Oncaeidae

Abstract: Feeding habits and gill raker morphology were examined for the three major planktivorous pelagic fishes, Japanese anchovy Engraulis japonicus, Pacific round herring Etrumeus teres and Japanese jack mackerel Trachurus japonicus, off the northern and western coasts of Kyushu, in the north-eastern part of the East China Sea in the summer months of 2001. Using fishes in the same size range (80-140 mm, standard length), the stomach contents of the three fish species were compared. The diet of the Japanese anchovy mainly consisted of Oncaeidae copepods, while the diets of the Pacific round herring and Japanese jack mackerel were dominated by calanoid copepods at all stations. Comparisons between prey size in the stomach, zooplankton size in the water and gill raker morphology suggested that the stomach contents of the three species were characterized mainly by the difference in the feeding behaviour between Japanese anchovy (filter-feeding) and the other two species (particulate-feeding), rather than by the difference in the morphology of feeding apparatus only. It was concluded that behavioural adaptations in the feeding of these pelagic fishes brought about trophic partitioning to some degree in this pelagic ecosystem in summer. Although the diets of these three species overlapped to some extent, there was still little likelihood of competition between the Japanese anchovy and the other two species. The potential for competition between the Pacific round herring and the Japanese jack mackerel is discussed.

Abstract: We describe two species from the genus Triconia Bottger-Schnack, 1999 in the family Oncaeidae from the northeastern equatorial Pacific, both belonging to the conifera-subgroup. Both sexes of T. derivata (Heron & Bradford-Grieve, 1995) are redescribed, including morphological features not noted in earlier descriptions, such as the posterior face of the labrum. Specimens of T. derivata from the northeastern equatorial Pacific differ slightly from the original descriptions in some morphometric characters, and their differentiation from T. furcula (Farran, 1936), which is closely related, is summarized. The male of T. hirsuta Wi, Bottger-Schnack & Soh, 2010 is described for the first time and the morphology of the female is redescribed. The female holotype of T. hirsuta from Korean waters is reexamined and discrepancies between text and figures in the original description regarding the endopodal spine lengths on swimming leg 2 are clarified. A revised version of the respective part of the original figure is included in the present paper. An indication of the variation in endopodal spine lengths on swimming legs 2 to 4 is provided for both sexes of the two species. It is pointed out that morphometric characters required for unequivocal identification of species of the conifera-subgroup are still not sufficiently well defined. The records of T. derivata and T. hirsuta to date are summarized and indicate a wide zoogeographical distribution of the former species.

Abstract: The pelagic marine copepod family Oncaeidae is highly diversified (over 100 species worldwide) and includes a great number of sibling species, which are difficult to identify morphologically, because of their very small size (0.18–1.2 mm total length as adults). Global investigations of oncaeid biodiversity are severely hampered by insufficient taxonomic knowledge, in particular for species which have first been described from the European Mediterranean Sea (type locality). Many of these species have been reported as key taxa of small-sized copepod communities in very distant oceanic regions. However, due to the taxonomic uncertainties it cannot be excluded that at least some of these allegedly cosmopolitan species in reality represent a complex of distinct, yet closely related, species. To improve the basis for the identification of Oncaeidae of Mediterranean origin, new diagnostic characters in combination with traditional methods were applied in the present study. Copepods were sampled with fine nets of 0.1 mm mesh size down to a maximum depth of 1,000 m on a west-east-transect in the Mediterranean Sea. Oncaeid species and form variants were predefined morphologically and the genetic identity of the morphospecies was analysed by about 650 and 500 bp region of the mitochondrial COI and 12S srRNA gene sequence, respectively (barcoding). A total of 67 individuals from 24 oncaeid species and forms were successfully analysed, including 12 species and one form of Mediterranean origin. For Oncaeidae, the 12S amplification turned out to be more successful (23 species) than the COI amplification (13 species and 1 form). Together, the morphological and molecular results are discussed with respect to three topics: (1) confirmation of a genetic distinction of three Triconia species, which have been interpreted as sibling species by morphological characters, (2) genetic distance of species within the ovalis-complex of oncaeids and (3) the taxonomic status of two form variants of Oncaea mediterranea (Claus).