Synalpheus

Individual of Synalpheus fritzmuelleri infected by bopyrid isopod, copyright Arthur Anker.

Belongs within: Alpheoidea.

Synalpheus is a pantropical genus of snapping shrimp, commonly found living in association with sessile marine animals such as sponges, corals or crinoids. The genus was erected on the basis of the absence of epipods on the thoracic appendages (Dardeau 1984).

Kneel before the shrimp queen
Published 5 January 2009
A pair of commensal snapping shrimps Synalpheus on their host crinoid. Photo from here.

As a child, snapping shrimp were one of my favourite things to find under rocks at the beach. The characteristic bang made by their enlarged pincer snapping shut never fails to fascinate. This ‘snap’ can often be heard for some distance, and the explosive force generated by it can be strong enough to stun small animals that get too close. Snapping shrimp form the family Alpheidae and Synalpheus, with well over a hundred described species and counting, is one of the larger genera in that family.

Synapheus has a pantropically-centred distribution. Though it seems to be more abundant in the east Pacific and Atlantic Oceans than in the Indian, I’d be a little suspicious of the role collection bias has played in this. The various species of Synalpheus are retiring animals by nature, and sequester themselves in cryptic habitats, all the better to defend themselves against would-be intruders with that impressive claw. The best-known species of Synalpheus live within the body cavity of other animals such as sponges or corals, and a few species live on the underside of crinoids (VandenSpiegel et al. 1998). It is debatable to what extent the relationship between Synalpheus and their host should be regarded as commensal (with the shrimp feeding on food particles brought in by the host) or parasitic (with the shrimp feeding directly on the host tissue), as evidence exists for Synalpheus individuals doing both. Dardeau (1984) suggested that Synalpheus species could be divided into three broad levels of host association, from group I (generally free-living or opportunistically commensal species with very low or no host specificity) to group III (invariably commensal species with high host specificity). Many commensal-living individuals will do so as male-female pairs, aggressively excluding any conspecific competitors that attempt to settle in their home. Other species may be more tolerant, with numerous individuals in a single host.

Colonial Synalpheus on a sponge. Photo from Biology-Blog—this would appear to be a laboratory colony, with dabs of identifying paint on the individuals.

The most remarkable association of all, though, is found in certain species of what is called the Gambarelloides species group (after the species Synalpheus gambarelloides). The Gambarelloides group is a morphologically distinct association of species (most notably, they have a dense brush of setae on the smaller pincer) that was separated by Ríos & Duffy (2007) from the remainder of Synalpheus as their new genus Zuzalpheus. This separation was debated by Anker & De Grave (2008), but the complaint does not seem to concern the integrity of ‘Zuzalpheus’ itself, but that of the remainder of Synalpheus if the Gambarelloides group species are not included. Some of the group III sponge-dwelling species in this group (using Dardeau’s grouping) form large colonies with hundreds of individuals in a single sponge. It was only recognised as recently as 1996 (Duffy et al. 2000, 2002) that these Synalpheus colonies actually qualify as eusocial, in the manner of bees and ants, representing the only known occurrence of eusociality outside insects other than mole rats. Reproduction within the colony is conducted by a single queen, though it remains unknown how the queen of a colony is established, and how she prevents other members of the colony becoming reproductive. The sexual ratio of the remainder of the colony remains unknown, as males are indistinguishable from non-egg-bearing females (and gender may be environmentally-determined rather than genetic), but the colony does include a number of larger individuals (called “males” by Duffy et al. 2002) that seem to be primarily responsible for the colony’s defense, moving about the sponge more than the smaller juveniles seemingly on the lookout for intruders. The queen plays little part in defending the colony, and in one eusocial species, Synalpheus filidigitus, she lacks the large snapping pincer of the other individuals (Duffy et al. 2002). It is not yet established how fertilisation of the queen occurs but allozyme analysis suggests that there may be only a single reproductive male in the colony (Duffy et al. 2000).

Phylogenetic analysis of the Gambarelloides group by Duffy et al. (2000) found that eusociality has evolved at least three times within the group. They suggested that it may have evolved as a response to severe competition for habitat. Where eusocial shrimps are found, almost all suitable hosts are home to a colony, so unoccupied homes are few and far between (offhand, how new colonies do become established is yet another unknown factor—eusocial Synalpheus lack a planktonic larval stage so hatching offspring remain in the parent colony). A colonial group may be more effective at defending their host against would-be usurpers than a solitary individual or pair would be. With the large “soldiers” defending her, the queen is able to spend more time feeding and reproducing, safely hidden within the sponge.

Systematics of Synalpheus
<==Synalpheus Bate 1888D84
    |--S. agelas Pequegnat & Heard 1979D84
    |--S. anasimus Chace 1972 [=S. anisimanus (l. c.)]D84
    |--S. androsi Coutière 1909D84
    |--S. apioceros Coutière 1909D84
    |--S. barahonensis Armstrong 1949D84
    |--S. bousfieldi Chace 1972D84
    |--S. brevicarpus (Herrick 1891)D83
    |--S. brevifrons Chace 1972D84
    |--S. brooksi Coutière 1909 [incl. S. brooksi eleutherae Coutière 1909, S. brooksi strepsiceros Coutière 1909]D84
    |--S. carinatus (de Man 1888)J90
    |--S. comatularum (Haswell 1882)MG-H11
    |--S. curacaoensis Schmitt 1924D84
    |--S. demani Borradaile 1900J90
    |--S. disparodigitus Armstrong 1949D84
    |--S. dominicensis Armstrong 1949D84
    |--S. filidigitus Armstrong 1949D84
    |--S. fossor (Paulson 1975)J90
    |--S. fritzmuelleri Coutière 1909D84
    |--S. gambarelloides (Nardo 1847) [incl. Alpheus laevimanus Heller 1862, S. laevimanus]D84
    |--S. goodei Coutière 1909 [incl. S. osburni Schmitt 1933]D84
    |    |--S. g. goodeiD84
    |    `--S. g. occidentalis Coutière 1909D84
    |--S. hastilicrassus Coutière 1905TSH09
    |--S. heardi Dardeau 1984D84
    |--S. hemphilli Coutière 1908D84
    |--S. heroni Coutière 1909J90
    |--S. herricki Coutière 1909 (see below for synonymy)D84
    |--S. iocosta de Man 1909J90
    |--S. longicarpus (Herrick 1891) (see below for synonymy)D84
    |--S. lophodactylus Coutière 1908MG-H11
    |--S. macclendoni Coutière 1910D84
    |--S. minus (Say 1818)D84
    |--S. neomeris (de Man 1897)MG-H11
    |--S. neptunus Banner & Banner 1975J90
    |    |--S. n. neptunusMG-H11
    |    `--S. n. germanus Banner & Banner 1975MG-H11
    |--S. nilandensis Coutière 1905MG-H11
    |--S. obtusifrons Chace 1972D84
    |--S. pandionis Coutière 1909 [incl. S. pandionis extentus Coutière 1909, S. grampusi Coutière 1909]D84
    |--S. paraneptunus Coutière 1909D84
    |--S. parfaiti Coutière 1898 [=S. laevimanus parfaiti]D84
    |--S. pectiniger Coutière 1907D84
    |--S. rathbunae Coutière 1909D84
    |--S. regalisKSM06
    |--S. sanctithomae Coutière 1909D84
    |--S. scaphoceris Coutière 1910D84
    |--S. sladeni Coutière 1908D84
    |--S. spongicola Banner & Banner 1981D84
    |--S. stimpsoni (de Man 1888)TSH09
    |--S. streptodactylus Coutière 1905MG-H11
    |--S. tenuispina Coutière 1909D84
    |--S. theano de Man 1910MG-H11
    |--S. townsendi Coutière 1909D84
    `--S. tumidomanus (Paul’son 1875)TSH09
         |--S. t. tumidomanusKK03
         `--S. t. africanus Crosnier & Forest 1965KK03

Synalpheus herricki Coutière 1909 [incl. S. herricki angustipes Coutière 1909, S. herricki dimidiatus Coutière 1909, S. tanneri Coutière 1909]D84

Synalpheus longicarpus (Herrick 1891) [=Alpheus saulcyi var. longicarpus, S. laevimanus var. longicarpus; incl. S. longicarpus approxima Coutière 1909]D84

*Type species of generic name indicated

References

Anker, A., & S. De Grave. 2008. Zuzalpheus Ríos and Duffy, 2007: a junior synonym of Synalpheus Bate, 1888 (Decapoda: Alpheidae). Journal of Crustacean Biology 28 (4): 735–740.

[D84] Dardeau, M. R. 1984. Synalpheus shrimps (Crustacea: Decapoda: Alpheidae). I. The Gambarelloides group, with a description of a new species. Memoirs of the Hourglass Cruises 7 (2): 1–125.

Duffy, J. E., C. L. Morrison & K. S. Macdonald. 2002. Colony defense and behavioral differentiation in the eusocial shrimp Synalpheus regalis. Behav. Ecol. Sociobiol. 51: 488–495.

Duffy, J. E., C. L. Morrison & R. Ríos. 2000. Multiple origins of eusociality among sponge-dwelling shrimps (Synalpheus). Evolution 54 (2): 503–516.

[KSM06] King, R. C., W. D. Stansfield & P. K. Mulligan. 2006. A Dictionary of Genetics 7th ed. Oxford University Press.

[KK03] Kocataş, A., & T. Katağan. 2003. The decapod crustacean fauna of the Turkish seas. Zoology in the Middle East 29: 63–74.

[MG-H11] McEnnulty, F. R., K. L. Gowlett-Holmes, A. Williams, F. Althaus, J. Fromont, G. C. B. Poore, T. D. O’Hara, L. Marsh, P. Kott, S. Slack-Smith, P. Alderslade & M. V. Kitahara. 2011. The deepwater megabenthic invertebrates on the western continental margin of Australia (100–1100 m depths): composition, distribution and novelty. Records of the Western Australian Museum Supplement 80: 1–191.

Ríos, R., & J. E. Duffy. 2007. A review of the sponge-dwelling snapping shrimp from Carrie Bow Cay, Belize, with description of Zuzalpheus, new genus, and six new species (Crustacea: Decapoda: Alpheidae). Zootaxa 1602: 1–89.

[TSH09] Titelius, M. A., A. Sampey & C. G. Hass. 2009. Crustaceans of Mermaid (Rowley Shoals), Scott and Seringapatam Reefs, north Western Australia. Records of the Western Australian Museum Supplement 77: 145–176.

VandenSpiegel, D., I. Eeckhaut & M. Jangoux. 1998. Host selection by Synalpheus stimpsoni (De Man), an ectosymbiotic shrimp of comatulid crinoids, inferred by a field survey and laboratory experiments. Journal of Experimental Marine Biology and Ecology 225 (2): 185–196.

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