Leptocaris: Living on the Edge

Some of the most remarkable faunal diversity in the marine environment is to be found in the interstitial spaces between grains of sand. Grazers, predators and scavengers can be found creating entire food webs at scales of less than one millimetre. The minute crustaceans known as copepods are among the more abundant inhabitants of the interstitial, and today's subject, Leptocaris, is among those interstitial copepods.

Dorsal habitus of female (left) and male Leptocaris ryukyuensis, from Song et al. (2012).

Leptocaris contains more than twenty-five species of extremely slender, cylindrical harpacticoid copepods growing to a bit over half a millimetre in length. Characteristic features of the genus include having the maxillipeds (one of the pairs of appendages making up the mouthparts) reduced or lost, and the proximal part of the endopod of the first swimming leg bearing a special anteriorly directed seta with a terminal comb (Song et al. 2012). Representatives of this genus have been collected from localities around the world though mostly in the Northern Hemisphere. Nevertheless, one can't help wondering how much of the genus' apparent rarity in the Southern Hemisphere is an artefact of low collection effort. This possibility should also be kept in mind when considering differences in the ranges of individual species: whereas many have only been collected from single localities (Song et al. 2012), the species L. trisetosus has been found from Finland to the Bahamas to South Africa, as well as in Korea with the last population being treated as a distinct subspecies (Lee & Chang 2008).

The majority of collections of Leptocaris have been from among sand but the genus has also been found in other microhabitats. In general, they are found in sediments with a high organic content. They are found in euryhaline and eurythermal habitats: that is, locations subject to wide variations in salinity and temperature. These may include beaches and brackish pools. They have been found among decomposing leaves in mangrove swamps (offhand, I haven't found if the diet of Leptocaris has been firmly established but I suspect they are probably detritivores). One species, L. kunzi, was described from an estuarine lake in Louisiana; another, L. stromatolicolus, is known from among stromatolites in Mexico. Two species, L. brevicornis and L. sibiricus, have even been found in continental fresh waters in Europe as well as in coastal brackish waters (Song et al. 2012). Overall, Leptocaris species seem to be most abundant in marginal habitats that may be too harsh and unstable for other copepods, making them fronteir harpacticoids.

REFERENCES

Lee, J. M., & C. Y. Chang. 2008. Copepods of the genus Leptocaris (Harpacticoida: Darcythompsoniidae) from salt marshes in South Korea. Korean Journal of Systematic Zoology 24 (1): 89–98.

Song, S. J., H.-U. Dahms & J. S. Khim. 2012. A review of Leptocaris including a description of L. ryukyuensis sp. nov. (Copepoda: Harpacticoida: Darcythompsoniidae). Journal of the Marine Biological Association of the United Kingdom 92 (5): 1073–1081.