Zosime carsteni, Pointner, 2017

Pointner, Karin, 2017, Description of two new species of Zosime (Copepoda: Harpacticoida: Zosimeidae), including remarks on its phylogeny and distribution on the Great Meteor Seamount plateau (North-east Atlantic Ocean), Journal of Natural History (J. Nat. Hist.) 51 (37 - 38), pp. 2283-2330 : 2324-2325

Differentiation of Zosime carsteni sp. nov. and Zosime eliasi sp. nov. within Zosime

Most of the Zosime species are poorly described, and therefore it is not easy to define apomorphies for either Zosime eliasi sp. nov. or Z. carsteni sp. nov. Nevertheless, some autapomorphies and derived features can be provided for both species, whilst stressing that it was not possible to include all characters for all known species.

In Zosime carsteni sp. nov., the spines on P2 – P4 exp1 – 3 are ornamented with overlapping, flat and blunt spinules, which are novel for Zosimeidae and not represented in Idyanthidae . The typical corresponding spines bear small pinnae. Thus, the P2 – P4 exp1 – 3 spines of Z. carsteni sp. nov. are most derived and are considered autapomorphies for this taxon. In Z. carsteni sp. nov., the outermost distal element of P1 enp2 and P2 – P4 enp3 is a robust spine, whilst in Z. paci fi ca the spine is longer, flexible and seta-like and in other species of Zosimeidae and in the Idyanthidae it is a seta. As these robust-shaped spines are unique within the taxon Zosimeidae , they are interpreted as autapomorphies for Z. carsteni sp. nov. These autapomorphies clearly support the erection of Z. carsteni sp. nov. as a new species and it is placed within the incrassata group, as Z. carsteni sp. nov. shares the incrassata apomorphies mentioned above. Additionally, the other members of the incrassata group can clearly be distinguished from Z. carsteni sp. nov., as all these species share the reduction of one outer spine on P1 exp3. As Z. carsteni sp. nov. has an eight-segmented antennule, the diagnosis given by Kim et al. (2016) has to be expanded from seven-segmented female antennule to seven- to eight-segmented female antennule.

Only one autapomorphy for the species Zosime eliasi sp. nov. was detected. The endopod of the maxilliped bears one small and flexible outer seta in Z. eliasi sp. nov. Zosime carsteni sp. nov. and Z. typica also exhibit one outer seta, but these are bare, slender and not flexible. Hence, this character is regarded as an autapomorphy for Z. eliasi sp. nov. In addition to the mentioned autapomorphy, Z. eliasi sp. nov. shares the apomorphies of the typica group and is therefore placed in this group. Moreover, it has some derived attributes in common with other species of the typica group: in Zosimeidae the male P2 enp3 is usually modified, terminating in an apophysis with an accompanying long seta. In Z. eliasi sp. nov., the seta is absent, a condition previously only described in Z. bergensis . Both species are easily distinguished by the female P5, which bears three setae on the baseoendopod in Z. bergensis , and four in Z. eliasi sp. nov. Moreover, Z. bergensis bears two setae on each side of the bilobed proximal endite of the maxilla, whilst Z. eliasi sp. nov. is armed with three setae each, the more plesiomorphic condition of that character. Hence, these two species can easily be separated from each other, but as the only species to date described with a reduction of the P2 enp3 terminal setae, they seem to be closely related. P1 enp2 of Z. eliasi sp. nov. is armed with three elements, two distally located setae (one spine and one seta) and one inner, medially located seta. Three elements are also present in, for example, Z. bergensis and Z. paramajor , whilst others bear four elements, such as Z. anneae , Z. paci fi ca and Z. paratypica . In the latter condition three of the elements are distal and one is located along the inner margin, whilst in those species with only three elements, the innermost distal seta is reduced, suggesting a close relationship between them.

Closely related species cannot be determined with certainty for Zosime carsteni sp. nov. or Z. eliasi sp. nov. Moreover, it is not possible to ascertain the relationships of the species within the two subgroups. Several characters are not comparable within the species, since some older descriptions lack corresponding information. Additionally, most characters are based on single-seta reductions only, which can easily evolve several times and therefore do not provide enough information to clarify the relationships of the species within each group. However, in addition to Z. carsteni sp. nov., Z. eliasi sp. nov. and Z. anneae , three undescribed species (author ’ s pers. obs.) of the plateau have an eight-segmented female antennule, which is the most ancestral state for this character ( Koller and George 2011).

As Koller and George (2011) and Kim et al. (2016) already stated, an extensive phylogenetic analysis, based not only on the type material but also on molecular data, is required to clarify the relationships in detail.