Mar Biol (2008) 153:953–963 DOI 10.1007/s00227-007-0867-x 123 RESEARCH ARTICLE Circadian shelter occupancy patterns and predator–prey interactions of juvenile Caribbean spiny lobsters in a reef lagoon Howard M. Weiss · Enrique Lozano-Álvarez · Patricia Briones-Fourzán Received: 8 August 2007 / Accepted: 8 November 2007 / Published online: 27 November 2007  Springer-Verlag 2007 Abstract The spiny lobster, Panulirus argus, is predomi- nantly nocturnal, remaining inside shelters during the day and foraging outside at night, presumably to minimize pre- dation risk. Predation risk generally decreases with increas- ing lobster size. Therefore, this study examined the hypothesis that size would inXuence this basic circadian pattern. Video cameras continuously recorded the shelter occupancy of juvenile lobsters (n = 72) having a carapace length (CL) of 30–62 mm that were tethered to shelters in a shallow reef lagoon. The lobsters’ shelter occupancy was 100% during the day, but declined linearly from shortly before sunset to a minimum of 50% shortly after midnight and then increased linearly, reaching 100% by 1 h after sunrise. The percent time the lobsters spent in the shelters followed a similar trend, but there was wide variability at night (0–100%) for individual lobsters. Lobsters left their shelters 2–30 times night ¡1 , with a majority of excursions lasting <10 min. These results suggest that juvenile P. argus minimize predation risk by remaining in their shel- ters as long as possible but oVset the energetic cost of this behavior by foraging close to their shelters for several short periods at night. This emergence pattern contrasts with those of early benthic phase lobsters (<15 mm CL), which seldom leave their shelters, and adults (>80 mm CL), which have a dusk/early evening peak in activity and leave the shelter for extended periods of time during the night. Fur- thermore, a minimum shelter occupancy in the middle of the night appears especially well adapted to avoid expo- sure to daytime predators. Videotaped observations also included interactions between lobsters and two dominant lobster predators, the triggerWsh, Balistes capriscus, and the octopus Octopus cf. vulgaris. Lobsters responded diVer- ently to these predators: remaining in the shelter when attacked by a triggerWsh and Xeeing the shelter when attacked by an octopus. TriggerWsh were nearly twice as likely to attack a lobster that was outside of the shelter than inside. Once under attack, however, a lobster had nearly the same chance of surviving if it was inside or outside. Results suggest that the patterns of shelter use and emergence change as lobsters grow, probably reXecting the interplay between perception of predation risk and the need to forage. Introduction Many marine animals dwell in shelters during some portion of the day or night. Shelters provide protection from preda- tors as well as a micro-habitat where harsh external envi- ronmental conditions such as intense sunlight, strong waves or currents, extreme temperatures, and toxic chemicals can be avoided or modulated (Sih 1980). However, most animals must spend some time outside of their shelters to forage for food, locate a mate, migrate, or carry out other essential biological functions. Thus, the advantages of being in a shelter must be balanced with the necessities of Communicated by J.P. Grassle. Electronic supplementary material The online version of this article (doi:10.1007/s00227-007-0867-x) contains supplementary material, which is available to authorized users. H. M. Weiss (&) Project Oceanology, Avery Point, Groton, CT 06355, USA e-mail: weissmail@aol.com E. Lozano-Álvarez · P. Briones-Fourzán Unidad Académica Puerto Morelos, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, PO Box 1152, Cancún, Quintana Roo 77500, Mexico