Diosaccinae

Diosaccus spinatus, copyright Florida Museum of Natural History.

Belongs within: Podogennonta.

The Diosaccinae: worldwide sediment dwellers
Published 16 May 2021

The harpacticoid copepods have been featured on this site a reasonable number of times now. These tiny crustaceans are among the most numerous animals in the world, both in terms of numbers of individuals and (in certain habitats) numbers of species. And among the most widespread representatives of the harpacticoids are members of the subfamily Diosaccinae.

Diosaccus tenuicornis, from Sars (1906).

The Diosaccinae are currently recognised as members of the family Miraciidae; earlier sources will usually refer to a family Diosaccidae but the recognition of the pelagic Miraciinae as derived members of this group (Willen 2000) requires use of the older name. Distinctive features of the Miraciidae compared to other harpacticoids include the presence of a relatively large, mobile rostrum and a number of distinctive arrangements of setae, including the inner seta on the basal endopodal segment of the first peraeopod (trunk leg) arising distally (Nicholls 1941, Willen 2000). Miraciids are also unusual in that females carry paired egg-sacs laterally; most other harpacticoid families carry only a single median egg-sac. Miraciids are divided between three subfamilies of which the Diosaccinae are the most diverse. Diosaccines are most readily distinguished by their retention of a number of plesiomorphic features such as crawling legs and relatively short caudal rami (Nicholls 1941b; this author divided the current diosaccines between two subfamilies, the Diosaccinae sensu stricto and Amphiascinae, based on the presence or absence, respectively, of a clear distinction in breadth between the metasome and urosome, or ‘trunk’ and ‘abdomen’, but this division does not appear to have been recognised at this level by any subsequent authors). The great majority of diosaccines are marine, free-living and benthic. A handful of species have been described as associates of lobsters, whether commensals or semi-parasites. A small radiation of species of the genus Schizopera is known from Lake Tanganyika, and Karanovic & Reddy (2004) described a species Neomiscegenus indicus from subterranean fresh water in India. Marine diosaccines are found at all depths from the intertidal zone to the deep abyss. I don’t know for sure but, though they are sediment dwellers, I don’t get the impression (I could be wrong) that they are strictly meiofaunal. As noted earlier, many do not have the vermiform body shape characteristic of interstitial copepods. Many species also are around the half-millimetre size range, which I think may be relatively large for meiofauna?

Four species of Schizopera collected from Korea, from Karanovic & Cho (2016). Left to right: S. yeonghaensis, S. daejinensis, S. gangneungensis, S. sindoensis.

The other two subfamilies of Miraciidae are the aforementioned Miraciinae and the Stenheliinae, which have the endopod of the first peraeopod adapted for swimming rather than grasping and longer caudal rami. Though potential synapomorphies of the Diosaccinae were identified by Willen (2000), they’re a bit weaksauce. There is a distinct possibility that further studies may identify the diosaccines as paraphyletic to the other two subfamilies. In particular, some diosaccines say a very unusual form of nauplius larva with the Stenheliinae, in which the body is strongly foreshortened and crab-like (Dahms et al. 2005). These nauplii also move sideways in a crab-like fashion and do not swim in the water column like the nauplii of other species. Practical considerations have lead most investigators of crustacean phylogeny to emphasis adult over larval morphology but the larval morphology of diosaccines raises some interesting questions.

Systematics of Diosaccinae
<==Diosaccinae
|--Diosaccopsis Brian 1925N41b
| |--*D. rubeus Brian 1925 [incl. D. amphiasculus Brian 1927]N41b
| `--D. ismaelensis Monard 1936N41b
|--Diosaccus Boeck 1872N41b
| |--D. ruber Brian 1923N41b
| |--D. sordidus Brady 1910N41b
| |--D. spinatus Campbell 1929N41b
| |--D. tenuicornis (Claus 1863)N41b
| `--D. truncatus Gurney 1927N41b
|--Pseudodiosaccus Scott 1906N41b
| `--*P. propinquus (Scott & Scott 1893) [=Diosaccus propinquus]N41b
|--Tydemanella Scott 1909N41b
| `--T. typica Scott 1909N41b
|--Ialysus Brian 1927N41b
| `--I. rufus Brian 1927N41b [=Tydemanella rufusN41a]
`--Parialysus Nicholls 1941N41b
`--*P. robusta (Nicholls 1941) [=Tydemanella robusta]N41b

*Type species of generic name indicated

References

Karanovic, T., & Y. R. Reddy. 2004. A new genus and species of the family Diosaccidae (Copepoda: Harpacticoida) from the groundwaters of India. Journal of Crustacean Biology 24 (2): 246–260.

[N41a] Nicholls, A. G. 1941a. Littoral Copepoda from South Australia. (I) Harpacticoida. Records of the South Australian Museum 6 (4): 381–427.

[N41b] Nicholls, A. G. 1941b. A revision of the families Diosaccidae Sars, 1906 and Laophontidae T. Scott, 1905 (Copepoda, Harpacticoida). Records of the South Australian Museum 7 (1): 65–110.

Willen, E. 2000. Phylogeny of the Thalestridimorpha Lang, 1944 (Crustacea, Copepoda). Cuvillier Verlag: Göttingen.

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