New freshwater shrimp lives on new sponge species

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A new species of strikingly-coloured freshwater shrimp from the genus Caridina has been discovered in Sulawesi, Indonesia, where it lives upon a species of freshwater sponge previously unknown to science.

The new shrimp, which is a member of the family Atyidae, was discovered in the ancient Malili Lake system of Sulawesi and has just been named Caridina spongicola by Kristina Zitzler of Humboldt University and Yixiong Cai of the Biodiversity Centre, Singapore.

Caridina spongicola is one of several species of atyid shrimps found in the complex of lakes, where the crustaceans form an endemic species flock of related organisms that occur nowhere else on earth. The new species was discovered in Lake Towuti, the largest of the five interconnected lakes that form the Malili Lake system, and lives upon a new species of freshwater sponge which is believed to be a member of the Spongillidae family.

Zitzler and Cai claim that the new shrimp lives at depths of 2-5m on the outlet of the lake where it sits on or within the sponges growing on the substrate. The authors carried out a preliminary gut content analysis on six of the shrimps to determine whether they were feeding upon the sponge:

"None of the six dissected guts contained traces of poriferean spicules. The spicules are presumably too big (0.2-0.3mm) to be consumed by the shrimps. On the other hand, a variety of different diatoms, which possibly accumulate on or within the sponge, were found in the guts. These findings suggest that the shrimps do not feed on sponge's tissue and thus do not parasitise their hosts. Instead, they appear to be commensals using the sponge's cavitities as shelter and the inherent accumulation of diatoms as a food supply."

The shrimps have a striking colour pattern consisting of three dark reddish brown tranverse bands, the first two of which are joined at the surface to form an n-shaped band in lateral view. The species is very small, reaching a size of just 1.8-2.8mm carapace length (the total length is a few centimetres) and lays very small quantities of eggs, with around a dozen per brood.

Zitlzer told Practical Fishkeeping that she and other people had already tried unsuccessfully to breed the species, but advises that the species be protected from commercial exploitation: "I know C. spongicola looks pretty, but the populations are really small and fragile, and thus should rather be protected. It is difficult to breed and the beautiful colour pattern fades in captivity, as it seems to be strongly dependent on the substratum composition in the lake."

Zitzler and Cai believe that Caridina spongicola is the only freshwater shrimp discovered to date that forms as association with sponges. However, a related species from Lake Tanganyika, Limnocaridina iridinae, does form a similar commensal relationship with a bivalve mollusc called Iridina. The authors believe that the discovery is an example of convergent evolution between the two forms of atyid shrimp.

For more information on the new freshwater shrimp see the paper: Zitzler K and C Yixiong (2006) - Caridina spongicola, new species, a freshwater shrimp (Crustacea: Decapoda: Atyidae) from the ancient Malili Lake system of Sulawesi, Indonesia. Raffles Bulletin of Zoology 2006 54(2): 271-276.