Ethology 94, 339-345 (19?3) 0 1993 Paul Parey Scientific Publishers, Berlin and Hamburg ISSN 0179-1613

Fisheries Laboratory, Kyushu University, Hakozaki, Fukuoka

Sex Change in Both Directions by Alteration of Social Dominance in Trimma okinawae

(Pisces: Gobiidae)

TOMOKI SUNOBE & AKINOBU NAKAZONO

SUNOBE, T. & NAKAZONO, A. 1993: Sex change in both directions by alteration of social dominance in Trimma oktnawae (Pisces: Gobiidae). Ethology 94, 339-345.

Abstract

The mating system of the gobiid fish Trimma okinawae is one of polygynous hermaphroditism, in which the largest female of a social unit changes sex following the removal of the dominant male. Histological observations of the gonads however, revealed that males have an ovarian tissue within a functional testis. The occurrence of ovarian tissue in the functional male suggests that 7. okinawae males should be able to revert back into being functional females. To test this prediction, we placed females in an aquarium and allowed them to change sex. After confirming sex change from female to male, we individually placed new males into another aquarium and added a larger male to each. O u r experiments revealed that females change sex and become males upon the removal of dominant males, and that those males changed sex again and became females in the presence of larger males. Sex change in both directions may be advantageous when a male is forced to become subordinate following the take over of the social unit by a larger male.

Corresponding author: Tomoki SUNOBE, Natural History Museum and Institute, Chiba, Chuo-ku, Chiba 260, Japan.

Introduction

Sequential hermaphroditic fishes change sex in response to alterations in social dominance that depends upon differences in relative body size (e.g., Serranidae, FISHELSON 1970; Labridae, ROBERTSON 1972; Pomacentridae, FRICKE & HOLZBERG 1974; FRICKE & FRICKE 1977; Pomacanthidae, MOVER & NAKAZONO 1978; Gobiidae, COLE 1983; Pinguipetidae [= Mugiloididae], NAKAZONO et al. 1985; Malacanthidae, BAIRLI 1988). Sex change can occur in two directions: protogyny, sex change from female to male; protandry, from male to female (SADOVY & SHAPIRO 1987). Sequential sex change in both directions, in which males or females can change from one sex to the other and back again, has ever been unknown in fishes (SHAPIRO 1989), however, KOBAYASHI & SUZUKI (1992)

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340 TOMOKI SUNOBE & AKINOBU NAKAZONO

have recently reported this phenomenon in the hawkfish Ciwhitichthys aureus (Cirrhitidae).

In a previous paper, we reported that Trimma okinawae (Gobiidae) has a polygynous mating system and that protogynous sex change occurred following the experimental removal of the dominant male from a social unit (SUNOBE & NAKAZONO 1990). Histological examination of gonads following the field obser- vations of behaviour however, revealed that males had ovarian tissue within functional testes. The results of this examination suggested that a male T. okinawae has the potential to change sex back and revert to being a functional female.

Here, we demonstrate experimentally sex change in both directions by T. okinawae in that the largest female in a social unit changes into a male in the absence of a dominant male, and then reverts to the female when subsequently placed in company of a larger-sized male.

Protogynous sex change can be explained by the size-advantage model proposed by WARNER (1975). This model however, does not predict sex change from male back to female in polygynous mating systems. We discuss this type of sex change in relation to the alteration of social dominance within a polygynous social unit.

General Biology

Trimma okinawae is a small (to 32 mm in total length [TL]) orange-coloured goby distributed from Kagoshima south to the Ryukyu Islands, Japan. Habitat associations include cave ceilings, slopes of rock, holes, or the underside of table corals. Individuals maintain home ranges in these habitats, where they feed upon planktonic copepods (SUNOBE & NAKAZONO 1990).

Field observations (SUNOBE & NAKAZONO 1990) revealed a polygynous mating system with social units consisting of a single dominant male and one or more females. This species is iteroparous, and the females spawn within a cycle of 4-5 days during a season that lasts from Jun. to Sep. Females visit spawning sites in the morning, where waiting males are found. After spawning, females return to their home ranges and males care for the eggs until they hatch.

Gonadal Structure

Materials and Methods

13 specimens were collected on Jun. 24, 1988 at Makurazaki, Kagoshima Prefecture, Japan (31"15'10", 130"15'40"). Although sexual dimorphism and dichromatism were unclear underwater (SUNOBE & NAKAZONO 1990), we were able to accurately sex individuals by the structure of the genital papillae: in males, papillae were long and tapered posteriorly, in females, papillae were bulbous with several processes at the papilla opening (Fig. 1).

Four males (25.3-29.2 mm TL) and 9 females (22.3-28.7 mm TL) were fixed in Bouin's solution for 4 d and then preserved in 70 '70 ethanol. Afterwards, the abdomens of each were embedded in paraffin and sectioned transversely at 15-20 pm. Sections were mounted and stained with hematoxylin and eosin.

Fzg. 1:

Scx Change in Both Directions in a Gobiid Fish 341

Urogenital papillae of Trimma okinawuc. A, male; B, female. Scale indicates 0.5 mm

A ..

I ANUS

B

Results and Discussion

The most remark.able feature of the gonadal structure of T. okinawae was that all individuals sampled had, simultaneously, an ovary, a testis and an accessory gonadal structure (AGS: COLE 1990), similar to T. unisquamis, Priolepis hipoliti and .P. eugenius (COLE 1990). Each gonad was separated into ovarian and testicular parts by a thin wall of connective tissue (Fig. 2 ) . In females, the ovary was occupied by oocytes in various stages of development. The testis and AGS were undeveloped (Fig. 2A). In males, the testis was filled with spermatozoa. A fully developed AGS and ovarian tissue with a few young oocytes were attached to the testis (Fig. 2B). The AGS, attached to the testis, has been found in male gobiid fishes examined thus far, but its function remains unknown (MILLER 1984; COLE 1990). The oviduct and vas deferens of T. okinawue were fused posteriorly at the urogenital papilla.

Fig. 2: Cross section of gonads. A, female; B, male. 0 - ovary, T - testis, AGS - accessory gonadal struc- ture, DT - digestive tract. Scale indi-

cates 0.1 mm

342 TOMOKI SUNOBE & AKINOBU NAKAZONO

Gonadal tissue of the initial sex disappears after sex change in many species (SADOVY & SHAPIRO 1987). However, in T. okinawae all 4 males examined had immature oocytes and mature testicular tissue. The hawkfish, Civvhitichthys aureus, has similar gonadal structure, and both sexes can change to the opposite sex (KOBAYASHI & SUZUKI 1992). These results suggest that males can revert back to being females by maturation of the oocytes. To test this prediction, we performed the following experiment in which male and female T. okinawue of different sizes were paired in the laboratory.

Pairing Experiment

Materials and Methods

8 specimens were collected on Jul. 13, 1988 at Makurazaki, Kagoshima Prefecture, Japan. Fishes were anesthetized with MS222, measured and sexed by visual examination of genital papilla structure. Between JuI. 14 and Sep. 14, 1988, fishes were kept in either one of two aquaria. Each aquarium measured 60 x 30 x 28 cm. The first (Aq I) held 7 females, and the second (Aq 11) a single male. Individuals in each were identified by colour patterns on head and nape, and were numbered from 1 to 8 according to decreasing body size.

Water was circulated and filtered by an air-lift system through a layer of gravel at the substratum. An opaque vinyl chloride pipe (2 cm inside diameter and 5 cm long) was placed on the substratum as a shelter. This shelter was also utilized for spawning. Fish were fed once a day with a diet of minced fresh fish and whole water fleas (Duphnia sp.).

In an earlier field study (SUNOBE ti NAKAZONO 1990), it was determined that female abdomens swollen with mature eggs later shrunk after spawning, and that males guarded eggs after spawning. Therefore, the abdomens of females held in aquaria were closely examined to monitor egg laying. When a female with a swollen abdomen on one day had a shrunken abdomen on the next, she was judged to have spawned on that morning. After fertilization had been confirmed, the egg guarding

II -1 ................................ 1 1 2 30.0 M

3 2 9 . 4 ~ I 1 ................................. I 1 II 1 4 2i3.6 F. ................................................... M- F.

5 2ao I 6 6,7

M.

8 27.4 F .................................................... 4 . . . . . . . 1 1 1 e ~ ~ 1 . . . . . 1 . . . . 1 . , .

........................................................................................... 8

I 5

7 265 F I 4 8-1. II ......................................................................... I 8 252 F ......................................................................... .? ................ 5.

Fig. 3: Results of aquarium experiments with sex change in both directions. All individuals, except No. 2, were initially kept in Aq I, No. 2 was kept in Aq 11. Dotted lines: females, shaded lines: males. M (= male) and F (= female) indicate that sex change to that sex had been completed on that day. Arabic numerals above F or dotted lines: individual No. of males spawning with that female (i.e., functional males); Arabic numerals under M or shaded lines: individual No. of females spawned with.

Solid circle: the day when the specimens were sacrified or had died (No. 2 and No. 3)

Sex Change in Both Directions in a Gobiid Fish 343

individual was judged to have changed sex from female to male in the aquarium. Those individuals in Aq I which had changed sex were moved to Aq 11, where a single male was kept. We then waited to confirm whether or not either of the males in Aq I1 changed sex back to that of a female.

Results and Discussion

The results of this experiment are summarized in Fig. 3 . All the egg masses in the 15 spawnings ohserved were fertilized. The eggs developed normally and hatched about 4 d after fertilization took place.

The temporal pattern of spawnings and sex change is summarized as follows. At the onset of the experiment, all 7 females, kept in Aq I, did not demonstrate any sign of reproductive behaviour. O n Jul. 19, the largest female (No. 1) occupied the nest site and began to perform courtship behaviour previously described by SUNOBE & NAKAZONO (1990). O n Jul. 26, female No. 3 laid an egg mass at the nest site which was then guarded by female No. 1; this behaviour indicated that sex change into a male by No. 1 had occurred. Subsequently, No. 1 spawned with No. 4 on Jul. 30. We then moved No. 1 to Aq I1 where another male (No. 2) was kept alone. Just after this introduction, No. 1 drove the resident No. 2 from the nest site, and became the dominant individual. Four days later, No. 2 had changed sex to a female, and spawned eggs at the nest site.

Just after the removal of No. 1 from Aq I, the largest of the remaining females, No. 3, began to dominate and guard the nest site; however, it died of an unknown cause on Aug. 6. Female No. 4 immediately assumed the dominant role in Aq I and occupied the nest on the same day. Subsequently, No. 4 guarded egg masses spawned by No. 6 and No. 7 on Aug. 18; sex change by No. 4 was indicated. Fish No. 4 was then transferred to Aq 11, and a struggle with No. 1 for dominance took place. The larger of the two fish, No. I , won against No. 4, and the latter assumed a lower status. O n Aug. 25, the abdomen of No. 4 began to be swollen, and it spawned an egg mass with No. 1 on Aug. 28.

In Aq I, both No. 6 and No. 7 had changed to males and had spawned with No. 5 and No. 8 on Aug. 24 and Sep. 1 respectively. We then removed No. 6 and No. 7 to Aq I1 on the same or following day in which sex change occurred. Both No. 6 and No. 7 were dominated by No. 1 and changed sex back into functional females on Sep. 2 and 13, respectively. In Aq I, No. 5, which was larger in size than No. o and No. 7 changed into a male and spawned with No. 7, the smallest fish on Sep. 13.

O u r results show that sex change in both directions occurs in T. okinawae. Females can change into males and then can revert back to their initial sex. O u r results also show that the pattern of sex change is determined by social rank relative to body size in all instances except a single case involving fish No. 5. We did not subsequently test to see if reverted males, as females, can become males again. The gonadal structure of individual specimens examined after the conclu- sion of the experiment was consistent with that of either sex illustrated in Fig. 2. This result strongly suggests that sex change could happen again.

The genital papillae of No. 1 and No. 2 examined after the conclusion of the experiment were male 2nd female, respectively, and their structures consistent

344 TOMOKI SUNOBE & AKINOBU NAKAZONO

with those illustrated in Fig. I . This result shows that the structure of genital papillae also changes with sex reversal.

Previously this form of sex change had been reported from the polychaete Ophryotrocha puerilis puerilis (BERGLUND 1986), the holothurian Polycheira rufescens (TOMARI & KUBOTA 1989), plants of the genus Arisaema (KINOSHITA 1986), and the hawkfish Ciwhitichthys aureus (KOBAYASHI & SUZUKI 1992).

General Discussion

WARNER’S (1 975) size-advantage model proposes that protogyny occurs in populations in which a female selects older or larger males. The mating system of Trimma okinawae is polygynous, and larger males can obtain more females than smaller males (SUNOBE & NAKAZONO 1990). O u r field experiments in which males were removed from dominance of the social unit, showed that the largest female of each social unit undergoes sex change and assumes dominance. Thus, the largest female may be expected to change sex when a dominant male is lost by predation, disease, senescence etc.

The size-advantage model predicts that protandry occurs in randomly mating populations (WARNER 1975). However, this model cannot explain the advantage of reverse sex changes, from male back to female, in populations of T. okinawae with a polygynous mating system. One possible explanation is that it is advantageous to change sex again when the social unit is dominated by the arrival or presence of another individual.

Although our field study (SUNOBE & NAKAZONO 1990) shows no direct evidence of a secondary sex change in males, indirect evidence suggests that the dominance in social units of T. okinawae changed frequently. This change depends upon size difference between the dominant male and the subordinate female. Males were usually larger than females within the same social unit. However, SUNOBE & NAKAZONO (1990) reported that body sizes of the largest females examined were equal to those of males in 4 of 10 social units sampled. If females grow faster than males in the absence of reproductive activity, they could easily assume dominance in social units. Sex change in both directions then, could be advantageous. O u r field experiment (SUNOBE & NAKAZONO 1990) has shown that a male forcibly introduced into the home range of another male is driven out by the resident. So that, an “unwillingly” expelled male has little chance to assume dominance of another social unit and reproduce as a male, but may still realize greater fitness by changing sex and spawning as a female.

Acknowledgements

We are indebted to Dr. T. J. DONALDSON for his kind review of this manuscript and valuable advice. Thanks are also due to Dr. Y. IWASA for his comments on the early draft of the manuscript. This study was partly supported by grants from the Ito Foundation for the Advancement of Ichthyology to T. SUNOBE.

Sex Change in Both Directions in a Gobiid Fish

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Received: September 29, 1992

Accepted: April 29, 1993 (W. Wickler)

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