Taxon of the Week: Some Copepods for your Reading Pleasure

The copepods are a widespread group of aquatic crustaceans, another one of those groups of minute animals that are all around us, yet attract little attention because of their tiny size. Thousands of copepod species have been described from every imaginable habitat involving a certain degree of water - thousands more doubtless remain to be described.

In the tiny world of copepods, members of the family Aegisthidae are relative giants, attaining massive sizes of more than 1.5mm in length (in contrast, the type specimen of their sister taxon, Romete bulbiseta, measures a mere 360 microns). Aegisthidae are one of the more basal members of the Harpacticoida, one of the largest orders of copepods. Basal members of the order are fusiform, but many of the more derived infaunal species have a more vermiform shape. Some three thousand harpacticoid species have been named to date, and they are second only to nematodes in abundance in the meiofauna (Seifried, 2003).

The relatively large size of the Aegisthidae is probably connected to their different lifestyle from other harpacticoids - rather than being epibenthic (living on the surface of the substrate) or infaunal (burrowing within the substrate), many Aegisthidae are hyperbenthic - that is, they live in the water column just above the surface of the substrate (there is a bit of confusion about correct terminology - other authors refer to "demersal zooplankton" or "benthopelagic plankton". Funnily enough, the choice of terminology is generally connected to which marine setting, whether tropical, temperate or deep sea, the author is mostly working with - Mees & Jones, 1997). There is also a tendency to lengthen the body shape, particularly the caudal rami, two spine-like extensions from the posterior end of the abdomen (the image of Aegisthus at top left of this post, from here, shows just how incredibly long the rami can get). A few species of Aegisthidae have gone the whole pelagic hog and become genuine members of the upper zooplankton, some of the relatively few harpacticoids (members of only three families) to have done so.

I had intended to centre this post on just one of the aegisthid subfamilies, the Cerviniopsinae. It is worth noting that Aegisthidae once referred to a much smaller collection of species, the current subfamily Aegisthinae (including the holoplanktic species). However, Seifried & Schminke (2003) argued that the 'Aegisthidae' of previous authors represented a derived subgroup of the previous family Cerviniidae, rendering the latter paraphyletic and calling for its sinking. The 'cerviniid' subfamily Cerviniopsinae was recognised as probably also paraphyletic with regard to Aegisthinae, and in particular a connection was suggested to the 'cerviniopsine' genus Pontostratiotes. Nevertheless, Seifried & Schminke did not formally remove the Cerviniopsinae from their classification, retaining it as a provisional grouping pending a proper phylogenetic analysis of the Aegisthidae. Most of the Cerviniopsinae do not appear to have been dealt with since Lang's major monograph of the harpacticoids back in 1948. Members of the Aegithinae share at least one distinguishing feature of the Cerviniopsinae, having the caudal furci opposed to each other rather than divergent as in the Cerviniinae (Montagna, 1979). Members of the Aegisthinae show a tendency to reduction of the mandibles in the males, with the adult males of some species such as Andromastax muricatus and Aegisthus mucronatus being entirely non-feeding.

REFERENCES

Mees, J., & M. B. Jones. 1997. The hyperbenthos. Oceanogr. Mar. Biol. Ann. Rev. 35: 221-255.

Montagna, P. A. 1979. Cervinia langi n. sp. and Pseudocervinia magna (Copepoda, Harpacticoida) from the Beaufort Sea (Alaska, USA). Transactions of the American Microscopical Society 98 (1): 77-88.

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

Seifried, S., & H. K. Schminke. 2003. Phylogenetic relationships at the base of Oligoarthra (Copepoda, Harpacticoida) with a new species as the cornerstone. Organisms, Diversity and Evolution 3: 13-37.