Since marine medusae and ctenophores harbor a wide variety of symbionts, from protists to fish, they constitute a unique community in pelagic ecosystems. Their symbiotic relationships broadly range from simple, facultative phoresy through parasitisim to complex mutualism, although it is sometimes difficult to define these associations strictly. Phoresy and/or commensalism are found in symbionts such as pycnogonids, decapod larvae and fish juveniles. Parasitism and/or parasitoidism are common in the following symbionts: dinoflagellates, ciliates, anthozoan larvae, pedunculate barnacles, anuropid isopods, and hyperiid amphipods. Mutualism is established between ctenophores and gymnamoebae, and between rhizostome medusae and endosymbiotic dinoflagellates. More information on symbiotic apostome ciliates, anthozoan larvae and hyperiid amphipods is definitely needed for further studies in consideration of their high prevalence and serious damage they can inflict on their hosts. The present paper briefly reviews previously published data on symbionts on these gelatinous predators and introduces new information in the form of our unpublished data.

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The seasonal variations in trophic relationship between the moon jelly Aurelia aurita s.l. and mesozooplankton were investigated in a brackish-water lake, Honjo District, Japan from June 2005 to August 2006. The medusae occurred abundantly (average abundance and biomass: 0.55 medusae m⁻³ and 58.8 mg C m⁻³, respectively) during warm seasons (i.e. June–November, 2005), but were very scarce or absent during the remaining seasons. The mesozooplankton biomass fluctuated from 1.3 to 150 mg C m⁻³ (overall average: 60.5 mg C m⁻³) irrespective of the medusa biomass variation. Mesozooplankton were preyed upon by medusae almost non-selectively; the small copepod Oithona davisae and bivalve larvae were the predominant prey, comprising 52–99% (average: 85%) of the gastric pouch contents. The medusa population ingestion rate on mesozooplankton varied from 0.11 to 12.8 mg C m⁻³ d⁻¹, which corresponded to 0.6 to 29% of the mesozooplankton biomass per day and to 1.6 to 47% of mesozooplankton daily production rate. A. aurita medusae were certainly a key component of the zooplankton community, but they did not exert any significant top-down control as to suppress mesozooplankton biomass in this eutrophic lake.

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The Turrid whelk Phymorhynchus buccinoides aggregates Okutani, Fujikura & Sasaki on only four outcrops in a grey and black sediment area (25 m²) at a depth of 1180 m at the Off Hatsushima Island seep site, Sagami Bay, Japan. To determine the food web structure of the deep-sea chemosynthesis-based community, we conducted the dietary food habits of the whelk by a combination of in situ behavioral observations, stable isotope compositions, symbiotic bacteria in their soft part body parts, in situ bait trap experiments, and anatomic examinations. The whelks had a δ¹³C value similar to that of the mytilid mussel Bathymodiolus platifrons Hashimoto & Okutani, no symbiotic bacteria, and very small radula (probably useless). In bait trap experiments, the whelk swarmed toward crushed B. platifrons. These results strongly suggest that P. buccinoides feeds on B. platifrons. The ecological niche of the whelk is that of a carnivore or scavenger. B. platifrons was distributed on other outcrops at this seep site, but P. buccinoides did not occur elsewhere. Thus, the distribution pattern of this whelk is not determined solely by its dietary habits.

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We conducted field surveys to monitor the population dynamics of short-neck clam, Ruditapes philippinarum, in Hichirippu Lagoon, in eastern Hokkaido, Japan, focussing on the negative impact of extremely low temperatures during the winter on juveniles in the post-settlement period. The clam population had a short annual breeding season between September and November. The newly settled juveniles experienced the severe winter as the daily mean temperature of the surface sediment descended below 0°C soon after settlement. The mortality of small juveniles (less than 3 mm in mean shell length) in the post-settlement period between October 24, 2006, and August 1, 2007, was estimated at 1.34% d⁻¹, which was approximately 5.6 times higher than after the post-settlement periods (0.24% d⁻¹ and 0.17% d⁻¹). We compared the mortality of the juvenile in and after the post-settlement period with the results of the population study conducted on Kikuchi River Tidal Flat, Kumamoto, Kyushu (Tsukuda 2008), where the daily mean temperature was approximately 6°C higher in the post-settlement period of the juveniles. On Kikuchi River Tidal Flat, the mortality in and after the post-settlement period of the juvenile was estimated at 1.62% d⁻¹, and 0.49% d⁻¹ and 0.44% d⁻¹, respectively. These facts indicate that the extremely low winter temperatures in this study area (eastern Hokkaido) does not bring about large-scale mortality, and this is true not only for juveniles after the post-settlement period but also small juveniles in the post-settlement period.

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The development of microsatellite markers specific to corals (especially those of the genus Acropora) is difficult for various reasons (e.g. the small genome size and low number of microsatellite loci within the genome). Therefore, we developed a novel microsatellite marker and also attempted to apply those developed from other coral species to investigations of our target species, Acropora digitifera and Acropora sp. 1. We found that six available markers, including the novel marker, could be used for these two target species, along with commonly observed repeat motifs corresponding to six microsatellite loci. These markers may be appropriate for use with other species of this genus.

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