Paramesochra acutata

Copepod taxonomy, it seems, is largely about counting setae. In his review of relationships within the interstitial harpacticoid family Paramesochridae, Huys (1987) recognised four species groups within the genus Paramesochra (which previously got a look-in at this site here). One of these groups, labelled the P. acutata-group, was characterised by reductions in numbers of setae, having lost the inner setae on the first segments of the endopods on the third and fourth legs.

Paramesochra taeana, a close relative of P. acutata, from Back & Lee (2010).

The group takes its name from the species Paramesochra acutata, described by Klie in 1935 from samples taken from coastal groundwater near the town of Schilksee on the northeastern coast of Germany, in the state of Schleswig-Holstein. Other notable features of P. acutata include the presence of four setae on the antennary exopod, well-developed narrow, triangular endopodal lobes on the modified fifth legs of the females, and conical caudal rami produced into spinose processes (Back & Lee 2013). I haven't been able to find whether P. acutata has been collected much beyond its initial locality but other members of its species group have been found around the world. One of these, P. hawaiensis, from (nach) Hawaii, is similar enough that it was until recently treated as a subspecies of P. acutata.

Appendages of female Paramesochra acutata, from Klie (1935).

So what, if anything, does all this mean? That, I'm afraid, is getting a bit beyond me. The fifth legs are used in spermatophore transfer and differences between species might presumably function in recognising suitable mates. Regarding the details of setation and ramus appearance, one wonders if there could be any relation to preferred micro-habitat. Are harpacticoids with fewer setae and more robust rami adapted for crawling among coarser sand grains? Honestly, I have no idea. Anyone care to find out?

REFERENCES

Back, J., & W. Lee. 2010. A new species of the genus Paramesochra (Copepoda: Harpacticoida) from Korean waters. Proceedings of the Biological Society of Washington 123 (1): 47–61.

Back, J., & W. Lee. 2013. Three new species of the genus Paramesochra T. Scott, 1892 (Copepoda: Harpacticoida: Paramesochridae) from Yellow Sea, Korea with a redescription of Paramesochra similis Kunz, 1936. Journal of Natural History 47 (5–12): 769–803.

Huys, R. 1987. Paramesochra T. Scott, 1892 (Copepoda, Harpacticoida): a revised key, including a new species from the SW Dutch coast and some remarks on the phylogeny of the Paramesochridae. Hydrobiologia 144: 193–210.

Klie, W. 1935. Die Harpacticoiden des Küstengrundwassers bei Schilksee (Kieler Förde). Schriften des Naturwissenschaftlichen Vereins für Schleswig-Holstein 20 (2): 409–421.