Tomoki Sunobe

Sex Change in the Gobiid Fish Is Mediated through Rapid Switching of Gonadotropin Receptors from Ovarian to Testicular Portion or Vice Versa

Sex-changing fish Trimma okinawae can change its sex back and forth from male to female and then to male serially, depending on the social status in the harem. T. okinawae is well equipped to respond to its social status by possessing both ovarian and testicular tissues even though only one gonad remains active at one time. Here we investigated the involvement of gonadotropins in sex change by determining the changes in gonadotropin receptor (GtHR) gene expression during the onset of sex change from female to male and male to female. The expression of the GtHR was found to be confined to the active gonad of the corresponding sexual phase. During the sex-change from female to male, initially the ovary had high levels of FSHR and LHR, which eventually went up in the testicular tissue if the fish was bigger. Changing of the gonads started with switching of GtHR expression discernible within 8-12 h of the visual cue. Further in vitro culture of the transitional gonads with a supply of exogenous gonadotropin (human chorionic gonadotropin) revealed that the to-be-active gonad acquired the ability to produce the corresponding sex hormone within 1 d of the activation of GtHR. Conversely, the to-be-regressed gonad did not respond to the exogenous gonadotropin. Our findings show that the gonads of successive sex-changing fish possess the intrinsic mechanism to respond to the social cue differentially. Additionally, this location switching of GtHR expression also could substantiate the importance of the hypothalamo-pituitary-gonadotropic axis.

Bi-directional sex change in the gobiid fish Trimma sp.: does size-advantage exist?

Summary The size-advantage model predicts that a large dominant female should become male in fishes where large males monopolize mating opportunities with multiple females. We investigated the sexuality of the gobiid fish Trimma sp. to determine if a size advantage exists for this goby. Histological examination of the gonads showed that individuals functioned only as one sex at a time, even though the gonads contained both ovarian and testicular tissues. Experiments in aquaria demonstrated that bi-directional sex change occurred in Trimma sp. In natural populations, males were larger than females and the sex ratio was strongly biased towards females. This indicates that Trimma sp. may be polygynous and function primarily as a protogynous hermaphrodite. However, sex-size distributions largely overlapped and contained small males. Furthermore, in many three-female groups, the largest fish remained female and the smaller fish became male. The population density of Trimma sp. was about 12 times that of T okinawae; high population density is likely to allow the possibility that small males attain some degree of reproductive success, and this reproductive success selects for the small male strategy. This is in agreement with the predictions of the size-advantage model and a new version of the size-advantage hypothesis.

Bi-directional sex change and gonad structure in the gobiid fish Trimma yanagitai

Bi-directional sex change in the deep-water gobiid fish Trimma yanagitai was examined. The gonads of all individuals consisted of ovarian and testicular elements, and an accessory gonadal structure. In no gonads were both testicular and ovarian parts simultaneously active. Bi-directional sex changes occurred during the rearing experiments in aquaria under conditions of which there was co-existence of two males or plural females. The sex of individuals could be determined by their relative body size or social dominance: the largest individuals acting as male and the remainder as female.

Random mating and protandrous sex change of the platycephalid fish Thysanophrys celebica (Platycephalidae)

The mating system and protandrous sex change of the platycephalid fish Thysanophrys celebica were examined by collection and observation in the field and by use of aquarium experiments. Male–female pairs formed in the afternoon, and performed spawning behavior at night during the warmest season from July to early September. The pair bond was temporary, and both sexes subsequently changed partners. In pairs, the female was always larger than the male, although there were no significant size-assortative relationships between males and females in the pair. Home ranges of females and males overlapped, indicative of an absence of territoriality. Tagged individuals did not stay in the study area. These results indicate that random mating occurs for this species. Protandrous sex change of T. celebica was confirmed by field observation of a tagged individual and by aquarium experiments. Males (≤116xa0mm total length) changed sex at 2xa0years old, and females (≥97xa0mm) at 3–5xa0years old. Because males changed sex even in the presence of larger females, sex change was not socially controlled. Consequently, reversed (female-to-male) sex change did not occur in aquarium experiments with rearing females only. For this species, it is suggested that random mating favors protandrous sex change at a fixed age and/or size, not depending on social status, as the original size-advantage model has predicted.

Accumulation of organotins in wharf roach (Ligia exotica Roux) and its ability to serve as a biomonitoring species for coastal pollution.

In this study, we measured the accumulation of tributyltin (TBT) in wharf roach (Ligia exotica Roux) and examined the species ability to be used for TBT biomonitoring in coastal environments. In an exposure test, wharf roach were exposed to TBT via diet for 2d. TBT was accumulated in wharf roach, and its metabolite dibutyltin was detected. The concentrations of these compounds gradually decreased during the depuration period, but they were still detected 12d after exposure ceased (TBT 290±140ng/g; dibutyltin 1280±430ng/g). The biological half-life of TBT in wharf roach was estimated to be about 4d. In a field study conducted in 2011-2012, wharf roach were collected from 15 coastal sites in Japan and 3 sites in Manado, Indonesia. TBT was detected in both Japanese and Indonesian samples. The highest concentration of TBT was found in wharf roach collected at Bitung ferry port, Manado (57.9±16.5ng/g), which is close to a shipyard, and the highest concentration at a Japanese site was 12.3±6.2ng/g. Thus, we were able to detect organotins in the coastal environments by testing wharf roach, suggesting that L. exotica might serve as a good bioindicator for monitoring organotin pollution.

Evolution of bidirectional sex change and gonochorism in fishes of the gobiid genera Trimma, Priolepis, and Trimmatom

Size-advantage and low-density models have been used to explain how mating systems favor hermaphroditism or gonochorism. However, these models do not indicate historical transitions in sexuality. Here, we investigate the evolution of bidirectional sex change and gonochorism by phylogenetic analysis using the mitochondrial gene of the gobiids Trimma (31 species), Priolepis (eight species), and Trimmatom (two species). Trimma and Priolepis formed a clade within the sister group Trimmatom. Gonadal histology and rearing experiments revealed that Trimma marinae, Trimma nasa, and Trimmatom spp. were gonochoric, whereas all other Trimma and Priolepis spp. were bidirectional sex changers or inferred ones. A maximum-likelihood reconstruction analysis demonstrated that the common ancestor of the three genera was gonochoristic. Bidirectional sex change probably evolved from gonochorism in a common ancestor of Trimma and Priolepis. As the gonads of bidirectional sex changers simultaneously contain mature ovarian and immature testicular components or vice versa, individuals are always potentially capable of functioning as females or males, respectively. Monogamy under low-density conditions may have been the ecological condition for the evolution of bidirectional sex change in a common ancestor. As T. marinae and T. nasa are a monophyletic group, gonochorism should have evolved from bidirectional sex change in a common ancestor.

Pollution of radiocesium and radiosilver in wharf roach (Ligia sp.) by the Fukushima Dai-ichi Nuclear Power Plant accident

Radionuclide concentrations in wharf roaches inhabiting coastal areas of Honshu, Japan, were investigated in October 2011 and June 2012. Relative high concentrations of 110mAg (2.1–127xa0Bqxa0kg-wet−1), 134Cs (2.6–61xa0Bqxa0kg-wet−1), and 137Cs (3.5–92xa0Bqxa0kg-wet−1) were detected in specimens from the eastern Honshu areas. Significantly lower 137Cs concentrations (0.7–1.6xa0Bqxa0kg-wet−1) were detected in specimens from western and northern Honshu. The decay-corrected 137Cs concentration was significantly inversely correlated with the distance from the Fukushima Dai-ichi Nuclear Power Plant. Thus, wharf roach may serve as a good bioindicator for monitoring radioactive contamination of its habitats.

Monogamous mating system and sexuality in the gobiid fish, Trimma marinae (Actinopterygii: Gobiidae)

The mating system and sexuality of the gobiid fish Trimma marinae were investigated in aquaria and by gonadal histological examination. The male to female sex ratio in the study aggregation was female biased (14:27), and females were larger than males. T. marinae were monogamous because they established continuous pairs and spawned repeatedly with the same individuals. Observations of aggressive behavior suggested that the monogamous mating system resulted from female mate guarding. We also performed a rearing experiment to test whether sex change occurs in this species. None of the males or females reared separately in aquaria for 63xa0days changed sex. Additionally, gonadal histology revealed that mature fish had unisexual gonads (testis or ovary). These results strongly suggest that T. marinae is gonochoristic. However, immature fish had a bisexual gonadal structure, indicating juvenile hermaphroditism.

Notes on protandry in the creediid fishes Limnichthys fasciatus and L. nitidus (Teleostei: Creediidae)

Fishes of the family Creediidae occur mainly on the sandy bottom of tropical and temperate shallow waters in IndoWest Pacific Ocean. Eight genera and 18 valid species are known (Fricke and Golani 2012; Nelson et al. 2016). For ecological information, Leis (1982) and Reader et al. (2000) describe egg and larval development of Limnichthys fasciatus and Limnichthys nitidus, respectively. Cozzi and Clark (1995) report darting behavior, exhibited by L. nitidus as quick movements out and back in the sand, suggesting that it was an escape behavior when they were disturbed. Biological information on the reproduction of Creediidae has been very limited. Langston (2004) histologically studied the sexuality of 10 species of creediids and showed that Crystallodytes cookei and L. nitidus, identified as Limnichthys donaldsoni in the original paper, which is a junior synonym of L. nitidus (see Yoshino et al. 1999; Shimada 2013), are regarded as protandry (sex change from male to female) by histological observation of gonads and size distributions of both sexes (female [ male). In addition to these two species, Chalixodytes tauensis, Crystallodytes pauciradiatus and L. fasciatus have gonads comprising ovarian and testicular parts divided by connective tissue in functional males, whereas those of females consist of only the ovarian part. Sadovy de Mitcheson and Liu (2008) indicate that functional hermaphroditism is confirmed in 27 families of teleost fishes in their review on hermaphrodite fishes. However, Creediidae is not included among these families; namely, hermaphroditism of Creediidae has been overlooked for a long time. To provide evidential data of functional hermaphroditism, we made histological observations on gonads of the two creediid fishes L. fasciatus and L. nitidus. Forty-two specimens of L. fasciatus were collected by a hand net using SCUBA at Banda Beach, Tateyama, Chiba, Japan (34 580 N, 139 460 E) on 13 April (n = 1), 14–15 May (n = 7), 17 June (n = 9), 18–16 July (n = 13) and 18–21 August (n = 12), 2013. The fish were brought to the laboratory, anesthetized in 100 ppm MS-222, measured for standard length (SL) to the nearest 0.1 mm with a digital caliper, fixed in Bouin’s solution for 24 hours, and then preserved in 70 % ethanol. The abdominal parts of the specimens were embedded in paraffin, sectioned to 5 lm, and stained with hematoxylin and eosin. We also examined gonads of seven L. nitidus specimens deposited at the National Science Museum, Tokyo: NSMT–P 71438 (n = 4, 16.0–20.5 mm SL, collected at Ambon Isl., Indonesia on 5 December 1998) and NSMT–P 77532 (n = 3, 14.9–26.8 mm SL, Okinoshima Isl., Kochi, Japan on 24 July 2007). We dissected and extracted the abdominal organs containing the gonads and prepared the tissues following the methods outlined above. The gonads of 21 specimens of L. fasciatus comprised both testis and ovary (Fig. 1a), which were apparently divided by connective tissue. The oocytes of all 21 hermaphroditic specimens were immature, while the testicular parts of seven specimens collected in July and August were & Toshiaki Shitamitsu shitamitsu@affrc.go.jp

Interspecific territoriality in males of the tube blenny Neoclinus bryope (Actinopterygii: Chaenopsidae)

Territoriality was investigated in the tube blenny Neoclinus bryope (Actinopterygii: Chaenopsidae) at rocky intertidal areas of Banda Beach, Tateyama Bay, central Japan. Males used small holes as spawning nests, usually staying at the nest and maintaining the area while showing exclusive behaviors. Their home ranges were limited to areas within 30xa0cm distance from the nest for over 2xa0months. Four heterospecific fishes were threatened when they approached to within 6–14xa0cm of the nest holes, and two species of carnivorous snails were removed at points 0–30xa0cm from the nest entrance. There were no significant differences between the distances from the nest entrances to the points defended against fish and those used for foraging. As the four heterospecific species have similar feeding habits to those of N. bryope, the area defended against fishes may function as a foraging territory. At 24xa0h following the removal of nest owners, carnivorous snails had gathered to actively prey on eggs, indicating that the area defended against snails may function as a territory for protection against egg predators. Although the positions of females where males started courtship displays were significantly farther than the foraging points and the positions of threatened fishes, males displayed no territoriality against conspecific males. The fact that males did not leave the nest hole during the courtship suggests that it may be costly to maintain a courtship territory. These results show that males of tube blenny maintain territories for egg guarding and for protecting food resources around their nest holes in the spawning season.