Halocypretta

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.