Synalpheus regalis

Synalpheus regalis is a member of the genus Synalpheus, the second largest genus in the snapping shrimp family (Alpheidae), with over 150 species worldwide. As of 2013, S. regalis is one of at least seven recognized species of eusocial shrimp. The other six are Synalpheus brooksi, Synalpheus chacei, Synalpheus elizabethae, Synalpheus filidigitus, Synalpheus microneptunus and Synalpheus rathbunae. ==Appearance==

Synalpheus regalis is a translucent orange. The distal portion of its major chela (the pincer) is brighter orange than the rest of its body and its embryos and ovaries are pale green. Scattered red chromatophores decorate the facial region of its carapace (the region between and lateral to the eyes). Its rostrum (forward extension of the carapace) is thin and is about the same length as the triangular ocular hood, which covers the shrimps' eyestalks. Ovigerous females are generally 2.6–3.7 mm in length and males and juveniles are approximately 2.8 mm. Externally, the females can be distinguished from the males by their ovaries; however, males and juveniles cannot be distinguished from each other. S. regalis is morphologically similar to several other species of Synalpheus including S. elizabethae, S. rathbunae, and S. filidigitus. It is distinguishable from its close relative, the S. elizabethae, as its non-ovigerous colony members have rounded abdominal pleura compared to the S. elizabethae that have pointed abdominal pleura. S. regalis, however, has a more acute abdominal pleura, less fixed teeth, and lacks a secondary armature on its major chela compared to the S. rathbunae. The S. filidigitus males have much more rounded abdominal pleura and longer scaphocerites (lateral stabilizing fin) compared to the S. regalis. ==Habitat==

S. regalis lives exclusively in association with sponges; however, it has only been found to associate with three sponge species: Neopetrosia proxima, Neopetrosia subtriangularis, and Hyatella intestinalis. In Jamaica, they are found in H. intestinalis, in depths of over six meters. In Belize, the only other known locality of the S. regalis, they are more commonly found in N. proxima and N. subtriangularis in addition to H. intestinalis. The shrimp feed on the host tissues as well as on the detritus, which includes bodies of dead organisms or fecal material. ==Life cycle==

Synalpheus regalis exhibits eusocial organization like many other species in the genus. There is one breeding female and members of the colony defend, forage, and take care of the colony. The young hatches from the eggs as a crawling larva and undergoes direct development. S. regalis goes through outbreeding, where at least one of its sexes leaves to find mates, but the details of this process are yet unknown. ==Behavior and ecology==

Eusociality J. Emmett Duffy, a primary investigator of S. regalis, uses the term "eusociality" to refer to the syndrome of multigenerational, cooperative colonies with strong reproductive skew (usually a single breeding female) and cooperative defense of the host sponge found in several Synalpheus species. The populations that live in sponges contain a few hundred individuals, each with two generations of kin. Also, by having just one reproductive female, colonies clearly fit the first criterion of reproductive division of labor. A unifying theory is still in progress at the moment. Colony organization S. regalis has the largest colonies and the largest reproductive skew of all eusocial shrimp. Its colonies can consist of up to 350 organisms, who are related to each other with an average of r=0.50, with one queen. This led researchers to conclude that outbreeding is common in the S. regalis, and at least one of the sexes leaves its natal home to find mates. The queen is the sole breeder of the colony. Studies show that there is a strong correlation between the size of the queen and the size of the colony, which implies that there is a parallel between growth of the breeding female and her colony. Furthermore, the queen is not as aggressive or active as the other adults in the colony, suggesting that she does not dominate other individuals and instead looks to them for protection. The reason behind the reproductive skew in S. regalis is currently unknown, but a theory that explains this phenomenon in other organisms, the "majority rules" model by Reeve and Jeanne, could explain the reproductive skew in S. regalis. Crespi (1994) argued that three conditions must be met to explain most cases of fortress defense: a coincidence of food and shelter in an enclosed habitat, a high value of food-habitat resources that renders inhabitants vulnerable to predatory attacks, and the ability to defend the resource effectively. The strong selective pressures of enemies on kin-structured aggregations may promote evolution of specialized defenders that raise their own and the breeders' inclusive fitness by defending the colony. S. regalis lives and feeds exclusively within their hosts, therefore meeting the first condition. Also, data shows that fewer than 5% of sponges sampled were unoccupied by shrimp, which means that sponges are in short supply and subject to strong competition. Finally, the large non-breeding defenders utilize the snapping claw, a potent weapon that produces a water jet intense enough to stun small animals. S. regalis appears to reach Crespi's (1994) three criteria. 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 and survive long enough to reproduce. Nestmate recognition S. regalis are exceptionally tolerant of conspecifics within their colonies, and aggressive towards conspecifics not of their own colony. These peaceful interactions are attributable to close genetic relatedness among nestmates. Allozyme data revealed that relatedness within colonies is high, averaging 0.50, indicating that colonies in this species represent close kin groups. The existence of such groups is an important prerequisite of explanations of social evolution based on kin selection. During the intruder experiment, resident shrimp contacted foreign conspecifics less and snapped more frequently than they did when faced with a nestmate. Because nestmates are generally close kin in S. regalis, this discrimination may reflect kin recognition and may help maintain the integrity of kin-structured social colonies. Nestmate discrimination likely involves both waterborne and contact chemical signals which have been shown to mediate sex recognition in other alpheids. The high frequency of intruder contacts with the queen may suggest that she produces pheromones like in social insects. ==See also==