Halocypretta Chavtur and Stovbun, 2008

Angel, Martin V., 2013, Planktonic ostracods (Myodocopa: Halocyprididae) from abyssopelagic depths in the Atlantic, North Pacific and Gulf of Oman: Chavturia abyssopelagica (n. gen., n. sp.), Halocypretta profunda (n. sp.), Halocypretta parvirostrata Chavtur and Stovbun, 2008 and Halocypretta striata (Müller, 1906), Zootaxa 3709 (5), pp. 401-431 : 410-429

Genus Halocypretta Chavtur and Stovbun, 2008

This genus has only recently been described from the North Pacific based on H. parvirostrata Chavtur and Stovbun, 2008 , which was designated as the type species. Chavtur and Stovbun (2008) included Halocypris striata Müller, 1906 in their new genus, while noting the inadequacies of the original description. Their material included only males of the H. parvirostrata , so its females have remained unknown.

Three specimens of this species have been identified in material from the North-east Pacific sent to me by Dr Moira Galbraith—an adult male, an adult female and an A- 1 female, which was undergoing its final maturation moult. The original description is supplemented with observations on the adult female, and the meristic characters of the two adults are used in the comparisons between all the species. These specimens of H. parvirostrata not only came from further north and east in the North Pacific than specimens in Chavtur and Stovbun (2008). but first this species caught in the Atlantic during the CMarZ cruise is described.

Zoogeography of Halocypretta

All records for the species of Halocypretta are summarised in Table 3 , and Figure 17 shows the known geographical distributions of the three species. H. parvirostrata is a North Pacific species that appears to be restricted to high northern latitudes where there is seasonal vertical mixing. H. profunda is restricted to low latitudes in the tropical Atlantic, and H. striata to the southern Indian Ocean where the upper water columns are not subject to seasonal turn over. The record of H. striata from the Atlantic ( Granata and Caporiacco 1949) is disregarded as the authors provided no information whereby this identification can be verified. The distribution data would suggest that, if their specimen was indeed a Halocypretta species, then it may have been H. profunda . However, no other specimens have been identified from numerous samples collected at abyssopelagic depths in the North Atlantic during the extensive sampling programmes carried out from RRS Discovery. The sedimentary fluxes, which are likely to provide the nutriment for such deep living species, are likely to be more seasonally pulsed at high latitudes than at the lower tropical and subtropical latitudes where the latter two species occur. H. striata has not been collected recently despite the extensive sampling that was undertaken during the International Indian Ocean Expedition and more recent expeditions (e.g. Drapun and Smith 2012), probably because sampling was seldom deep enough and the samplers will not have filtered enough water to catch these relatively uncommon animals. From the known distributions of these species, they would seem to be associated with the deep circulation and water masses. If so, then further abyssopelagic sampling is likely to extend these ranges considerably.

Comparisons between the species

Table 2 summarises the meristic characters of the four species described here, based on measurements of the type specimens for C. abyssopelagica n. sp. and H. profunda n. sp., and the new specimens of H. parvirostrata from the Northeast Pacific and of H. striata from the Gulf of Oman. The substantive differences between the two genera are the relative breadth of the carapace, the structure of the frontal organs, the lengths of the e-seta of the first antenna, the lengths of the swimming setae on the second antenna, the lengths of the f- and g-setae on the endopodites of the second antenna, and the lengths of the copulatory appendages. The differences between the Halocypretta species, as might be expected, are more subtle. There are differences in the shapes of the carapaces as well as H. striata being smaller than the other two species. The length of the rostrum and the depths of the incisures differ. The longer e-seta of the first antenna distinguishes H. profunda from the other two species, although the difference between males of H. profunda and H. striata is trivial. The copulatory appendage in H. profunda is relatively longer and broader than in the other two species. The number of muscles distinguishes H. parvirostrata from the other two. The meristic data presented have an inherent weakness in that there is a lack of information about the extent of interspecific variability, but without data from many more specimens the significance of any of the differences cannot be established. However, they do provide a useful starting point for identification, and in time the measurement of more specimens may confirm which of the differences are significant.

Key to distinguish the species

1. Adult carapace length <3 mm ............................................................. Halocypretta striata

-. Adult carapace length> 3 mm ............................................................................ 2

2. Carapace breadth: length ratio <41 %.................................................. Chavturia abyssopelagica

-. Carapace breadth: length ratio>49 %...................................................................... 3

3. First antenna e-seta †>64 % CL......................................................... Halocypretta profunda

-. First antenna e-seta †<61 % CL..................................................... Halocypretta parvirostrata

† Table 3 lists several other distinguishing meristic characters.

Figures

FIGURE 17. Map showing the global distribution of all records of Halocypretta species. Note the record of H. striata by Granata and Caporiacco (1949) has not been plotted because there is no means of verifying this record and it seems probable that their specimen may have been H. profunda.