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We described and compared the mating behavior and morphology of the reproductive system in chromodorid nudibranchs, then, examined how extensively the unique usage of the penis (autotomy and sperm removal) evolved among Chromodorids. In addition to Ardeadoris egretta, all of examined five species in Glossodoris autotomized their penises at the last stage of copulation. An interspecific difference was seen in penis autotomy in Noumea and Goniobranchus. A spiral structure was recognized in the vas deferens of autotomized species. This structure is supposed to be undifferentiated “next penises” stored in the vas deferens, which ensure successive copulation in autotomized species. Though the basic mechanism of penis replenishment was consistent, the reason to autotomize their penises may not be the same. Goniobranchus reticulatus is reported to remove sperm already stored in the mating partner’s sperm storage organ(s) with backward-pointing spines on the surface of its penis. Contrary to G. reticulatus, all of ten species that autotomized their penises in the present study did not have thorny but smooth penises. When they autotomized their penises, the tip of the penises still remained in the vagina of the partners. This suggests that autotomized penises in these nudibranchs function as a kind of copulatory plugs.

Simultaneous hermaphroditism is, at least initially, favoured by selection under low density — and therefore it can be assumed that sperm competition has little importance in this sexual system. However, many simultaneously hermaphroditic nudibranchs have both an allo-sperm storage organ (the seminal receptacle) and an allo-sperm digesting organ (the copulatory bursa), suggesting the possibility of the occurrence of sperm competition. A nudibranch, Chromodoris reticulata, autotomizes its penis after every copulation and replenishes it within about 24 h to perform another copulation. We observed that the surface of the autotomized penis was covered with many backward-pointing spines and that a sperm mass was often entangled on the spines. This suggests that the nudibranch removes sperm that is already stored in a mating partner’s sperm storage organ(s) with its thorny penis. Using six microsatellite markers, we determined that the sperm mass attached to the penis were allo-sperm originating from individual(s) that had participated in prior copulations. We revealed that C. reticulata performed sperm removal using the thorny penis. These results suggest that competition in fertilization is quite intense and mating frequency in the wild is relatively high in this species.

The article A nudibranch removes rival sperm with a disposable spiny penis, written by Ayami Sekizawa, Shin G. Goto and Yasuhiro Nakashima, was originally published Online First without Open Access.

The occurrence of polygyny requires specific environmental conditions such as female aggregation or patchy resource distribution. However, it is difficult to determine the factors responsible for polygyny in species in which the territories of both sexes overlap. To overcome this, we performed female removal experiments in the polygynous triggerfish Sufflamen chrysopterum (Balistidae) in the Okinawa coral reef. Both sexes defended their territories exclusively against consexuals of the same species, and female aggregation was absent. Each male territory included 1–3 female territories, and nonterritorial males were significantly smaller than territorial males. Further, the body size of territorial males was positively correlated with that of the largest female in their territories, and larger males tended to mate with more females. The results of the female removal experiments (n = 10 females) indicated that females competed for better territories rather than larger mates. In contrast, males abandoned the territories once the females emigrated. These results strongly suggest that males defend females rather than sites and compete for larger and a greater numbers of females. Thus, in S. chrysopterum, female defense polygyny occurs in the absence of female aggregation.

In many protogynous wrasses, large males with bright coloration (terminal phase males, TP males) establish mating territories and pair-spawn with females. In contrast, small primary males with drab coloration (initial phase males, IP males) are non-territorial and adopt three alternative reproductive tactics—group spawning, streaking, and sneaking. We investigated how IP males of the threespot wrasse Halichoeres trimaculatus use these tactics in different situations. The mating frequency of the IP males was positively correlated with their courtship frequency, but not with their body size. Larger IP males tended to attack the smaller ones at the mating sites. This indicates that the larger IP males attempted to exclude the smaller ones from mating with the intention of minimizing the number of IP males involved in group spawning and ultimately leading to pair spawning (sneaking). However, the larger IP males were unable to completely exclude the smaller males because the intensity of the attack by the larger IP males was weak. Consequently, the smaller IP males could easily streak into the sneaking of larger IP males, thereby resulting in group spawning.

To examine how a change in an individual's social status could influence its behavioural sex, we conducted male "removal-and-return" experiments in the polygynous wrasse, Halichoeres melanurus. This coral-reef fish is a protogynous hermaphrodite: the largest female (LF) living in a male's territory typically completes functional sex change within 2–3 weeks after the male's disappearance. In this experiment we removed males from their territories just prior to spawning time, about 1 h before sunset. In 12 of 30 trials, the resident LF spawned in the male role with smaller females, 21–98 min after male removal. Previous research suggests the LF should readily adopt male sexual behaviour to retain smaller females as future mates. However, the LFs of smaller body size were less likely to immediately perform male-role behaviour. This could be related to females' preference for larger mates: smaller LFs would be less likely to be chosen by other females, even if they could complete sex change and defend a territory. When a male was returned immediately after an occurrence of female–female spawning, the LF subsequently spawned in the female role with the returned male (6 of 12 trials). It could be adaptive for the LFs to accept a larger male as a mate rather than to fight against it. Thus, behavioural sex is reversible in H. melanurus, changing rapidly with social status.

The influence of predation risk on the opportunity for female mate choice was investigated in the tailspot wrasse Halichoeres melanurus at two sites on a coral reef in Okinawa, Japan. Females mated repeatedly with the nearest males, but they also changed mates frequently at both sites. Mate changes were seen not only in the context of spatiosocial changes (mate disappearance or shifts in male territories) but were also probably the result of actual mate choice by females. Females at one site (site A) changed mates more often and conducted longer spawning trips from their home ranges to male territories than at the other site (site B). Fish at site A were faced with a higher frequency of predators than that of site B. However, fish of site A suffered fewer attacks from predators because they had more shelter, suggesting lower predation risk in that site. These results suggest that females under higher predation risk had less opportunity to choose preferable mates and that they had to mate with the same, nearest males in most cases.

Morphological sexual differences and their effects on male mating success were investigated in the tailspot wrasse, Halichoeres melanurus, on a subtropical coral reef of Okinawa, southern Japan. Male mating territories were established along the reef edge from late April to early October, during which time pair-spawning occurred just before sunset in the male territories. Territorial males were larger and brighter than females in this protogynous fish. The males had longer, colorful ventral fins and colorful caudal fins, both of which were used during courtship. Multiple regression analysis of the effects of male characteristics on male mating success revealed that the yellow spot at the pectoral fin base was the most effective. It is suggested that large body size is favored in male–male competition for establishing mating territories and that conspicuous body color is favored in female mate choice.

The capacity for insemination and competitive ability of small males of the partially protandrous alpheid shrimpAthanas kominatoensis Kubo which lives in symbiosis with the Japanese purple sea urchinAnthocidaris crassispina (A. Agassiz) were examined. Although male shrimps 1.5 mm in carapace length or larger produced sperm, it was not until they grew to 2.4 mm that they became functional. Males lost their insemination ability when their endopodite (or the whole) of the 2nd pleopods were amputated. The amputation of the 5th pleopods did not affect the insemination ability. The males also lost insemination ability after being infected by 2 species of Isopod ectoparasites, probably because of the transformation of the 2nd pleopods. Small males were shown to be able to achieve copulation in cohabitation experiments with amputated or parasitized large males. The total parasite infection rate in the field was highest in December and lowest in May. From August to October, when the small and large males coexisted, on the average 15.9% of large males were infected and transformed by the parasites. The presence of parasites offers the small males an opportunity for reproduction and is thought to play an important role in the evolution of protandry in this shrimp.

Gastropods are generally known to be able to follow mucous trails. They follow the trails of their own for homing (McFarlane 1981; Chelazzi et al. 1987), of the same species for mating (Townsent 1974) or of other species for predation (Paine 1963; Gonor 1967). But according to Cook (1977), "trail following may be widespread [but] it need not necessarily be a general feature of a gastropod's normal behavioural repertoire." Trail-following behavior in opisthobranchs, especially for mating, has been referred to in only a few studies (Hadfield & SwitzerDunlap 1984) and not certified experimentally. For example, Stylocheilus longicauda (SwitzerDunlap & Hadfield 1979) and Bursatella leachii pleii (Lowe & Turner 1976) were observed to follow conspecific trails in the field. Todd (1979) considered that Onchidoris bilamellata might find mates by following trails, but the results of his laboratory tests were inconclusive. Dendrodoris nigromaculata and D. nigra are common sponge feeders in the intertidal zone of southern Japan. They are annual species, as is common among nudibranchs, and are sometimes observed in aggregations during the spring mating season. As nudibranchs are simultaneous hermaphrodites, they need not distinguish the sex of other individuals for mating. In the present study, I examined whether they utilize water-borne substances of the same species or mucous trails to find their mate.

An alpheid shrimp,Athanas kominatoensis, inhabiting a Japanese purple sea urchin was shown to be a partially protandrous hermaphrodite. They settled mainly in summer and spent the first reproductive season as males. Smaller males changed into females the following spring, but larger ones remained males throughout their lives. Sex change was socially controlled, as has been known in some fishes. In contrast to the fishes, however, subordinates changed their sex as the best of a bad situation in this shrimp. Most larger individuals lived singly or in sexual pairs, not tolerating others of the same sex on a host. The larger a male was, the more likely he was to be found with a female. On the contrary, males smaller than 3 mm were tolerated by larger males to some extent and had chances to cohabit and copulate with larger females. This made it advantageous to be functional as a male at first. Though the secondary sex change from female to male is theoretically expected, it is hardly realized because of the short life span. Hermaphroditism in this shrimp can be explained by the size-advantage model as a special case.