce the risk of predation and thus facilitate the continued coexistence of predator and prey are also briefly considered.

The role of learning and memory in the feeding behaviour of the fifteen-spined stickleback, Spinachia spinachia L.

As a result of experience, fifteen-spined sticklebacks began to attack Gammarus and Artemia more efficiently. This was caused by changes in reaction distance, speed of attack, handling time and success in apprehending and handling prey. How quickly foraging efficiency improved depended on the type of prey. Handling times for Artemia decreased more than those for Gammarus; attack and handling efficiency increased correspondingly. Sticklebacks feeding on Artemia also had shorter and less complex foraging sequences. The fish learned about the characteristics of prey items. Gammarus, whose legs lay head-to-tail, were most easily swallowed head-on, and foragers modified their behaviour accordingly. When two types of prey were presented alternately, individuals improved their foraging skills less effectively and handling times decreased less than when on pure diets. Foragers did not retain learned foraging skills indefinitely and became less efficient at dealing with specific prey when these were encountered less frequently. Handling times for prey offered in trials separated by 8 days were similar to those of naive fish. By decreasing handling time, learning increased the profitability (yield of energy/time) of specific prey. Gammarus could thus become more profitable than Artemia, if it predominated in the diet for several feeding bouts, and vice versa.

From cells to colonies: at what levels of body organization does the 'temperature-size rule' apply?

SUMMARY An inverse relationship between temperature during ontogeny and final body size is widespread in ectotherms, but poorly understood. Evidence suggests that within organs, this “temperature‐size rule” (TSR) may also apply to cell size with no change in numbers. So how closely do reductions in size and number of cells and other repeated structures correlate with size reduction at higher levels of organization? We examine this in the context of a proposal that size and/or number changes at various organizational levels are adaptive responses to temperature‐ and size‐dependent oxygen supply. We subjected two clones of the modular colonial bryozoan, Celleporella hyalina, to orthogonal combinations of two temperatures and two oxygen concentrations during ontogeny, observing effects on sizes of colonies and larvae, and sizes and numbers of cells, tentacles, and modules (autozooids). We found that the size:number responses varied among cell types and among structures at different levels of organization, with the inverse temperature‐size relationship applying only to larval parenchymal cells and colony modules. Using our findings and other evidence we propose a unifying adaptive hypothesis that predicts how temperature affects the sizes of mitochondria, cells, organs, modules and organisms, and their relationships with processes that determine the functional capacity of aerobic metabolism.

Mating trials validate the use of DNA barcoding to reveal cryptic speciation of a marine bryozoan taxon.

Despite increasing threats to the marine environment, only a fraction of the biodiversity of the oceans has been described, owing in part to the widespread occurrence of cryptic species. DNA-based barcoding through screening of an orthologous reference gene has been proposed as a powerful tool to uncover biological diversity in the face of dwindling taxonomic expertise and the limitations of traditional species identification. Although DNA barcoding should be particularly useful in the sea, given the prevalence of marine cryptic species, the link between taxa identified through DNA barcodes and reproductively isolated taxa (biological species) has rarely been explicitly tested. Here, we use an integrated framework comparing breeding compatibility, morphology and mitochondrial (cytochrome c oxidase 1) and nuclear (elongation factor-1-alpha) DNA sequence variation among globally distributed samples of the cosmopolitan marine bryozoan Celleporella hyalina (L.). Our results reveal that C. hyalina comprises numerous deep, mostly allopatric, genetic lineages that are reproductively isolated, yet share very similar morphology, indicating rampant cryptic speciation. The close correspondence between genetic lineages and reproductively isolated taxa in the context of minimal morphological change suggests that DNA barcoding will play a leading role in uncovering the hidden biodiversity of the oceans and that the sole use of morphologically based taxonomy would grossly underestimate the number of marine species.

The influence of hunger on feeding behaviour and on the acquisition of learned foraging skills by the fifteen-spined stickleback, Spinachia spinachia L.

Increased hunger and associated feeding motivation increased the foraging efficiency of sticklebacks already experienced with Gammarus or Artemia as prey. Behavioural components additional to the basic foraging sequence were performed less frequently and reaction distance and speed of attack increased as handling time decreased. Foraging efficiency also increased as naive fish gained experience with prey, but the rate of learning was modified by the foragers initial hunger level. Fish deprived of food for 24 h acquired attack and handling skills faster and more thoroughly than those fed to 50 and 95% satiation. Learning increased the profitability of both Gammarus and Artemia. Whether a less profitable prey type can become more profitable than another will, however, depend critically on the prey-specific intrinsic rates of increase and decrease of handling time associated with the independent effects of learning and satiation, and on the foragers initial level of hunger.

Behavioural mechanisms of prey selection in crabs

Laboratory experiments were designed to investigate key stimuli used by blue crabs, Callinectes sapidus, when selecting prey. Results were compared with behaviour predicted, or assumed, by optimal foraging theory. Both live prey and simple models were employed to monitor the responses of blue crabs foraging either in tactile mode, as when feeding on marsh mussels, Geukensia demissa, attached to the substratum, or in visual mode, as when foraging on fiddler crabs, Uca spp., moving over the substratum. We suggest that foraging blue crabs simply respond to the strongest stimuli received from the prey. Items are rejected after a certain persistence time if the attack is proving unsuccessful. Persistence time is adjusted, through experience, to expectations of local prey quality or availability. The integrated result of this key-stimulus mechanism of prey selection will be a close match to diets predicted by other behavioural models, including that based on optimal foraging theory.

Chelal morphometry, prey-size selection and aggressive competition in green and red forms of Carcinus maenas (L.)

Abstract Carcinus maenas exhibits a range of adult colour forms, from green to red. Coloration is at least partially correlated with the length of intermoult. There are no external morphometric differences between colour forms, but red crabs have a greater chelal dry mass per unit chelal size, shown particularly in the chelal integument, apodemes, and to a lesser extent the closer muscle. Sarcomere length of the closer muscle of the crusher chela is not significantly different in the two colour forms. Strain gauges mounted in a model mussel showed that red crabs, of ≈21 mm chelal height, exerted a mean force of 21.5 N and green crabs, of similar size, 16.7 N. Red crabs exerted a greater proportion of slow chelal contractions and maintained them for longer than green crabs. Red crabs preferred larger mussels and usually dominated green crabs in aggressive disputes over prey. The wider range of potential prey and aggressive dominance of the red crabs probably places them at a competitive advantage over the green forms, which find refuge in marginal habitats, such as high shore-levels and estuaries.