A 25-month field survey was conducted to investigate the life cycle and seasonal population fluctuations in the poecilostomatoid copepod Hemicyclops spinulosus in the burrows of the ocypodid crab Macrophthalmus japonicus in the mud-flats of the Tama-River estuary, central Japan. On the basis of sample collections in the water column and from the crab burrows, it was confirmed that H. spinulosus is planktonic during the naupliar stages and settles on the bottom during the first copepodid stage to inhabit the burrows. Furthermore, the copepods' reproduction took place mainly during early summer to autumn with a successive decrease from autumn to winter. A supplementary observation on the burrows of the polychaete Tylorrhynchus heterochaetus suggested that these burrows are another important habitat of H. spinulosus. There were additional discoveries of male polymorphism and precopulatory mate guarding behavior by males, suggesting an adaptation in the reproductive strategy of this copepod to their narrow habitat spaces and low population densities, in contrast to the congeneric species H. gomsoensis, which co-occurs in the estuary but attains much larger population sizes and is associated with hosts having much larger burrow spaces.
Parasitic copepods usually have one or more free-swimming larval/juvenile stages for dispersal or infection. However the present study has revealed that adults and chalimus stages of caligiform copepods have often been discovered in plankton samples collected from East Asia. This is the first report on free-swimming adults of caligiform copepods from Japanese and Korean waters. Adults of the following species have been discovered in this study. In Japanese waters: Caligus coryphaenae (4♂♂), Caligus sp. (1♂), Pandarus sp. (4♂♂, 16 chalimi) off Nansei Islands in May 2003 and 2006; C. sclerotinosus (1♀) near a fish farm, Ehime Prefecture in December 2006; C. undulatus (1♀) off Ube, the Seto Inland Sea in July 2006; and C. undulatus (1♂) in the Ariake Sea in June 2007. In Korean waters: C. orientalis (1♀, 1♂) from brackish waters of the Mankyong River and C. undulatus (1♂) from the Seomjin River in October 2006. It is interesting to point out that C. undulatus has never been recorded as a parasite infecting any host, but has been collected from plankton samples in East Asia, India and Brazil. It is also noteworthy to mention that males (80%) were found more frequently than females (20%) in this study. We considered the following possibilities for the occurrences of caligiform adults originally infecting fish: (1) escaping from irritation or diseases in the host, (2) looking for an opportunity to switch hosts, (3) change in their life mode, and (4) accidental detachment. In the case of chalimi, it could be accidental, because they would be tightly attached to the host using a special organ called the “frontal filament”, and cannot grow up to the adult stage without nutrient supply from the host.
To understand coral-reef zooplankton ecology on a diel basis, zooplankton abundance, biomass and size composition from a fringing coral-reef of Redang Island, Malaysia, were investigated at three-hour intervals for a 48 hour period. Zooplankton was collected vertically and size-fractionated into three size-classes; 100–200 μm, 200–335 μm, and >335 μm. Diel variation of the zooplankton showed that the catches at night were always higher than those in the daytime and the nocturnal increase occurred most strongly in the large fraction (>335 μm). Zooplankton abundance steeply increased just after sunset, showed an abrupt increase 1.5 hour later and declined sharply thereafter. Observation of diel variation in coral-reef zooplankton from three-hour intervals revealed that the temporal variation was large in this study.
The aim of this study is to describe jellyfish fisheries (JF) in Thanh Hoa, the northern part of Vietnam. Information was accumulated based on an interview with the owner of a private jellyfish processing factory (JPF) and fishermen, sampling animals, and through reports of fishery statistics. The JF season begins in April and finishes in May. Two species, Rhopilema hispidum and Rhopilema esculentum are confirmed as commercially exploited, with the former species being caught in much higher abundance than the latter. Cyanea, Chrysaora, Sanderia, and Aequorea were also by caught but not used for processing. Jellyfish are cut into three parts, the bell, the oral-arms, and the stem (fused part of the oral-arm), which are processed separately using salt and alum at the JPF. The number of Rhopilema jellyfish collected by fishermen is estimated as 800,000–1,200,000 indiv. per fishery season, suggesting that the fishery may have an impact on jellyfish populations in the area. On the other hand, the JF has resulted in substantial economic benefits to fishermen, the JPF and thus the local economy. In a jellyfish-rich year, the income of fisherman can reach 31–75 USD day−1 or 1,200–3,000 USD during the JF season, which could sustain their living for the rest of the year. However, the JF also put fishermen into a financially unstable condition because they have to rely on jellyfish occurrence, which shows high inter-annual variation, and market price set by brokers who purchase their product. Statistics provided by the Sea Product Processing Company indicates that the export amount of jellyfish tripled from a total of 1,500 tones in 1995 to 4,600 tones in 2005.
To specify the aestivation site of Apostichopus japonicus, an individual tracking survey was carried out in the jetty in Yoshimi Bay, western Yamaguchi Prefecture. Ten marked animals were released and continuously tracked for one summer. Their aestivation sites were not in the mud that was once considered as the main aestivating habitat of this species, but in the narrow space between shells on the steel sheet-pile wall in the bend of the jetty, and the undersurface of the overhang-structure in the quay. We conclude that underneath of the rock tracts and boulder stones are the natural aestivating habitats.
A new marine species of chironomid genus Tanytarsus van der Wulp, 1893, collected from the Iriomote Island (Okinawa Pref., Japan) is described and figured on the basis of adult male morphology. This species is closely related to T. maginihamatus Tokunaga, 1933 in the structure of hypopygium, but is easily distinguishable from the latter in having well developed anal point, large egg-shaped superior volsella and apically twisted median volsella.