Large mating aggregations and daily long-distance spawning migrations in the bluehead wrasse,Thalassoma bifasciatum

Synopsis The mating system of the labrid fishThalassoma bifasciatum has been extensively described in the literature, and the species has formed the basis for much work in ecology and behavior. However, most work in the past on this species has concentrated in small patch reef habitats. Here I describe a different mating pattern in bluehead wrasses occurring on a large, linear barrier reef in St. Croix, U.S. Virgin Islands. On this reef, very large group-mating aggregations (> 10000 fish) form daily at a single area on the forepart of the reef. Tagging studies showed that fishes are faithful to particular feeding schools arrayed along the forereef, and that they migrate to the spawning grounds along a specific path from distances more than 1.5 km away. No mating appears to occur in other upcurrent areas of the forereef. One-way migration times averaged 52 min from the farthest site studied, and feeding does not occur en route. Despite large differences in time spent in migration, no significant differences existed in fecundity or frequency of spawning among females living at different distances from the mating aggregation. The only apparent differences was a significantlyhigher growth rate in the fishes residing farthest from the aggregation. This higher growth rate corresponded to a higher overall feeding rate in the distant location, suggesting that food intake can more than compensate for the increased costs of migration.     

Role of the tidal cycle in the gonadal development and spawning of the tropical wrasse Halichoeres trimaculatus

Many wrasses on coral reefs exhibit daily spawning that peaks around daytime high tides. In this study, we examined tidal-related ovarian development in the threespot wrasse, Halichoeres trimaculatus, a species common on coral reefs in the Indo-Pacific Ocean. When the fish were collected in the morning at different tidal phases, the gonadosomatic index (GSI) and ovarian histology changed; concomitant with increases in GSI towards high tide, a clutch of the most advanced oocytes developed from vitellogenic to maturation stages. Ovulated eggs and post-ovulated follicles (POF) existed in most ovaries around high tide, but only POF remained around ebb tide, suggesting that spawning occurred during or after high tide. We noticed that tidal-related spawning was considerable in the morning and that most ovaries collected on the afternoon high tide exhibited post-spawning features. This suggests that certain labrid species possess plasticity with regard to their spawning time and utilize potent environmental cues to ensure their reproductive success. When pieces of ovary were incubated with precursor steroids, high conversion of testosterone to 17beta-estradiol occurred during high and ebb tides, while that of 17alpha-hydroxyprogesterone to 17alpha,20beta-dihydroxy-4-pregnen-3-one and 17alpha,20beta,21-trihydroxy-4-pregnen-3-one was observed during low and flood tides. Incubation of pieces of ovary with human chorionic gonadotropin resulted in similar fluctuations in the steroid hormones with tidal phase. Production of these steroid hormones correlated with oocyte development in the ovaries and was probably regulated by gonadotropin. These results demonstrate that the daily cycle is fundamental for oocyte development, and that the tidal cycle is superimposed on this process.     

Endocrine control of diurnal oocyte maturation in the kyusen wrasse,Halichoeres poecilopterus

The present study examined diurnal cycles of oocyte development and maturation in the kyusen wrasse, Halichoeres poecilopterus, and investigated the sensitivity of oocytes to maturation-inducing hormone (MIH) and gonadotropic hormone (GTH). Female fish were sampled at fixed intervals throughout the day, revealing that final oocyte maturation and ovulation were completed by 6:00 hr, and that spawning occurred daily between 6:00 and 9:00 hr. In vitro experiments showed that the steroids 17,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P) and 17,20beta,21-trihydroxy-4-pregnen-3-one (20beta-S) were equally potent and highly effective inducers of germinal vesicle breakdown (GVBD) in kyusen wrasse oocytes. Additionally, circulating levels of 17,20beta-P and 20beta-S increased around the time of GVBD and ovulation, suggesting that 17,20beta-P and 20beta-S act as MIHs in the kyusen wrasse. Moreover, in vitro experiments clearly showed that kyusen wrasse oocytes had a daily developmental cycle of GTH and MIH sensitivity, and that oocytes that completed vitellogenesis acquired GTH-induced maturational competence. An endogenous GTH surge likely occurs between 12:00 and 15:00 hr, and this daily pre-maturational GTH surge probably controls the diurnal maturation cycles of kyusen wrasse oocytes.     

Ovarian degeneration during the female-to-male sex change in the protogynous hermaphroditic fish, Halichoeres poecilopterus

Halichoeres poecilopterus (Labridae) is a protogynous, hermaphroditic fish common in temperate waters around Japan. Although oocyte degeneration is an established event at the onset of sex change, there is little information on the process. We examined cell numbers and proportional ratios of ovarian germ cells in sex-changing gonads of H. poecilopterus. This confirmed a lack of YV (yolk vesicle stage) and Y (yolk stage) oocytes, a rapid decrease of P (perinucleolus stage) oocytes, and a slight increase of CN (before/at chromatin nucleolus stage) germ cells. We discuss gonadal conditions necessary for the protogynous sex change of H. poecilopterus.     

Group spawning results from the streaking of small males into a sneaking pair: male alternative reproductive tactics in the threespot wrasse Halichoeres trimaculatus

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.     

Vertical migrations of hungry imagoes of the tick Dermacentor marginatus

The ticks of D. marginatus display activity at a temperature of 3 to 16 C and relative humidity of 20 to 75%. At a low temperature higher than 19 C their activity is inhibited throughout the whole autumn period at any time of the day. Active appearance of ticks on the stems of vegetation was recorded at 8C and higher in August and the first decade of September (in the morning 16%, in the day time 7.5% and in the evening 74% of ticks become active). As a rule the most high percentage of ticks (56 to 85%) recorded from upper parts of stems was observed in August and first decade of September at evening hours at 12 to 17 C and relative humidity of 50 to 70%.     

Male morphological characteristics and mating success in a protogynous coral reef fish, Halichoeres melanurus

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. Received: June 16, 1999 / Accepted: September 22, 1999     

Benefits of heterospecific foraging by the Caribbean wrasse,Halichoeres garnoti (Pisces: Labridae)

Synopsis Terminal-phase yellowhead wrasses, Halichoeres garnoti, foraged alone or in association with foraging goatfish, Pseudupeneus maculatus and Mulloides martinicus. Whereas H. garnoti did not dig for benthic infauna, the goatfish foraged almost exclusively on and in sand substrata. Wrasses in the company of goatfish made significantly more strikes on sand substrata than did solitary wrasses, although there were no significant differences in the frequency of strikes on sand and hard substrata combined. The frequencies of other behaviors (searches, scratches, flights, chases, rests, yawns, and cleanings) were not significantly different between solitary and associated wrasses. H. garnoti are attracted to foraging goatfish, which provide access to an otherwise unavailable food resource. To whom reprint requests should be sent     

Social factors of group spawning as an alternative mating tactic in the territorial males of the threespot wrasse <Emphasis Type=BoldItalic>Halichoeres trimaculatus</Emphasis>

In most labrid fishes, large males with bright terminal phase (TP) coloration normally defend mating territories and pair spawn after following (i.e., courting) solitary females. It has also been reported that the TP males may abandon their territories and participate in group spawning when mating groups of small non-territorial males with drab initial phase (IP) coloration frequently intrude into the territories of TP males. In the similar situation of the present study, however, the territorial TP males of the threespot wrasse, Halichoeres trimaculatus, did not always participate in group spawning but continued pair spawning. Only the TP males that had few opportunities to encounter solitary IP individuals participated in group spawning, frequently following the mating groups of IP individuals. It is therefore suggested that group spawning of the territorial TP males is regarded as a tactic of making the best of the bad situation by the TP males with few opportunities to pair spawn in an area where group spawning dominates.     

Terminal phase males stimulate ovarian function and inhibit sex change in the protogynous wrasse Thalassoma duperrey

Under experimental conditions, the probability of sex change in the protogynous wrasse Thalassoma duperrey is determined largely by an individual's relative size within a social group. Natural populations, however, contain two distinct male phenotypes that may also play a role in regulating sex change. To investigate potential effects of male phenotype, the ability to change sex, ovarian histology and serum estradiol-17 beta levels were examined in females maintained under controlled social settings. Large females housed with smaller or larger terminal phase males had significantly larger gonadosomatic indices than females housed singly, with other females or with smaller initial phase males. Similarly, ovaries of females housed with terminal phase males showed no histological evidence of sex change, whereas large females from other social groupings were in advanced stages of sex change. These results demonstrate terminal phase males inhibit sex change regardless of their size relative to the female. Furthermore, gonadosomatic indices, ovarian histology, and serum estradiol-17 beta levels of females housed with terminal phase males indicate normal ovarian function whereas ovaries of other treatment groups appear quiescent or are undergoing sex change. Consequently, terminal phase males may be required for normal ovarian development which may, in turn, inhibit sex change in T. duperrey.     

Importance of sandy bottoms in coral reefs to the oscillation of daily rhythms in the tropical wrasse Halichoeres trimaculatus

Most wrasse species swim during the day and bury themselves in the sandy bottoms of shallow reefs at night. This study aimed to evaluate the importance of sandy bottoms to the day-active/night-inactive rhythmicity of the tropical wrasse Halichoeres trimaculatus. Actogram analysis revealed that fish were active during the photophase and inactive during the scotophase in aquariums with both sandy and bare bottoms. When fish were kept in aquariums with bare bottoms, rhythmicity was maintained under constant dark conditions (DD) but became obscured under constant light conditions (LL), suggesting that a day-active/night-inactive rhythmicity is regulated by the circadian system. Robust fluctuations in Period1 (wPer1) and Period2 (wPer2) expression were observed in the pectoral fin tissue under light–dark conditions (LD). Similar fluctuations in wPer1 expression persisted under DD. When fish were kept under LD conditions for 7 days and then DD for 20 days, the emergence of fish from the sandy bottom was delayed gradually. At the same time, the peak time of wPer1 expression under DD was retarded from 06:00 to 10:00. Although wPer2 expression was dampened under DD, it increased after exposing fish to light. These results suggest that wPer1 and wPer2 are differentially involved in the day-active/night-inactive rhythmicity, and that blocking light with a sandy bed at night and exposing fish to light during emergence in the morning play important roles in maintaining consistent activities in wrasse species.     

Androgens stimulate sex change in protogynous grouper,Epinephelus coioides: spawning performance in sex-changed males

We examined the efficacy of androgens (1.0 mg/kg body mass), testosterone (T), 11-ketotestosterone (11-KT), 17alpha-methyltestosterone (MT), testosterone propionate (TP) or androgen mixture (T, MT and TP in an equal ratio), for induction of sex change in protogynous orange-spotted grouper, Epinephelus coioides. The spawning performance in sex-changed males was also investigated. MT and androgen mixture at a dose of 1.0 mg/kg BW induced a sex transition and completion of spermatogenesis up to the functional male phase. The androgen mixture was most effective. Significantly, higher plasma T levels were found in MT and androgen mixture groups compared to control and other androgen implantation (T, TP or 11-KT) groups. We found that plasma levels of estradiol-17beta (E2) or 11-KT were not different among treated groups. Sex-changed males could successfully fertilize mature eggs. Fertilization and hatching rates were of 23.5-70.4% and 8.4-44.6%, respectively. The data demonstrated that induction of sex change by exogenous androgens in groups could apply to the aquaculture field for seed production.     

Human chorionic gonadotropin (hCG) induces gonad reversal in a protogynous fish, the bluehead wrasse, Thalassoma bifasciatum (Teleostei, Labridae)

Female to male successive hermaphroditism (protogyny) is common in several groups of marine fish. Thalassoma bifasciatum, the bluehead wrasse (Labridae), found in the reefs of the Caribbean normally undergoes sex reversal after receiving behavioral cues. This report deals with the successful use of human chorionic gonadotropin (hCG) in inducing gonad reversal in this species. Eighty percent (n = 40) of the treated fish showed signs of reversal in 1-6 weeks; only 11% (n = 54) of the control (nontreated) group showed signs of reversal during the same period. The number of fish undergoing reversal increased with the length of the treatment period, 55% after 1 week, 100% after 6 weeks. A bluehead color pattern, typical of the terminal male phase, also appeared more frequently in the longer treated groups. To determine the efficacy of hCG in inducing gonad reversal, morphological criteria for reversal had to be established. For the majority of treated fish, the presence of both spermatogenic cysts and degenerating oocytes was sufficient to label these gonads unambiguously as undergoing reversal. However, at the extremes of this process, i.e., onset (early) and endpoint (late) stages, ambiguity could arise in identifying a gonad as undergoing reversal, and, therefore, key criteria were established for these stages. In our hands, the most consistent and reliable criterion for the early stage reversal was appearance of male germ line cells, clusters of "spermatogonial-like" cells. For the late stage, recognizable remnants of late-state oocytes had to be present. Some details of the histological changes that occur during early, middle, and late stages of reversal are also described. It is suggested that these results with hCG shed new light on the endocrine control of gonad reversal in T. bifasciatum.     

Female nuptial signal advertises spawning readiness in a population of <Emphasis Type="Italic">Halichoeres margaritaceus</Emphasis> on the Great Barrier Reef,Australia

Females of some polygynous fish species display female nuptial signals (FNSs), advertising spawning readiness to potential mates. Halichoeres margaritaceus, a polygynous coral reef fish, reportedly display red belly color and a unique bobbing behavior as FNSs advertising spawning readiness. In this study, I examined whether FNSs were observed in an H. margaritaceus population in Australia. Females in this study displayed red belly color as an FNS, although at a lower intensity than that reported in a Japanese population. Contrary to observations made in the Japanese population, bobbing behavior performed by females in this study showed no temporal trend as spawning approached, indicating bobbing behavior did not function as an FNS in the Australian population. The role of local selective pressures on FNS intensity and lifetime reproductive benefits H. margaritaceus may potentially gain in utilizing FNSs are briefly discussed.     

Histological and ultrastructural evidence for the role of gonadal steroid hormones in sex change in the protogynous wrasse Thalassoma duperrey

Synopsis The process of sex change in the protogynous wrasse, Thalassoma duperrey, was investigated through histological and ultrastructural observations on the gonads of females changing sex to male. Changes in plasma steroid levels concomitant with structural changes were measured by radioimmunoassay. The process of sex change from ovary to testis was divided into six stages on the basis of changes in the structure of the germinal and somatic elements. Ovaries of females were filled with vitellogenic oocytes during the breeding season, but contained no spermatogenic tissue (Stage 1). At the commencement of sex change (Stage 2), vitellogenic oocytes began to degenerate, and were ingested by macrophagous cells. This stage was accompanied by a rapid drop in plasma levels of estradiol-17. Thereafter, previtellogenic oocytes (Stage 3) also began to degenerate, and aggregations of stromal tissue, and loose connective tissue were observed in the central region of the lamellae. Steroid producing cells (Leydig cells), developed at the border of this loose connective tissue. Presumed spermatogonia proliferated on the periphery of the lamellae, and Leydig cells increased in size and number (Stage 4). Spermatogonia formed cysts, and underwent spermatogenesis (Stage 5). Finally, sex change to male was considered complete, with the beginning of active spermatogenesis and spermiation (Stage 6). Plasma levels of testosterone remained low throughout the sex change, but a second androgen, 11-ketotestosterone increased gradually in parallel to the increased numbers of Leydig cells and spermatogonia. Preliminary in vitro incubation of gonads with salmon gonadotropin, revealed that sex-changed males had higher levels of 11-ketotestosterone production than did females, while females had higher levels of estradiol-17 production than did males. Production of both these steroids increased in a dose-related fashion with increasing doses of gonadotropin.     

Daily spawning and development of sensitivity to gonadotropin and maturation-inducing steroid in the oocytes of the bambooleaf wrasse, Pseudolabrus japonicus

The cycle of oocyte development of the bambooleaf wrasse, Pseudolabrus japonicus, was studied to elucidate the endocrinological mechanism of oocyte maturation in a marine teleost. A single female reared with two males spawned every day for 17 days in captivity, indicating that this species is a daily spawner. Ovarian histology revealed that germinal vesicle migration of the largest oocytes progressed from 12:00 to 3:00 h, and germinal vesicle breakdown (GVBD) was completed at 6:00 h. Ovulation and spawning occurred between 6:00 and 9:00 h. The effectiveness of human chorionic gonadotropin (HCG) and 17,20-dihydroxy-4-pregnen-3-one (17,20-P), which is one of the most potent steroidal inducers of GVBD in bambooleaf wrasse oocytes, in inducing final oocyte maturation was examined at eight different times of the day. The responsiveness of the oocyte to HCG and steroid differed at different times of the day. The GVBD could be induced by HCG but not 17,20-P at 9:00 h. Between 12:00 and 18:00 h, not only HCG but also 17,20-P induced GVBD. Both GVBD and ovulation spontaneously occurred between 0:00 and 6:00 h without any hormonal treatment. These results clearly showed that the oocyte of the bambooleaf wrasse possessed a diurnal maturation cycle. Responsiveness of oocytes to HCG appeared earlier than responsiveness to 17,20-P. This suggests that sensitivity to 17,20 -P is induced by gonadotropic hormone (GTH).