Euphausiids of Southeast Asian waters
Euphausiids collected by R/V Stranger during 1959-61 were examined with respect to 1) seasonal change in distribution, abundance and recruitment in the Gulf of Thailand and South China Sea and 2) range through the Indo-Australian seas. Discussion of the species is preceded by a description of the distribution of zooplankton biomass. In the Gulf of Thailand, biomass was found to be three to five times greater than in the open South China Sea. In eastern waters of the Gulf, the peak appeared during the intermonsoon period following northeast winds. The western and northern parts of the Gulf were richest under the influence of the southwest monsoon. Northern coastal waters of South Viet Nam were richest toward the end of each season: 1) the northeast monsoon when coastal upwelling and southerly flow from the Gulf of Tonkin took place and 2) the southwest monsoon season of northerly flow. The southern shelf and the basin region of the South China Sea yielded the greatest biomass during the southwest monsoon season when flow was from the direction of adjacent neritic waters to the southwest. The Sunda Shelf waters, including the Gulf of Thailand, constitute the extensive neritic province in which Pseudeuphausia lives. The South China Sea basin, though the largest basin in the inter-ocean region, is essentially a cul-de-sac with respect to its oceanic euphausiid assemblage, the species of which are derived from both the tropical and subtropical Pacific to the northeast. They are transported southwestward as far as the Sunda Shelf. During the northeast monsoon season many of the oceanic species tend to extend into shelf waters. Evidently the presence over the shelf of a dense neritic plankton, together with the shallowness of the water which prevents the normal range of euphausiid vertical migration, precludes presence of the oceanic species. Only Stylocheiron species, which do not migrate vertically, extend much beyond the edge of the shelf, rarely entering the Gulf of Thailand. The taxonomic diversity of the predominant species of the South China Sea (each belongs to a different species group) indicates that this assemblage is part of a regional plankton community. The annual reproductive cycles of the oceanic species were like that of neritic Pseudeuphausia, with larvae produced year-round but with maximum spawning taking place during, and at the end of, the northeast monsoon season. Production of Pseudeuphausia in the Gulf of Thailand shifted seasonally from southwest to northeast, evidently in response to coastal enrichment processes initiated by southwest and northeast monsoon winds respectively. Of the parameters measured, only low salinity (‰) correlates with the paucity of larvae. Local occurrences of the species are considered in relation to the global distributions in a study of the effectiveness of the inter-ocean seas in maintaining or interrupting flow between Pacific and Indian Ocean populations and in providing local habitats. The pelagic connection between the two oceans is limited to a succession of deeper seas: Celebes-Molucca-Banda and Flores-Timor. The large number (46) of euphausiid species in this inter-ocean region apparently reflects complexity in the oceanography rather than in the plankton community. Regularly occurring species are either endemic (e.g. Stylocheiron insulare), Indo- Pacific equatorial (e.g. Euphausia diomedeae) or broadly ranging warm-water cosmopolites (e.g. epipelagic Stylocheiron carinatum and mesopelagic Nematobrachion boöpis). Species associated primarily with central water masses of the Indian and Pacific Oceans are sparse though seasonally consistent in occurrence. The most widely-ranging species in the global sense (e.g. Nematobrachion boöpis, Stylocheiron carinatum) exhibit inter-oceanic and inter-hemispheric continuity that appears to have long been firmly established. These species do not have close relatives, with the exceptions Stylocheiron longicorne and S. maximum, each a member of a sub-generic species group. This suggests that the speciation process (population isolations + re-invasions) in Stylocheiron, a genus consisting of members which do not migrate vertically, each occupying a discrete depth interval, may differ from that process in strongly migrating genera such as Euphausia and Thysanopoda. The importance of the Indo-Australian region in the evolution of sub-tropical and tropical species is indicated by 1) the large number of species found there, 2) the present partitioning of central water mass species distributions (e.g. Euphausia brevis exists globally in five sub-populations), and of Indo-Pacific equatorial distributions (E. paragibba exists in separated Pacific and Indian Ocean populations), 3) the patchy confluence of some central and subtropical species through this waterway (e.g. E. mutica), and 4) the existence of Indo- Australian endemics (e.g. E. fallax, E. sanzoi, Nematoscelis lobata). The species of the region, including larval stages of most South China Sea species, are illustrated and briefly described. Thysanopoda subaequalis Boden is synonymized with T. aequalis Hansen, the type material of T. aequalis having been found to possess the characteristics defining T. subaequalis. The species presently called T. aequalis is redescribed as T. astylata. A third geographical form of Stylocheiron longicorne is described as the "North Indian Ocean Form."