The calanoid copepod Acartia steueri Smirnov, 1936 is a dominant embayment species in the temperate coastal waters of Sagami Bay, Japan and food concentration in the embayment area varies abruptly. In this study, we examined the biological responses of A. steueri in the embayment following an abrupt decrease in food concentration and subsequent starvation in the laboratory. Unlike other neritic species, A. steueri survived for 18 days and continued to produce eggs for 15 days under starvation. Dry weight, carbon weight, and C/N ratio of A. steueri under the high food condition were much higher than those of other Acartia copepods, while prosome length and nitrogen weight of A. steueri were similar. The length–weight relationship of A. steueri was atypical for marine planktonic copepods. Oil-like droplets, similar to the oil sac in oceanic copepods, were observed in the body cavity of A. steueri raised at the high food condition for more than 10 days. In the field, the rapid accumulation of lipids during sporadic high food conditions might contribute to continue the metabolism and egg production under low food conditions until they encounter favorable food conditions again. Thus, the survival strategy to abrupt changes in food concentration or starvation might support the population of A. steueri in the embayment.
The study was conducted to identify the effect of different types of tube traps on the sampling efficacy and accuracy for invasive golden mussels, Limnoperna fortunei (Dunker, 1857), at four sites in Japan. The traps consisted of PVC tubes (diameter: 5.5 cm; length: 20 cm) inside which a PVC plate was fixed horizontally as settlement substrate, and we examined two trap characteristics: mesh covering at the tube ends (present or not) and substrate texture (even or uneven). Mesh covering had a negative effect (P<0.001) and uneven substrate had a marginally positive effect (P=0.08) on mussel settlement density. A positive relationship was found between larval and settlement density in traps without mesh. These results imply that the most effective type of tube trap had no mesh covering and used uneven substrate. The settlement density of the mussels was also examined on several surfaces of the tube trap and the rope used to suspend the trap. The mussel density on the rope was higher than that on the PVC plates and tubes. The best correlation between settlement density and larval density occurred on the outside of the PVC tubes.
Envenomation by toxic box jellyfish species is known to be a serious problem to public health. In order to elucidate the problem, it becomes necessary to predict the occurrence of box jellyfishes, as well as understanding their ecology and life cycle. Mature medusae of Carybdea brevipedalia (Cubozoa: Carybdeida), which is a common species of box jellyfish in Japan, were collected from northern Japan to observe its early life history, including polyp formation. Fertilization occurred externally, and blastulae developed into planulae. Free swimming planulae settled and metamorphosed into tiny primary polyps with two forms, i.e. settled and creeper. Adult polyps formed cysts at temperatures below 15°C or when water replacement and/or feeding was stopped. Budding occurred in four-tentacled polyps, and the buds were released after commencement of budding. Complete metamorphosis of a whole polyp into a single medusa occurred at stable temperatures between 18 to 25°C (18, 20, 23, 25°C, respectively) or when temperatures were raised from 20 to 25°C. Newly released medusae had four tentacles. Our study demonstrated that polyps of C. brevipedalia survive and propagate over a wide range of water temperatures and that developmental features resemble closely those of some tripedaliid species, namely Tripedalia cystophora and Copula sivickisi, rather than Carybdea marsupialis. The morphological affinities of polyp in C. brevipedalia, T. cystophora and C. sivickisi support recent molecular results. However, further studies are needed to confirm the morphological contradiction between C. brevipedalia and C. marsupialis in the future.
Pegea confoederata is a salp with a pinkish-brown body. The color was retained in the mantle tissue, while the tunic was transparent. We examined the fine structures of the mantle tissue to clarify the cytological basis of the body coloration. Apical cytoplasmic bulges of the epidermal cells were associated with dense tunic fibers, suggesting an involvement in the secretion of the tunic. Light microscopy analysis of the mantle revealed pigment cells that are dendroid-shaped hemocytes filled with brown granules. Five types of hemocytes were recognized in the hemocoel, based on their ultrastructure, and the pigment cells in the present species were hemocytes classified as ‘storage cell’. Additionally, some hemocytes were seen to have migrated into the tunic through the epidermis, and were supposed to be presumptive tunic cells.
The species Epizoanthus planus is the only known described zoantharian with an association with echinoderms, and it is known to live on the spines of living sea urchins from the family Cidaridae. This species has been reported from 741 to 1019 m in the Indian Ocean and the East China Sea. However, diagnostic characters are lacking in both the original description and subsequent studies. The present study reports the first record of E. planus from Japanese waters (300–400 m) based on a newly collected specimen, and this record slightly extends the species distribution range in the northwest Pacific Ocean. This new record increases the number of Epizoanthus species in Japanese waters to at least five species. Although many taxonomy and diversity studies have focused on specimens from SCUBA-based collections in Japanese waters, few studies have focused on zoantharians in deeper waters below SCUBA diving limits. The results of the present study highlight the importance of examining specimens from deeper waters to better understand the overall diversity of the order Zoantharia.