Shelter scaling regulates survival of juvenile Caribbean spiny lobster Panulirus argus

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Abstract
Marine habitats with limited refugia from predation but adequate food may support increases in prey abundance if artificial shelters placed in these habitats reduce predation-induced mortality. Moreover, the protective capacity of shelters may vary according to the scaling between shelter size and prey size. We tested these hypotheses with field tethering experiments in Bahia de la Ascension, Mexico, by examining the impact of different-sized artificial shelters upon mortality rates of 3 juvenile size-classes of the Caribbean spiny lobster Panulirus argus at 2 sites (inner-bay sand-seagrass flat and outer-bay seagrass bed adjacent to coral reefs). The artificial shelters were sunken concrete structures (casitas) that simulate lobster dens. We also quantified potential predators and estimated the physical features of casitas that influence den choice by juvenile spiny lobster. In the tethering experiments, spiny lobster survival was (1) higher in casitas than seagrass meadows, irrespective of casita size; (2) generally higher in smaller than larger casitas, though the effect depended upon the relationship between lobster and shelter size; and (3) independent of site. Thus, spiny lobster survival depends not only upon the availability of shelter, but also on the scaling between shelter size and lobster size. Predator observations indicated that the size range, maximum size and species diversity of predators increased with casita size, thereby imposing higher predation intensity in larger casitas. Furthermore, since shelter appears to limit spiny lobster abundance in habitats such as reefs and seagrass meadows, placement of appropriately-scaled artificial shelters (e.g. casitas) in nursery areas like the study site is likely to augment habitat carrying capacity by increasing protection from predators.