A red tide causative dinoflagellate, Karenia mikimotoi, appeared in a dense concentration in Kusudomari, Kujukushima waters, Japan in June 2015. A PVC-made mesocosm was installed during the red tide period to examine possible factors controlling diel vertical migration of K. mikimotoi, and the following parameters were measured during 19–20 June: vertical profiles of water temperature, salinity, chlorophyll fluorescence, photon flux density, Fv/Fm, concentrations of DIN and PO4-P, cell densities of K. mikimotoi and Prorocentrum shikokuense. During the cloudy daytime, cells of K. mikimotoi were mainly found in the subsurface (0.5–2 m), while the distribution layer went down to the deeper layer (4–5 m) under intense sunlight. Interestingly, under such strong light conditions, the K. mikimotoi population always stayed beneath the P. shikokuense layer. Averaged photon flux density at the K. mikimotoi layer, where the highest chlorophyll fluorescence (>15) were observed, was 100±43 µmol m-2 s-1, a value almost consistent with the suitable range for K. mikimotoi growth. Meanwhile, at night, cells of K. mikimotoi were mostly found in the nutrient-rich bottom. This study confirmed the suggestions of previous resarchers that K. mikimotoi cells accumulate at a depth that provides a suitable light regime in daytime and remain at the bottom layer at night to absorb nutrients. Moreover, the current study demonstrated K. mikimotoi stayed beneath the dense P. shikokuense layer, which might provide a shade-effect for K. mikimotoi and enable them to grow in shallower layers.
We investigated the morphology of cynoglossid larvae with elongated dorsal double rays collected in Osaka Bay. Larvae were divided into three types based on the distribution of melanophores on the lateral surface of the tail. Type A had melanophores along the midline of the tail. Type B had no melanophores along the midline, but had melanophores scattered near the center of the tail. Type C had no melanophores on the lateral surface of the tail. Type A, B, and C were collected in August–November, August–October and July–October, respectively. The range of the body length (BL) of each type were as follows, A: 4.0–12.4 mm; B: 3.4–7.8 mm; C: 2.1–9.6 mm. There were no individuals larger than 8 mm BL in type B. In type C, all but two individuals larger than 9.5 mm BL, were smaller than 5.2 mm BL. Types A and B had similar numbers of dorsal, anal and caudal fin rays (D ≤ 115; A ≤ 88; C 10) and myomeres (M 48–50) but could be distinguished by ratios of the body depth and preanal length to BL. In type A, notochord flexion had not begun at 7 mm BL but in type B, notochord flexion was complete at ca 7 mm BL. The numbers of fin rays and myomeres of type C larger than 9.5 mm BL (D 125–130, A 101–102, C 10, M 58–59) differed from type A and B; however, type C smaller than 5.2 mm BL had similar numbers of myomeres and proportion to type A and B. Our analysis of cynoglossid larvae and the characteristics of cynoglossid fishes occurring in Osaka Bay i.e., Arelia bilineata, Cynoglossus abbreviates, C. interruptus, C. joyneri, C. robustus and Paraplagusia japonica, suggest the following; type A: C. interruptus; type B: C. joyneri; type C (larger than 9.5 mm BL)：C. robustus. Type C smaller than 5.2 mm BL were likely to be C. joyneri or C. interruptus. Results of our study provide a first attempt at a description of C. interruptus larvae. Our results differ from some previous reports, which described the morphology of C. joyneri and C. robustus larvae and it is possible that individuals identified as C. joyneri larva in previous studies included C. interruptus or C. robustus larva. Discrepancies between the results of the present and previous studies suggest that further study will be required before the cynoglossid larvae can be positively identified.
Plankton are aquatic organisms unable to swim against the current, and they include diverse taxa of different phylogenetic origins. The taxonomy, phylogeny and ecology of nine plankton groups are reviewed in this paper, in order to comprehensively understand the latest information and current situation of plankton studies. The order-level classification of dinoflagellates was re-arranged, but the classification system is still not well organized at the family-level. The taxonomy of raphidophytes and dictyochophytes was partly confused, however, molecular studies provided clear categorization between these groups. The diatoms could be identified by observing some important morphological characteristics. Yet, these characteristics are sometimes not observable because of inappropriate specimen treatments, and furthermore, the morphological terms are not enough unified, resulting that the species-level identification is complicated and difficult. Recent studies revealed the cryptic diversity and high abundance of some microalgae, such as haptophytes and prasinophytes. The diversity and ecology of planktonic foraminifers have been clarified, but those of radiolarians and phaeodarians are still wrapped in mystery. The classification needs to be re-arranged especially for collodarians, phaeodarians and acantharians. The phylogeny of copepods has been elucidated, and this group was re-classified into 10 orders. Future studies should clarify their evolutionary process and create useful databases for easier identification. The methods to reveal the larva-adult correspondence are established for decapods, and further clarification of the correspondence is expected. The classification system of chaetognaths has been updated, and the intra-species diversity is also being studied. The species diversity of scyphozoans has not been well clarified especially for deep-sea species, and their classification still involves problems such as cryptic species. The dataset including DNA sequences and different types of images (taken in the field and under the microscope, etc.) should be accumulated for comparing the data from different methods (e.g., direct microscopy, optics-based survey and environmental DNA analysis).