Some shrimps live with their family
Introduction
What kind of animal would you call 'social'? In ecology, sociality can exist at different scales. The most basic social interaction is, of course, mating. But some species interact further: they organize themselves in groups, sometimes with different roles, and assist with raising offsprings. When a species organizes itself in complex groups where 1) not everybody reproduces but everybody takes care of the young an 2) generations overlap, we deem that species 'Eusocial'. It is the highest form of sociality found in the animal reign. Eusociality was, for a long time, thought to be restricted to terrestrial species only, mostly arthropods like ants, termites and bees, and a couple mammal species – mole rats for example (Ruxton, 2014). However, in 1996, J.E. Duffy described the first marine eusocial specie: Synalpheus regalis. S. regalis is a snapping, sponge-dwelling shrimp living in central America, in colonies of over three hundred individuals with a single breeding female (Duffy, 1996).
Ecology of Synalpheus
The genus Synalpheus is part of the Alphaidae family, more commonly known as snapping shrimps. Snapping shrimps received their name from their one or two large claws (the chela or chelae) that they can “snap” fast, projecting hot and fast water in front of them. They use their claw to communicate and defend themselves against enemies (Duffy 1996). Synalpheus is a large taxa, containing over a hundred and fifty species located in the Indo Pacific and the Western Atlantic. A group of around 20 species of Synalpheus shrimps from the Caribbean Sea is named the gambarelloides group and is characterized by some morphological criteria, and a sponge-dwelling lifestyle: the individuals live in the canals inside sponge hosts (Coutière 1909; Duffy & Macdonald 1999). It is in this group that we can find the most eusocial Synalpheus species, as well as species with other forms of social structures. The simplest being monogamous pair-living, that sometimes evolved towards communal group living with several monogamous pairs sharing a common sponge host (Brooks et al, 2017). In the Caribbean Sea, close to a hundred percent of host sponges are already occupied by Synalpheus colonies or individuals. This leads to intense competition for host sponges among Caribbean Synalpheus (Duffy 1996).
Eusociality in Synalpheus
Eusocial Synalpheus shrimps live in the internal canals of sponge host, just like most eusocial species (ants, bees...), who also live in nest with cavities for protection from predators and food (Duffy 1996). They feed either on the host sponge’s tissue or on the debris and microorganisms floating inside the canals (Duffy et al, 2000). Inside one sponge host, there can be a dozen to hundreds of shrimps, with a single breeding female – the queen (Duffy 1996).
The queen is almost always the only breeding female in the colony, and the other large adults defend the colony with their enlarged chela. As almost all sponges are occupied in the Caribbean, competition for a host is tough, and many intruders try to chase other snapping shrimps away (Duffy 1996). The absence of the large, defensive claw in Synalpheus queens suggests that they do not maintain their status through intimidation or violence with other colony members, but rather that they are protected enough by the rest of the colony to afford to lose their main defence mechanism (Duffy & Macdonald, 1999).
How did eusociality evolve in Synalpheus?
There are two factors explaining the presence of social structures in snapping shrimps out of all marine animals (Duffy & Macdonald, 2009):
(1) they live and feed themselves through sponge hosts that are limited in number, leading to fierce competition, and (2) Just like many insects living in nests, they have a powerful defence mechanism through their claw. But then why are some Synalpheus species in the Caribbean still not social?
Because shrimps in communal groups are not closely related, but shrimps in eusocial communities are, we can assume that those two social structures evolved independently from each other, instead of group living being the first step towards eusociality (Chak et al, 2017; Brook et al, 2017). The two evolutionary paths depend on the dispersion of larvae in each species:
In Synalpheus, there are species with swimming larvae and juveniles that can only crawl. Eusociality is only found in the latter: as the larvae cannot disperse far from its parents’ host, the juveniles crawl out of the egg and stay in the colony. This allows said colony to keep relatives inside of the same host sponge, creating a closely related eusocial community. There is a very strong correlation between the presence of an elaborate social structure and non-dispersing larval development in Synalpheus snapping shrimps (Duffy & Macdonald 2009, Brook et al 2017, Chack et al 2017).
References
- Brooks K.C., Maia R., Duffy J.E., Hultgren K.M., Rubenstein D.R. (2017) Ecological generalism facilitates the evolution of sociality in snapping shrimps. Ecology Letters, 20(12), 1516-1525.
- Coutière, H. (1909) The American species of snapping shrimps of the genus Synalpheus. Proceedings of the United States National Museum, 36, 1–93.
- Chack S.T.C., Duffy J.E., Hultgren K.M., Rubenstein D.R. (2017) Evolutionary transitions towards eusociality in snapping shrimps. Nature Ecology & Evolution, 1, 0096.
- Duffy, J. E. (1996) Eusociality in a coral-reef shrimp. Nature, 381, 512–514
- Duffy, J. E. & Macdonald, K. S. (1999) Colony structure of the social snapping shrimp Synalpheus filidigitus in Belize. J. Crustacean Biol. 19, 283–292.
- Duffy, J.E. & Macdonald, K.S. (2009) Kin structure, ecology and the evolution of social organization in shrimp: a comparative analysis. Proceedings of the Royal Society B, 277, 575-584.
- Duffy, J. E., Morrison, C., & Rios, R. (2000) Multiple origins of eusociality among sponge-dwelling shrimps (Synalpheus). Evolution, 54, 503–516.
- Ruxton G.D., Humphries S., Morell L.J., Wilkinson D.M. (2014) Why is eusociality an almost exclusively terrestrial phenomenon? Journal of Animal Ecology, 83(6), 1248-1255.
- Wilson, E. (1971) The Insect Societies. Cambridge, MA: Belknap Press of Harvard University.
Written 27.07.2025