Predation Sites Research Articles

Discriminating between Hungry and Satiated Predators: The Response of Guppies (Poecilia reticulata) from High and Low Predation Sites1

AbstractFixed anti‐predator activities are costly because they limit the ability of the prey to take advantage of short term temporal patchiness in predation pressure. The ability to discriminate between hungry and satiated predators and a flexible response to the differential threat can help to lower the costs of anti‐predator behavior. In this study Trinidadian guppies (Poecilia reticulata) were found to distinguish between hungry and satiated predators. In Trinidad, populations of guppies experience different levels of predation from piscivorous fish. Individuals taken from populations with chronically high predation pressure responded stronger to the hungry predator than those from low predation sites.

Local and Latitudinal Variation in Predation on an Herbivorous Marine Snail

Intertidal grazing molluscs show striking local and geographic variation in distribution and body size. Their density is generally greatest in the upper and mid—intertidal zones on rocky shores of the Pacific coastal of North America, and least in the low intertidal zone. However, the elevation of the lower limits of their shore—level distribution varies considerably among sites. In addition, the lower limits are generally higher south of central California than in the north, and the average body size is usually smaller in the south than farther north. This is exemplified by the commonest molluscan grazer in the mid—intertidal region of many rocky shores on this coast, the trochid snail Tegula funebralis. A hypothesis to explain these patterns, that they are due to variation in predation, was tested. At sites where predator density was high, regardless of latitude, the lower limit of Tegula's intertidal range was higher on the shore, compared with areas containing few predators. Shell width was usually smaller, and increased with elevation wherever predators were abundant. Predators were more abundant at southern than at northern sites. The primary predators of Tegula are octopuses (Octopus bimaculoides and O. bimaculatus), the starfish Pisaster ochraceus, and crabs of the genus Cancer. The octopuses occur only at the southern sites, whereas Pisaster and Cancer are found throughout the geographic range of the study. Reciprocal transplants of marked Tegula, done between several southern and northern sites, showed that the lower limit of distribution of adult Tegula on the shore at a given site is influenced by the snails' defensive behavior. Where predators were abundant, both southern and northern Tegula placed low on the shore rapidly climbed to higher levels than in locations where predators were scarce. The probable cause of this upward migration is an avoidance response to chemical exudates from predators. Southern Tegula also have an inherently different response than northern Tegula; southern snails always climbed upwards sooner and reached higher levels than northern snails. This resulted in predators killing more of the northern snails in these experiments. The behavioral difference between southern and northern Tegula may be the result of long—term differences in the intensity of predation by octopuses, which are much rarer in the intertidal north of central California than south of there. Young—of—the—year Tegula were found mainly in the mid—intertidal region. The shift in the direction of shore—level size gradients of Tegula populations between high and low predation sites may be caused by movements of adult snails to levels above or below the less mobile juveniles. The relative scarcity of Tegula and other grazing molluscs in the low intertidal zone is probably due both to upward migration and high mortality rates, with predation being a major cause of both.

Effects of Sampling Method on the Estimation of Population Parameters in Hermit Crabs

Hermit crabs were collected from intertidal and shallow nearshore habitats in the vicinity of Alligator Harbor, Florida, either by visual search of prescribed areas along transects or by collection of crabs that were attracted to baits of gastropod flesh (mimicking natural predation sites). The two sampling methods produced collections that were closely similar in crab species composition and relative abundance and in the relative abundance of the shells of the common gastropod genera that the crabs inhabited. The two collections differed in the following respects: (1) more individuals were collected at baits, (2) crabs attracted to baits probably were more secretive and less mobile, (3) significantly more crabs collected at baits occupied damaged shells (72.2%) than did those located by visual search (29.9%), (4) crabs attracted to baits occupied shells of more gastropod genera (22) than did those collected on transects (14), and (5) more individuals of small size and crabs of a wider size range were collected at baits. Our data suggest that the two methods sample different types of individuals with different efficiencies. Small secretive crabs that occupy damaged shells arc underrepresented in collections made by visual search, while larger crabs in intact shells are underrepresented at baits. Estimates of hermit crab population parameters probably should be based on a combination of sampling methods.

Specificity of hermit crab attraction to gastropod predation sites

Chemical cues are important to hermit crabs in location of shells. The type of gastropod flesh at a predation site (area where a gastropod is being consumed) influences species of crabs attracted to the site. Additionally, amount of flesh at a site is an important factor influencing relative size of the site attendants. Chemical information broadcast from natural predation sites appears highly specific and may serve to separate shell searching of various sizes and species of hermit crabs in time and space.

Enzymatic production of small molecules attracting hermit crabs to simulated gastropod predation sites

Predatory marine gastropods commonly capture and feed upon gastropod prey. This event is of importance to hermit crabs as the result is a shell available for occupation. Previous studies of gastropod predation sites (McLean, 1974, 1975; Rittschof, 1980) have shown that hermit crabs are attracted to the sites via chemicals released from the prey snail flesh. This study was conducted to further characterize the nature of the chemical signal and to investigate possible mechanisms of its production. Simulated gastropod predation sites were initiated and observed on ebb tide in the low intertidal zone in Sarasota Bay, Sarasota, Florida. Flesh from the muscular foot was used to construct three major types of gastropod sites: fresh flesh, flesh which had been frozen-thawed, and fresh flesh plus trypsin. The tissue was placed in dialysis bags to limit the diffusion of all but small molecules. The first hermit crabs came to the trypsin sites 16 ± 3 min post initiation, to freeze-thaw sites 33 ± 11 min, and to flesh sites after about 2 hr. The average number of crabs attending each type of site was 0.3, 11.6, and 12.6 for the flesh, freeze-thaw, and trypsin sites, respectively. The time of initial arrival of hermit crabs to the freeze-thaw and trypsin sites was similar to that observed at natural predation sites. These data provide indirect evidence that hermit crabs are responding to peptides produced by enzymatic degradation of proteins in gastropod flesh, and support the hypothesis that the protease(s) responsible for producing the signal peptides originates from the predatory gastropod. Finally, a tabulation is presented of all gastropod prey species (13 species in 12 genera) and hermit crab attendants (8 species in 4 genera) for which the pheonomenon has been observed.

Chemical attraction of hermit crabs and other attendants to simulated gastropod predation sites

Simulated gastropod predation sites were observed in the gulf intertidal near the Edward Ball Marine Laboratory, Sopchoppy, Florida,Fundulus similis, Callinectes sapidus, Melongena corona, Clibanarius vittatus, andPagurus longicarpus were attracted to the sites by small molecules released passively from the flesh of wounded or dead animals. Flesh consumers (F. similis, C. sapidus, andM. corona) were attracted to molecules released from the flesh of bivalves, gastropods, and crabs. Shell users (C. vittatus andP. longicarpus) were attracted only to small molecules from gastropod flesh, withP. longicarpus attendingP. duplicatas sites whileC. vittatus preferentially attendedM. corona, Busycon contrarium, andBusycon spiratum sites. Flesh consumers and shell users may be attracted to the sites by different sets of small molecules. The addition of proteolytic enzymes to the flesh increased the attendants at a site, indicating that the attractant molecules might be amino acids or small peptides. Flesh consumers were attracted to the sites primarily in the first 12 hr while the shell users were attracted from 2 hr to several days postinitiation. New shells were attractive to hermit crabs up to 12 hr after entry by a hermit crab. The shell species of the attendantC. vittatus were in different proportions than the generalC. vittatus population, and the shell fit of theP. longicarpus attendants was not as good as the general population ofP. longicarpus. Significantly moreC. vittatus attended than P. longicarpus, and it was speculated that there were moreC. vittatus in the area with a poor shell fit thanP. longicarpus.