Synalpheus regalis

Synalpheus regalis is a species of snapping shrimp that commonly live in sponges in the coral reefs along the tropical West Atlantic. They form a prominent component of the diverse marine cryptofauna of the region. For the span of their entire lives, they live in the internal canals of the host sponge, using it as a food resource and shelter. It has been shown that colonies contain over 300 individuals, but only one reproductive female. Also, larger colony members, most of which apparently never breed, defend the colony against heterospecific intruders. This evidence points towards the first known case of eusociality in a marine animal.

The species name "regalis" comes from the Latin regalis which means royal. This likely stems from the structural hierarchy of the colonies in which only a single female produces all of the offspring.

Synalpheus regalis is a member of the genus Synalpheus, the second largest genus in the snapping shrimp family (Alpheidae), with over 150 species worldwide. The species was discovered by J. Emmet Duffy in 1996 on the Belize Barrier Reef. 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.

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.

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.

They rarely cohabit sponges with other shrimps, suggesting that competition in the habitat is fierce and would explain why they are found associated exclusively with only two species of sponges. For co-inhabitance situations, S. regalis appeared to dominate the sponge, leaving the other species severely under-represented compared to cases in which S. regalis was not present. In some places such as the Bocas del Toro Archipelago, S. regalis co-occurs with close relative S. elizabethae.