Paramesochra

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

Belongs within: Paramesochridae.

Life in sand
Published 15 December 2010
Paramesochra mielkei, from Huys (1987).

Paramesochra is a genus of minute marine copepods found around the world. Over twenty species are currently assigned to the genus, but it is likely that many more await description. The extremely small size of paramesochrids (most are less than half a millimetre in length) reflects the interstitial habitat of most species described to date, i. e. they live among the grains of sand beneath the surface of their substrate. Also related to their choice of habitat is their vermiform (worm-like) shape and reduced setation compared to other copepods. These features also mean that they would be poor swimmers so they probably do not often emerge above the substrate surface. Most of the species described so far are from shallower waters, but this possibly reflects a lack of study of deep-sea species rather than reflecting true diversity. For instance, a survey of deep-sea Paramesochridae in the southern Atlantic and Antarctic Oceans by Gheerardyn & Veit-Köhler (2009) identified four species of Paramesochra, none of which corresponded to previously described species. These species probably do not have the same lifestyles as the shallow-water interstitial species due to the deep-sea substrate being fine mud rather than sand. Vasconcelos et al. (2009) suggested that another deep-sea paramesochrid, Kliopsyllus minor, might burrow in fluid mud or live in the ‘organic fluff layer’ (wonderful words) on top of the sediment. Deep-sea Paramesochra would probably be similar.

For the most part, genera of copepods have generally been distinguished mechanistically—different genera have different combinations of key features (usually related to the number of setae or segments on appendages)—without an explicit consideration of how those characters relate to phylogeny. However, Huys (1987) did propose a phylogenetic arrangement for the genera of Paramesochridae in which he suggested that Paramesochra formed a clade with the genera Kliopsyllus and Kunzia on the basis of their possessing single-segmented exopodites on the antennae and mandibles. However, while his tree shows Paramesochra as a monophyletic sister group to a clade of the other two genera, he did not identify any synapomorphies for Paramesochra. Instead, the features distinguishing it from the other two genera (two-segmented endopodites on the second to fourth legs, four setae on the distal exopodite segment of the first leg and two setae on the distal exopodite segment of the fourth leg) are resolved as plesiomorphies relative to the other clade. So if any of you feel inspired to spend your time dissecting and examining the legs of animals about 0.3 of a millimetre in total length, I know a potential research project going begging…

Paramesochra acutata
Published 19 July 2022

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. 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.

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. Once again, anyone care to find out?

Systematics of Paramesochra
Paramesochra Scott 1892S03
|--P. acutata Klie 1934W86
| |--P. a. acutataW86
| `--P. a. hawaiiensis Kunz 1981W86
|--P. borealis Geddes 1981W86
|--P. brevifurca Galhano 1970W86
|--P. denticulata Rao & Ganapati 1969W86
|--P. dubia Scott 1892W86
|--P. helgolandica Kunz 1937W86
| |--P. h. helgolandicaW86
| `--P. h. galapagoensis Mielke 1984W86
|--P. kunzi Mielke 1984W86
|--P. laurenticaW67
|--P. longicaudata Nicholls 1945W86
|--P. mielkeiS03
|--P. ornata Krishnaswamy 1957W86
|--P. pterocaudata Kunz 1936W86
|--P. similis Kunz 1936W86
`--P. unaspina Mielke 1984W86

*Type species of generic name indicated

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.

Gheerardyn, H., & G. Veit-Köhler. 2009. Diversity and large-scale biogeography of Paramesochridae (Copepoda, Harpacticoida) in South Atlantic Abyssal Plains and the deep Southern Ocean. Deep-Sea Research I 56: 1804–1815.

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.

[S03] Seifried, S. 2003. Phylogeny of Harpacticoida (Copepoda): Revision of “Maxillipedasphalea” and Exanechentera. Cuvillier Verlag: Göttingen.

Vasconcelos, D. M., G. Veit-Köhler, J. Drewes & P. J. Parreira dos Santos. 2009. First record of the genus Kliopsyllus Kunz, 1962 (Copepoda Harpacticoida, Paramesochridae) from Northeastern Brazil with description of the deep-sea species Kliopsyllus minor sp. nov. Zootaxa 2096: 327–337.

[W67] Wells, J. B. J. 1967. The littoral Copepoda (Crustacea) of Inhaca Island, Mozambique. Transactions of the Royal Society of Edinburgh 67: 189–358.

[W86] Wells, J. B. J. 1986. Copepoda: marine-interstitial Harpacticoida. In: Botosaneanu, L. (ed.) Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) pp. 356–381. E. J. Brill/Dr W. Backhuys: Leiden.

Leave a comment

Your email address will not be published. Required fields are marked *