LOMBARDO, R.; CHRISTY, J. H.*; RISSANEN, R.; Smithsonian Tropical Research Institute: Social monogamy in a simultaneous hermaphrodite
The pulmonate limpet Siphonaria gigas, lives in the mid to upper rocky intertidal zone in the tropical Eastern Pacific. Except when foraging during nocturnal low tides, individuals occupy scars, fixed positions on the rock surface to which the edges of their shells fit exactly. Off-scar, limpets are easy prey of both aquatic and terrestrial predators. On-scar, their predation rates are very low. During neap tides, individuals produce one or more mucous ribbons in concentric rings containing about 75,000 eggs. Egg mortality is extremely high. Within one tidal cycle after deposition fish typically eat all eggs except those deep in crevices but many of those protected embryos die from heat or desiccation before they hatch a week later. On a rocky shore in Panama adults appeared to be most common in fissures and many lived in pairs with their shells touching. Samples along five transects totaling 450 m showed that 73% of the population lives in fissures, a highly significant preference for this habitat type. Of 200 adults in 26 fissures totaling 219 m, 150 lived in pairs, a significant deviation from the number of pairs expected given a random arrangement. Paired limpets were size matched. Singletons were significantly larger and they often were next to vacant scars indicating that they were widowed recently. Limpets mate on falling tides, often during the day and usually with the other limpet in their pair suggesting social monogamy. High egg mortality may favor heavy investment in eggs over sperm leading to diminished prowess in sperm competition. Limpets that live and mate in pairs may largely avoid sperm competition and maximize the rate they produce fertilized eggs from relatively small sperm stores. Mate guarding for paternity assurance may give rise to social monogamy in this species. Singletons may produce only eggs favoring occasional highly profitable extra-pair sperm donations from paired individuals.
