The biology of siphonariid limpets (Gastropoda : Pulmonata)

The Siphonariidae are regarded as a primitive family of basommatophoran pul monate limpets which probably have a marine ancestry. They have a broad glo bal distribution but are most abundant on rocky shores in lower latitudes o f the Indo-Pacific, especially within the southern hemisphere. This review examines a number of aspects of the physiology, behaviour, ecology and repr oduction of these pulmonates in relation to their intertidal existence. Despite limpets being poorly designed to minimize heat stress and desiccati on, siphonariids are particularly abundant in warmer climates, unlike many prosobranch limpet taxa which decline towards the tropics. The success of s iphonariids in lower latitudes, when compared with patellogastropods, does not appear to be due to greater temperature or desiccation tolerances, alth ough important data from subtropical and tropical species are lacking. Howe ver, a number of physiological adaptations that may contribute to their suc cessful occupation of lower latitudes are discussed including: a) efficient respiration in air and water; b) facultative metabolic rate depression; c) anaerobiosis; d) rapid re-hydration after water loss. Siphonariids are osm oconformers, but have a wide tolerance to hypo- and hyper-osmotic condition s. Such tolerance has enabled these limpets to exploit habitats (e.g. rock pools and estuaries) generally not utilized by patellogastropods. The behavioural adaptations of siphonariids also help them reduce the physi cal and biological stresses of the intertidal zone. These adaptations (with few exceptions) include rigid homing to a scar or crevice, and confining f oraging activity to moist or humid conditions often when wave activity is m inimal. High shore species tend to be active when exposed at night, whereas lower shore species are more active when awash. Foraging is adjusted both in duration and distance in relation to day/night and spring/neap cycles. A lthough the timing of foraging may also serve to minimize predation, nearly all siphonariids are unpalatable to predators, possessing multicellular ep idermal glands that produce polypropionate chemicals. This ability to avoid the many predators of lower latitudes may be a major factor in explaining the success df siphonariids in these habitats. Siphonariids primarily feed on foliose macroalgae, cropping the algae but n ever removing them completely. On the whole they do not influence algal com munity structure in the same way as patellogastropods. Even though siphonar iids and patellogastropods generally do not compete for the same food resou rce, on many temperate shores direct and indirect interactions between the two limpet taxa occurs. Because of their weaker radula, siphonariids are of ten out-competed, but never completely eliminated, by the patellogastropod fauna. There are, however, examples of siphonariids outcompeting the patell ogastropod fauna. Siphonaria spp. can also interact with fissurellid gastro pods, the outcomes of these interactions being variable. The grazing activi ties of Siphonaria may be crucial to the survival of other limpets as well as other intertidal invertebrates (e.g. barnacles). These are discussed. Like all pulmonates the Siphonariidae are hermaphrodites which have interna l fertilization. Eggs are laid in egg capsules which are embedded in a prot ective gelatinous matrix. Ribbons of eggs are usually cemented to rocks wit hin the intertidal, although in two species, pelagic egg ribbons are produc ed. Two reproductive strategies have evolved; most species produce numerous small eggs which hatch into planktotrophic larvae within seven days of bei ng laid; several species deposit fewer, larger eggs which undergo direct de velopment, crawling larvae emerging from the egg ribbons within 35 days. Bo th adaptive and phylogenetic hypotheses have been proposed to explain these two life history strategies and these are discussed. Whatever the reproduc tive strategy, most species studied have a seasonal pattern of reproduction with individuals laying more than one ribbon per season. Copulation and sp awning are often linked to tidal and/or lunar cycles which are not only tho ught to provide the maximum time for egg Laying, but may also improve the c hances of survival of the eggs and the larvae.