Temporal activity patterns in reef-dwelling stomatopods: a test of alternative hypotheses

Abstract

Observations showed that two species of stomatopods, Gonodactylus oerstedii Hansen and G. spinulosus Schmitt, from shallow reefs in Saint Croix, Virgin Islands, U.S.A., exhibited a strong daily activity pattern. These mantis shrimps emerged from their burrows for longer periods of time and moved further from their burrows at dawn and dusk than during midday. They also exposed their bodies as far as the carapace or the abdomen significantly more frequently at dawn and dusk than during midday, although the overall number of times they appeared at the entrance of their burrows did not vary with time of day. All sealed themselves inside their burrow all night. We hypothesized that these activity patterns resulted from the availability of prey, activities of fish predators, or visual capabilities of stomatopods. Stomatopods fed primarily on small crustaceans, gastropods and polychaetes. Observations on feeding behaviors, the volume of food in stomachs and the rate at which it was processed indicated that these reef predators did not feed selectively at a particular time of day. Quantification of benthic prey at different times of the day confirmed that the availability of prey items did not determine the crepuscular activity pattern of these stomatopods. The extreme abundance of benthic prey and data on prey eaten suggest that food does not limit populations of these mantis shrimps. What is known of the structure and function of the eyes of gonodactyloid stomatopods indicates that they are adapted to relatively bright light conditions. Our data produced no evidence that stomatopods were active during crepuscular periods because they could see to feed and avoid predators better at twilight than during midday, or that they were differentially responding to the observer during midday. The absence of nocturnal activity in these stomatopods, however, probably reflects inadequate vision. Repeated close observations indicated that other physical factors (temperature, tides, wave action) had no apparent effect upon activity patterns. Censuses showed that all fishes were significantly less abundant on the study sites at dawn and dusk than in midday. Invertebrate-eating fishes were significantly less numerous at dawn than midday and dusk. The body sizes of “invertivorous” fishes and of all fishes were smallest at dusk, reducing the potential for predation on stomatopods at that time. A crepuscular “quiet period”, where diurnal fishes have retired but nocturnal fishes have not yet emerged to feed (or the converse), is known from previous studies of coral reef fishes, but the present study provides the first documentation of inversely correlated activity patterns for benthic reef organisms.