Densely packed invertebrate communities have been observed around deep-sea hydrothermal vents and cold seeps, despite the high concentrations of hydrogen sulfide present in these environments. The species that inhabit these environments have been reported to accumulate the sulfur-containing amino acid, thiotaurine. This non-toxic amino acid is synthesized from its precursor, hypotaurine and ambient hydrogen sulfide. In this study, we compared the levels of thiotaurine and hypotaurine in the gills and mantles of two deep-sea mussels (Bathymodiolus septemdierum and B. platifrons) and a shallow-water mussel (Mytilus galloprovincialis) to investigate their degree of dependence on the thiotaurine/hypotaurine system. The levels of thiotaurine and hypotaurine in the gill were higher than those present in the mantle in the two deep-sea species but lower or similar in M. galloprovincialis. The levels of thiotaurine and hypotaurine were the highest in the gill of B. septemdierum, estimated at 6.56 and 13.50 µmol/g wet weights, respectively. Moreover, B. septemdierum exhibited a higher thiotaurine/(hypotaurine+thiotaurine) ratio in the gill, B. septemdierum than that did B. platifrons and M. galloprovincialis. These results suggest that the levels of thiotaurine and hypotaurine in Bathymodiolus may be related to the ambient sulfide levels in their habitats.

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The occurrence of gnathiid isopods, a group of parasitic crustaceans on fish, is reported from low-salinity habitats in Miyako-jima Island, Ryukyu Islands, southwestern Japan. In total, 1,096 gnathiid larvae were collected from four of ten surveyed sites, but mostly at two subterranean water sites connected with anchialine waters and/or the sea. Three stages of parasitic zuphea larvae were collected, but no free-living adults. Morphological observations and DNA barcoding show that these larvae are closely related to, and most likely conspecific with, Gnathia limicola a dweller of mud burrows in brackish-water habitats. Our study suggests that G. limicola actively enters into low-salinity subterranean habitats, presumably to search for euryhaline fishes to use as temporary hosts.

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Although population dynamics of the egg and planula stages of jellyfish are key factors that predict the abundance of polyps and subsequent jellyfish blooms, information about predation on these stages of jellyfish is lacking. We examined whether threadsail filefish Stephanolepis cirrhifer, an efficient predator of Aurelia sp. medusae in the wild, could feed on eggs or planulae of Aurelia under laboratory conditions. S. cirrhifer larvae at 17 days post hatching (dph) with 4.9±0.8 mm standard length fed on significantly more eggs or planulae (maximum value: 151 inds. per 15 min.) than other growth stages of S. cirrhifer larvae (0–18 inds. per 15 min.). Considering prey-size selectivity in fish larvae, the mouth size of S. cirrhifer at 17 dph matches well with consuming jellyfish eggs or planulae as prey. These results suggest that S. cirrhifer larvae, during their free-swimming stage or possibly at the beginning of their jellyfish-commensal stage, can be potential predators of eggs or planulae of jellyfish.

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Jellyfish possess venomous cnidae on their tentacles to capture and consume marine zooplankton. Nevertheless, the planktonic larvae of the smooth fan lobster (Ibacus novemdentatus), known as phyllosoma, prey on jellyfish and successfully ingest both tentacle tissue as well as constituent cnidae, despite the presence of the venom-filled explosively penetrant cnidae or nematocysts. In the present study, we hypothesized that phyllosomas have mechanical and/or physiological resistance to internal envenomation by ingested nematocysts. To test this hypothesis, we examined the feces of phyllosomas (n=5) that were fed with Japanese sea nettle (Chrysaora pacifica) and found both undischarged as well as discharged cnidae surrounded by peritrophic membrane. We surmise that this membrane may mechanically insulate the lining of the midgut from stinging nematocysts to avoid injection of jellyfish venom into the phyllosomas’ body by nematocyst tubule penetration. We then tested physiological sensitivity of the phyllosomas (n=10) to crude extract of tentacle cnidae injected into their bodies. For this experiment, we used a crude venom extract prepared from nematocysts isolated from tentacles of a rhizostome jellyfish (Nemopilema nomurai) after exposure to high salt which disrupted tentacle integrity, and phosphate-buffered saline as a control. Nine out of 10 animals died after the injection of crude venom extract, while none of the animals died in the control group. These results indicate that the defense of phyllosoma larvae against the toxin of jellyfish is a combination of mechanical inactivation of the ingested nematocysts and chemical digestion of the toxin in the midgut rather than physiological resistance against the toxin.

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