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HERMIT CRAB SHELL SELECTION.
Term Paper ID:23946
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Essay Subject:
Examines crabs' complex decision making process in picking shells to dwell in.... More...
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5 Pages / 1125 Words
5 sources, 7 Citations,
MLA Format
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Paper Abstract:
Examines crabs' complex decision making process in picking shells to dwell in.
Paper Introduction:
INTRODUCTION
Research is currently being pursued in shell selection by the hermit crab. Hermit crabs usually are found in empty gastropod shells, seeking protection from predators, desiccation, and other hermit crabs. The hermit crab does not seem to select a place to stay at random but rather appears to have a selection process. Precisely what this process is and how the crab makes a decision as to what shell to select and what shell to reject is the subject of much research.
Hermit crabs are considered ideal for research into decision making of this sort. When an animal stops performing one activity, it must decide what activity to perform next, and this is called a "decision point." At any time, causal factors for several different potential activities are likely present, and
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Adult examples of Pagarus hirsutiusculus, and especiallythe males, have a tendency toward reduced shell living and often occupyshells that do not completely cover the vulnerable abdomen. Neil. A better approach issuggested by framing the hypothesis as a comparative statement of testablepredictions. The hermitcrab does not seem to select a place to stay at random but rather appearsto have a selection process. Wilberaddresses a procedural problem in terms of whether larger crabs are beingforced to use smaller shells in experiments, which would affect the outcomeof the research, and he finds a strong correlation between the length ofthe shell and crab shield length. Wilber noted that hermit crabs have been studied by ecologistsbecause the shells they occupy are potentially a limiting resource, andthis makes hermit crabs a good group on which to study the effects ofresource limitation on behavior and the biology of the population. Thecausal factors may be "weighted" in terms of their consequences for thefitness of the animal and for the animal to select that activity for whichthe highest level of causal factors exists. The authors assume that adult behavior reflects the importance ofshells to postlarval fitness and that megalopal delay capabilities areregulated by requirements at the postlarval stage, and they then predictthat Pagarus granosimanus is more likely to delay metamorphosis thanPagarus hirsutiusculus. W., and S. "Effects of Shell and Food Availability on Metamorphosis in the Hermit Crabs Pagarus hirsutiusculus (Dana) and Pagarus granosimanus (Stimpson)." Journal of Experimental Marine Biological Ecology 25 (1993): 237-249.Mesce, Karen A. Colasurdo. Theseshells differ greatly in their external appearance and their internalconfiguration. Harvey and Colasurdo examine the effects of shell and foodavailability on the process of change in two species of hermit crab, thePagarus hirsutiusculus and Pagarus granosimanus. In most animals,assessments can only be inferred based on the decisions the animals make,but, with hermit crabs, the assessment procedure itself can be observed,the sequence of investigatory acts can be recorded, and the timing ofdecisions can be noted. In this way, motivational modelscan be developed to explain the timing of decisions over empty shells, andthese can in turn be incorporated into models created to explain agonisticinteractions (Elwood and Neil 25). This suggests thatshell volume is more important than weight in the selection of a shell.Other experiments shifted the angle of the shell axis and position of theshell's center of gravity, and this often resulted in an aversion to thealtered shells by hermit crabs. CURRENT RESEARCH Shells are an important resource for the hermit crab, and differentshells have different effects on the fitness of the inhabitant. Certain common sense answers suggest themselves. Various comparisons were made among shells as to size, weight, amountof epifauna, and so on. They also have an extensive repertoire of behaviors makinglocating, assessing, and retaining suitable shells easier. The researcher can alter the shells so they conveydifferent information to the animal, and then the consequences can bedetermined. Hermit crabs are known to fight over the ownership of shells,and many of the resource assessment procedures seen in a fight are similarto those used in selecting empty shells. "Pseudoreplication in Hermit Crab Shell Selection Experiments: Does It Occur?" Bulletin of Marine Science 43 (1993): 838-841.----------------------- 6 The crabs wereremoved. The crabs were collected in Tampa Bay, south of St.Petersburg, Florida. Hermit crabs lend themselves to this research. CONCLUSION The various experiments on hermit crabs show the variety of elementsthat have to be considered when trying to ascertain the reasons for animalbehavior and the way researchers can manipulate these elements in order tochange behavior and demonstrate a relationship between a stimulus and aresponse. Visual and chemical stimuli have been found to have adifferent effect on each of the two closely related species of crab, andthis may explain their different choices. The authors examine the combined effects of shell and foodavailability on the survival of the megalopal stage and on the duration ofthis stage in the two species noted. Each speciesshowed a preference for either Tegula funebralis shells (in the case of P.samuelis) or Nucella canaliculata shells (for P. Another approach is found in the state spacemodel, which assumes that all activities are mutually exclusive and thateach combination of causal factors will determine a particular activity(Elwood and Neil 15-17). "The Shell Selection Beahvior of Two Closely Related Hermit Crabs." Animal Behavior 41 (1993): 659-671.Wilber, Pace. Both species did relyheavily on the chemical cues in order to locate and uncover shells thatwere partially buried, and both species rely on shell cues that aredistinguishing features of the shells they prefer. The first acquires new shells two to four times morequickly than the second and can evict Pagarus hirsutiusculus from high-quality shells. Both types of crab responded to the chemical composition of theshells, especially to the calcium content, and the application of variouscoatings was tried to remove the calcium from the cover of the shell andthus remove it as a cue. The delay capabilitiesof Pagarus granosimanus, however, are relatively trivial compared to otherinvertebrates (Harvey and Colasurdo 237-249). Disinhibition may occur intimes of conflict between tendencies toward two opposing behaviors, leadingto displacement activities. Records were kept of all collected gastropod shells as to speciesand whether or not they were inhabited by a hermit crab. The hermit crab is ideal for this type of research because theshells selected by the crab can be changed in a number of ways for purposesof experimentation and because the behavior is so predictable. An excess of shells was available in this areabecause only 53 percent of the available shells were occupied by hermitcrabs. The crabs had a large variety of unoccupied shells from which tochoose. When the weight of the shell was increased by 25percent or so either by its natural epifauna or artificially, hermit crabscontinued to occupy shells of the same size and volume. "The Importance of Various Shell Characteristics to the Shell-Selection Behavior of Hermit Crabs." Journal of Experimental Marine Biological Ecology 11 (1978): 131-142.Elwood, R. The behavior of Pagarus granosimanus is affectedby the availability of food and by whether or not the crab has a shell.Shells are more important for Pagarus granosimanus. The larvae stage of manyspecies of marine invertebrates delays their metamorphosis until theydetect particular cues, which often can be related to elements in theenvironment that have a strong effect on the fitness of the postlarvalstage. Researchhas found that a number of different elements may be involved or thatdifferent species of hermit crab may respond to different stimuli. London: Chapman and Hall, 1992.Harvey, Alan W., and Elizabeth A. Assessments and Decisions. This means thatunderlying sensory and other neural mechanisms make possible each speciesdetecting and responding quickly to the type of shell it prefers (Mesce 659-671). Mesce examines the shell selection behavior of two closely relatedhermit crabs, Pagarus samuelis and Pagarus hirsutiusculus. INTRODUCTION Research is currently being pursued in shell selection by the hermitcrab. hirsutiusculus). Hermit crabs are prime candidates for the comparativeapproach because adult and juvenile crabs have immediate and essentialrequirements for a limited resource, namely empty shells to be used ashousing. Simply demonstrating the existence of apattern and a potential process is not enough. The crabmust have an optimal shell, and, in encounters with other crabs, this isseen as the crab typically engages in a series of activities ending when heeither rejects that shell or moves out of the old shell and into the newone. Works CitedConover, Michael R. Conover reports on a study on shell selection by the hermit crab thatfocused on the Pagurus pollicaris in a natural population and in laboratoryexperiments. Few studies have addressed the selective disadvantage that mightcause a delay in metamorphosis. This suggested that the crabs selectshells on the basis of several shell traits bearing on protection providedby the shell or by the ease with which the shell can be carried (Conover131-142). He finds that the relationship betweenthe size of the crabs and the size of the shells could bias statisticaldescriptions (Wilber 838-841). This series of activities seems to be the process of informationgathering and decision making for the crab (Elwood and Neil 53). Once this cue was removed, Pagarus hirsutiusculusalone did not begin shell exploratory behavior. Pagarus hirsutiusculus, on the other hand, did notshow this tracking behavior. When an animal stops performing one activity, it must decidewhat activity to perform next, and this is called a "decision point." Atany time, causal factors for several different potential activities arelikely present, and the question arises as to how the animal selects whichactivity to perform. Pagarus samuelis relied onvisual cues for shells election, and this species also displays visually-mediated tracking of objects resembling the shell of choice in terms ofcolor, shape, and size. Hermit crabs usually are found in empty gastropod shells, seekingprotection from predators, desiccation, and other hermit crabs. J. Pagarus granosimanus is more focusedon finding, assessing, and retaining high quality shells than is Pagarushirsutiusculus. Hermit crabs are considered ideal for research into decision makingof this sort. Precisely what this process is and how thecrab makes a decision as to what shell to select and what shell to rejectis the subject of much research.
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