Context-dependent behavioural plasticity compromises disruptive selection of sperm traits in squid.

Sperm morphology is generally uniform within a species due to selective pressures that act to achieve better fertilization outcomes under postcopulatory competitive circumstances. Therefore, polyandry that intensifies post-mating sperm competition should constrain intraspecific sperm polymorphism. Contrary to this paradigm, we previously found that a polyandrous squid, Heterololigo bleekeri, produces dimorphic eusperm (flagellum length dimorphism; FLD), which is closely associated with alternative reproductive tactics (ARTs); large males (consorts) transfer their spermatophores inside the female's mantle cavity, while small males (sneakers) do so outside the mantle. Thus, FLD was considered as the consequence of different insemination strategies that arise from different modes of sperm competition, sperm storage and the fertilization environment. However, in other squid species showing ARTs, the choice of mating behaviour is rather conditional (i.e., switching mating tactic between consorts and sneakers), which poses the question of whether sperm FLD could have evolved. Here, we investigated five species in the family Loliginidae that exhibit ARTs and found that all species showed sneaker-biased FLD. However, in a species with conditional ARTs, we found FLD rather ambiguous and the testicular somatic index to be nearly continuous among individuals at transitional state, suggesting that plasticity in mating behaviour compromises the disruptive selection on a sperm morphological trait.

The role of the isolation of the marginal seas during the Pleistocene in the genetic structure of black sea bream Acanthopagrus schlegelii (Bleeker, 1854) in the coastal waters of Japan.

The black sea bream Acanthopagrus schlegelii (Bleeker, 1854) is a commercially important species in Japanese waters. Assessing its population structure is essential to ensure its sustainability. In the Northwestern Pacific, historical glacial and interglacial periods during the Pleistocene have shaped the population structure of many coastal marine fishes. However, whether these events affected the population of black sea bream remains unknown. To test this hypothesis and to assess the population structure of black sea bream, we used 1,046 sequences of the mitochondrial control region from individuals collected throughout almost the entire Japanese coastal waters and combined them with 118 sequences from populations distributed in other marginal seas of the Northwestern Pacific Ocean. As in other coastal marine fish with similar distribution, we also found evidence that the glacial refugia on the marginal seas prompted the formation of three lineages in black sea bream. These lineages present signatures of population growth that coincided with the interglacial periods of the Pleistocene. While the origin of Lineages B and C remains unclear, the higher relative frequency of Lineage A in the southernmost location suggests its origin in the South China Sea. The non-significant pairwise ΦST and AMOVA of Japanese populations and the presence of these three lineages mixed in Japanese waters; strongly suggest that these lineages are homogenized in both the Sea of Japan and the Pacific Ocean. Our results indicate that the black sea bream should be managed as a single stock in Japan until the strength of connectivity in contemporary populations is further addressed using non-coding nuclear markers.

Infestation of the Parasitic Isopod Mothocya parvostis on Juveniles of the Black Sea Bream Acanthopagrus schlegelii as an Optional Intermediate Host in Hiroshima Bay.

In Hiroshima Bay, parasitic isopods of the genus Mothocya infest the black sea bream Acanthopagrus schlegelii (Bleeker, 1854) and the Japanese halfbeak Hyporhamphus sajori (Temminck and Schlegel, 1846), two fish species that are abundant and commercially important in the Seto Inland Sea of Japan. Immature and mature Mothocya individuals can infect both juveniles and adults of H. sajori, while immature Mothocya are known to parasitize juveniles of A. schlegelii; i.e., no Mothocya parasites are found in adult A. schlegelii. The identification of the immature Mothocya parasitizing juveniles of A. schlegelii remains uncertain, because Mothocya species are morphologically identifiable only based on adult females. Also, the biological/ecological relationship between the hosts and parasites has not been studied. Here, we identified the parasites on A. schlegelii as Mothocya parvostis Bruce, 1986 by molecular sequence analyses along with other parasites obtained from H. sajori, the latter being morphologically confirmed by comparison with paratype materials of M. parvostis as well as the similar congener Mothocya sajori Bruce, 1986. The growth rates of the infected A. schlegelii juveniles from June to September in the years 2013-2015 and 2018 were significantly lower than those of the uninfected ones, suggesting a negative effect of the infection on the hosts. Our data on the prevalence and duration of the infection, as well as the body size gain of the hosts and parasites, corroborate a hypothesis that M. parvostis would utilize A. schlegelii as an optional intermediate host before it reaches the final host, H. sajori.

Toxicity and Toxin Composition of the Greater Blue-Ringed Octopus Hapalochlaena lunulata from Ishigaki Island, Okinawa Prefecture, Japan.

The toxicity of the greater blue-ringed octopus Hapalochlaenalunulata, whose bite is fatal to humans, was examined to better understand and prevent deaths from accidental bites. Living specimens were collected from tide pools on Ishigaki Island, Okinawa Prefecture, Japan, in November and December of 2015, 2016, and 2017. The specimens were examined for the anatomical distribution of the toxicity, which was expressed in terms of mouse units (MU), by the standard bioassay method for tetrodotoxin (TTX) in Japan. Paralytic toxicity to mice was detected in all of the soft parts. The posterior salivary glands exhibited the highest toxicity score with a maximum level of 9276 MU/g, which was classified as "strongly toxic" (more than 1000 MU/g tissue) according to the classification of toxicity established by the Ministry of Health, Labor and Welfare of Japan, followed by the hepatopancreas (21.1 to 734.3 MU/g), gonads (not detectable to 167.6 MU/g), arms (5.3 to 130.2 MU/g), and other body areas (17.3 to 107.4 MU/g). Next, the toxin from the salivary glands was partially purified by a Sep-Pak C18 cartridge and an Amicon Ultra Centrifugal Filter with a 3000-Da cut-off, and analyzed by liquid chromatography-mass spectrometry (LC-MS) equipped with a φ2.0 × 150-mm (5 µm) TSKgel Amide-80 column (Tosoh, Tokyo, Japan) with a mixture of 16 mM ammonium formate buffer (pH 5.5) and acetonitrile (ratio 3:7, v/v) as a mobile phase. This study aimed to clarify the toxicity and the composition of TTX and its derivatives in this toxic octopus. The main toxin in this toxic octopus was identified as TTX, along with 4-epi TTX, 4, 9-anhydroTTX and 6-epi TTX. Further, the toxicity of this species is also significant from a food hygiene point of view.

Eleven novel polymorphic microsatellite Loci for oval squid Sepioteuthis lessoniana (Shiro-Ika type).

The oval squid Sepioteuthis lessoniana is one of the most economically important squid species in Japan; however, its population structure is poorly understood due to the lack of hypervariable markers. Such information is critical for managing sustainable fisheries, as well as for ensuring the existence of wild S. lessoniana stocks. Eleven candidate microsatellite loci were isolated from a small insert genomic DNA library. Polymorphisms in these 11 loci were screened in 24 wild individuals. The number of alleles per locus was found to range from 5 to 19 alleles, and the observed heterozygosity ranged from 0.292 to 0.958. No evidence for linkage disequilibrium was detected among all the loci. The genotypic proportions conformed to Hardy-Weinberg equilibrium, except at one locus. In conclusion, these polymorphic microsatellite loci may be used to develop a genetic framework to manage S. lessoniana in the future.