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Substrate Selection and Use by a Deposit‐Feeding Crab
Author(s) -
Zimmer-Faust Richard K.
Publication year - 1987
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.2307/1938367
Subject(s) - foraging , burrow , biomass (ecology) , substrate (aquarium) , ecology , habitat , biology , environmental science , abundance (ecology) , crustacean , fishery
Foraging by the deposit—feeding crab Scopimera inflata was examined for 1 yr at three sandy beaches in southeastern Queensland, Australia, in order to determine which factors influence foraging paths, substrate selection, and substrate use. Crabs emerged from burrows only on the ebb tide in daylight and remained close to their burrows (< 30 cm radius). The initial position and direction in which foraging crabs moved were random relative to cues provided by biotic (substrate biomass concentration) and physical (wind direction, sun position, beach slope, magnetic north, landmarks) factors. Each crab foraged in a systematic fashion along a unidirectional, radial path around its burrow entrance. This minimized distances traveled and reduced the chances of reharvesting areas already grazed. Because crabs feed on a wide range of micro—flora and —fauna, biochemical assays (chlorophyll a, ATP, and ash—free dry mass) were used to estimate the abundance of food on the substrate. Biomass concentrations were always low and unpredictable in space within the foraging areas of individual crabs. Crabs did not prefer to feed where biomass was most abundant and instead foraged in an even manner. Experiments demonstrated that foraging crabs distinguished among organic and inorganic materials, but that the threshold concentration for substrate rejection was lower than that normally encountered in the natural habitat. Crabs did not sample local distributions of biomass. They exhibited stereotyped, all—or—nothing responses to feeding stimuli even when habitat quality was varied experimentally. A model was developed for a forager that harvests substrates in order of descending biomass concentration but is otherwise identical to Scopimera. Total daily ingestion by the model forager was <10% greater than that by Scopimera, even though costs of searching and sampling were ignored in the model. The benefits potentially gained by selective grazing are small. Foraging crabs need water and refuge, which are typically provided only by the burrow. Consequently, grazing by Scopimera must balance conflicting demands. The foraging behavior of this crab approaches an optimal trade—off in which: (1) path lengths and distances traveled from burrows are minimized, and (2) use of substrates immediately adjacent to burrows is nearly maximized.