A marine dwelling eusocial organism: synalpheus regalis  (Page 6/8)

Synalpheus regalis: fortress defenders

Sponge-dwelling shrimp are fortress defenders ( [link] ). The risk of predation is not most severe outside of the nest, but rather inside of it from intruders. Thus larger colony members, most of who do not breed, defend the sponge against any intruder impinging on the sponge. The defenders of the colony are large and overtly aggressive, possessing snapping claws to use against intruders, and tend to be older than the rest of the colony. This age related polytheism is a common trait in labor specializing social insects that also express eusociality (Wilson 1971; Oster and Wilson 1978). These large shrimp allocate their energy to protecting the nest, rather than parenting ( [link] ). The attacks of predators against shrimp can lead to wounds and even death. By allocating the fighting to just the male defenders, it is ensured that the reproductive individual will be protected and will survive to reproduce (Robinson 1992). These fortress defenders are also protecting the most valuable resource of the nest, the sponge itself that provides food and nourishment for the juveniles. Since most of the defenders do not breed, the only way to secure their genes in future generations is to protect their juvenile siblings, allowing them to grow to adulthood free from predation.

Examples of eusocial fortress defenders

Synalpheus regalis has been accepted as a eusocial species due to the vast importance division of labor has provided for populations. For all populations, the juvenile is a precious commodity that must be brought to adulthood to ensure the spread of gene copies. With a caste system, S. regalis is capable of doing just this, having a defender caste that ensures the safety of the nest’s juvenile as well as the queen. Other eusocial species have also classified as either fortress defenders or life insurers; with the primary distinction being the importance of the nest as a food source seen in fortress defenders. The eusocial termites also classify as fortress defenders (Thorne 1997). Similar to sponge-dwelling shrimp, nests of termites provide nourishment for juveniles and adults, reducing the need for foraging. Termites are similar to S. regalis due to the presence of a soldier caste to defend the colony against intruders—in both organisms the nest is very important since it provides nourishment, houses juveniles and reproductive individuals, and serves as protection. However unlike S. regalis , in the termite species Zootermopsis angusticollis female soldiers are also present. Normal female pre-soldiers (callow soldiers that will molt into soldiers) have oviducts, a seminal receptacle, and eggs, thus is capable of reproduction. Once the pre-soldier molts into a mature female soldier, the reproductive organs ceases development, making the female infertile (Thorne 1997). Using molecular analysis, all species of termites have been found to differentiate into castes not genetically, but via developmental instructions that allow them to become any one of the castes depending on hormonal stimuli ( [link] ). The first developmental pathway is the sexual line recognized by the presence of wing buds; the second pathway, apterous, leads individuals to become workers (Watson et al. 1985). Further research is necessary in Synalpheus regalis to determine what developmental pathways, or any other mechanism of differentiation, is used in the formation of soldiers versus the reproductive individual.