Male Mouth Brooding in Jawfishes (Opistognathidae)

BULLETIN OF MARINE SCIENCE, 52(2): 806-818,1993 CORAL REEF PAPER

MALE MOUTH BROODING IN JA WFISHES (OPISTOGNATHIDAE): CONSTRAINTS ON POLYGYNY

Helen C. Hess

ABSTRACT The mating systems of four Caribbean jawfish species were studied in natural populations. The type of parental care in this family of fishes, male mouth brooding, limits male mating success to some extent in all four species with the result that polygyny is constrained or prohibited. Monogamy is apparently the predominant mating system in Opistognathus max- iIlosus and Opistognathus aurifrons, and it occurs irregularly in Opistognathus macrognathus. Mouthbrooding precludes caring for the eggs of more than one female at a time and may be associated with other costs due to reduced feeding rates of parental males. Mouthbrooding may also strengthen other factors, such as spawning synchrony by females and a relatively long development time of embryos, in constraining polygyny or stabilizing monogamy. In the non-monogamous species, Opistognathus whitehursti, in which spawning is not synchronous, the long development time of the embryos limits the number of clutches that can be sequentially brooded by males; thus mouthbrooding limits male mating success in this species as well. All four species exhibit sexual dimorphism for larger mouths in males. Larger male mouths may be important because enhanced ventilation of embryos may result from a larger brood space. The fact that development time is more rapid in 0, macrognathus, which has the highest ratio of male mouth volume to clutch volume, suggests that larger male mouths may lead to more efficient ventilation of embryos and more rapid development.

Monogamy is not common within the animal kingdom, and is expected to evolve only in systems in which there are constraints on male polygynous mating success (Em len and Oring, 1977). A polygynous mating system has evolved in most species of fishes in which parental care is provided by the male alone; a single male typically cares for the demersal clutches of several females simultaneously (Perrone and Zaret, 1979; Baylis, 1981; Blumer, 1982; Gross and Sargent, 1985). A number of conditions can favor or impose monogamy in fishes (reviewed in Barlow, 1984). The need for biparental care of free-swimming fry has led to monogamy in several families of fishes. In systems where care is uniparental or absent, ecological factors which make the formation of stable pairs advantageous (e,g., joint defense of resources, low population density) create conditions favoring monogamy. There are other fish species in which none of these factors appear to apply, yet have a monogamous mating system. When parental care consists of bearing the developing embryos on or in the body of the adult, the mating system or sex of the care-giver may depart from the typical profile of male parental care and polygyny. Care by the female alone occurs in most mouthbrooding cichlids (Fryer and lies, 1972; McKaye, 1984). In cichlids where both parents mouthbrood or guard free-swimming fry, the mating system is monogamous (Barlow, 1974; Kuwamura et aI., 1989). Monogamy also occurs in some pipefishes, in which the male bears the offspring in a pouch on his body and is unable to accommodate the eggs of more than a single female simultaneously (Gronell, 1984). When egg-bearing is performed by the male, limits on mating opportunities imposed by this form of parental care can be expected to constrain polygyny. Male mouth brooders can usually care for only one brood at a time, so providing care for a brood prohibits a male from remating until that brood completes development. Thus, male mouthbrooding may be associated with a high mating cost in addition to any physiological costs that mouthbrooding may confer. Yet,

806 HESS: JAWFISH MATING SYSTEMS 807

in spite of these potential costs, male mouth brooding has evolved independently in at least six families of fishes: Apogonidae, Ariidae, Belontiidae, Cichlidae, Cyclopteridae, and Opistognathidae (Oppenheimer, 1970; Blumer, 1982). The effects of male mouth brooding on other aspects of the species biology are not well-studied. Jawfishes (Opistognathidae) are a circumtropical family of marine fishes con- sisting of three genera and approximately 70 species (Nelson, 1984), and all species studied to date are male mouthbrooders (Bohlke and Chaplin, 1957; Leong, 1967). Species living in shallow water dwell in narrow vertical burrows in the sand and rubble on shallow back reefs or reef slopes, and each fish excavates and maintains its own burrow. The mating system in natural populations has not been described for any species of jawfish. The only published report of mating in a jawfish species concerns a single pair of Opistognathus aurifrons observed in an aquarium (Leong, 1967). Only two species of jaw fishes, O. aurifrons and Lonchopisthus micrognathus, have been studied extensively in the field (Colin, 1971, 1972; Colin and Arenson, 1978), and these studies focused on general aspects of the natural history of these species. Most other work on this family concerns species descriptions and documenting the occurrence of male mouthbrooding. In this paper, I describe the reproductive biology and mating systems of four species of jawfish from studies of natural populations. I also compare rates of embryonic development and document patterns of sexual dimorphism for body size, jaw morphology, and mouth volume, which may be of particular importance to mouthbrooders.

MATERIALS AND METHODS

Four species of jawfish, Opistognathus rnaxillosus, Opistognathus whitehursti. Opistognathus rna- crognathus, and Opistognathus aurifrons were studied at the Smithsonian Tropical Research Institute (STRI) field station in the San Bias Archipelago off the Caribbean coast of Panama. All four species were monitored during April-September, 1988. In addition, two O. rnaxillosus populations and one O. whitehursti population were monitored during November-December, 1987. Populations of each species were studied on at least two reefs. All the jawfish on the reef comprised the study population for each species except O. whitehursti. For this species, densely populated areas that had a border of at least 10m from which jawfish were absent were treated as study populations. Jawfish burrows were marked with a small, numbered piece of surveyor's tape attached to a nail and later mapped by measuring distances between individual burrows to the nearest 0.05 m. Population density and spatial patterns of individuals within populations were determined from these maps. To census these study populations, a diver swam a routine circuit through the population to determine if each fish was present in its burrow, whether it was mouth brooding an egg mass and the color (estimated age) of those eggs. Censusing was done in a variety of weather conditions and times of day. The census data were used to determine the mating success, defined as the number of egg masses received per unit time, for each male in the study, temporal patterns of clutch deposition, and the degree of site fidelity exhibited by individuals of each species. Behavioral observations were also made during the course of these censuses. Typically, jawfishes either perch at the entrance of their burrow with only the head exposed or, in the case of O. aurifrons, hover in the water column close to the burrow entrance where they feed on plankton. Each individual was observed for at least I to 2 min per census, and behaviors other than perching and hovering, such as burrow-building activity and social interactions were noted. Initially, populations were censused daily to determine the development time of the eggs for each of the four species. It was possible to determine the stage of development of the eggs by their color. Early eggs are creamy yellow or orange, and as development proceeds, they gradually darken, ultimately becoming shiny silver as yolk is consumed by the embryo and pigment and eyes develop. After development times had been determined, populations were censused less frequently, typically every 2-3 days. Given that the minimum development time of jawfish eggs is 5 days (see results), all clutches that completed development would have been noted at least once during regular censusing. Occasionally, bad weather caused longer intervals between censuses, but only once during 165 days of monitoring was the interval between sequential censuses longer than 5 days. Opistognathus rna- 808 BULLETIN OF MARINE SCIENCE, VOL. 52, NO.2, 1993

crognathus was censused using SCUBA. The other three species were shallow enough to census while snorkeling. Individuals of O. maxillosus, O. whitehursti. and O. macrognathus were identified using detailed drawings of natural markings on the fish's head. These markings were stable over the course of the study. A small number of Opistognathus aurifrons, which does not have mottled coloration as do the other three species studied, were identified from unique markings, usually small black spots on the head (26 of 216 individuals, 12%). Jawfishes were collected to determine the sex of brooding individuals, correlates of male mating success, and patterns of sexual dimorphism. Collections of the two larger species, O. macrognathus and O. maxillosus. were made by spearing individuals. Individuals of the smaller species were captured by placing a clear plastic bag over the entrance to the burrow and driving the occupant out using a length of flexible tubing. Individuals outside the study populations were collected throughout the study. Also, many of the censused individuals were collected at the end of the study. Collected animals that were not killed in the course of collection were killed within a few hours in isopropyl alcohol, and all specimens were fixed in 10% formalin then transferred to alcohol. Egg masses that were collected were fixed in 5% formalin and transferred to alcohol. To measure volumes of preserved egg masses, which were very soft, an egg mass was briefly blotted to allow the alcohol in the interstices of the egg mass to drain and [hen measured directly in a graduated cylinder. Individual eggs were ellipsoidal, and the lengths of the two axes of preserved eggs were measured with an ocular micrometer under a dissecting microscope to calculate egg volume. Preserved jawfish specimens were measured with calipers for standard length and maxilla (upper jaw) length. Specimens were weighed wet on a top-loading balance. The gonads of each individual were removed and examined under a dissecting microscope to determine the sex of collected individuals. Ovaries were yellow to orange, had a granular appearance, and were round in cross-section, while testes were white, smooth, and triangular in cross-section. Mouth volume, standard length, and maxilla length of males and females were compared to determine patterns of sexual dimorphism for these characteristics in each species. Mouth volume of preserved specimens was measured using 3 mm glass beads. The maximum number of beads that could be put into the mouth with the lips still able to close over the beads was determined, and this number was multiplied by a conversion factor to estimate mouth volume. To calculate the conversion factor, the number of beads occupying a given volume was determined by counting the number of beads required to fill a 5-ml graduated cylinder. This method allowed the total volume occupied by the beads (beads plus interstitial space) to be determined. The only species observed feeding in this study was O. aurifrons, which feeds on zooplankton (Colin, 1971, 1972). To quantify differences in feeding rate between brooding and non-brooding O. aurifrons, individuals were observed for 3-min intervals, and the number of bites taken and the time spent hovering in the water column versus inside the burrow were recorded. To estimate the sex ratio in study populations, the sex of collected individuals was determined by examining the gonads, and gender assignments for individuals that were not collected were made using several criteria: I) Opistognathus macrognathus is sexually dimorphic, and the sexes could be distinguished on the basis of jaw morphology. 2) Because all jawfish species studied to date are reported to have paternal care (Bohlke and Chaplin, 1957; Leong, 1967), and because all brooding individuals subsequently collected during this study were male (N = 12 O. maxi/losus. I O. aurifrons, 9 O. macrognathus, and 21 O. whitehursti), any individual seen brooding eggs was assumed to be male. 3) Individuals never seen tending eggs were assumed to be female if they were observed during censusing for as long as the average inter-clutch interval for the species (the mean time period between sequential clutches for a male) plus two standard deviations. To avoid biases in assigning sex to uncollected fish, only individuals that were observed longer than this interval were assigned a sex. The rest of the uncollected individuals in the study populations were considered to be of unknown sex. For censused individuals that were subsequently collected, all the individuals that were assumed to be female using criterion (3) were, indeed, female (N = 15 O. maxi/losus. 8 O. macrognathus, and 6 O. whitehursti). The sex ratio of populations was compared to I: I using a Chi-square test (Zar, 1984). To determine if the sex of nearest neighbors was random, the frequencies of nearest-neighbor pairs of each type (male-male, male-female, male-unknown, etc.) were compared to the frequencies expected given a random distribution using a G-test (Zar, 1984). A nearest neighbor was defined as the individual whose burrow entrance was closest to the burrow entrance of the individual in question. Spatial patterns of individuals within populations were quantified by comparing the mean distances between nearest and second-nearest neighbors with the distances expected given a random distribution of individuals using the nth-nearest-neighbor analysis of Thompson (1956). The main study sites most intensively censused are listed in Table I. For a detailed map of the area and the locations and names of reefs see Robertson (1987). In addition to the populations listed in Table I, O. maxi/losus was censused during 1987 on Aguadargana and Korbiski, and during 1988 on Korbiski and Tiantupo; O. whitehursti was censused during 1987 on Stritupo and during 1988 on Stritupo and Aguadargana; and during 1988, isolated pairs or individuals of O. macrognathus were HESS:JAWFISHMATINGSYSTEMS 809

Table 1. Characteristics of jawfish populations censused intensively during 1988. Reefs are referred to by the first letter in their name: Aguadargana, Korbiski, Guigalatupo, and Wichubhuala. The mean of first- and second-nearest neighbor distances (NN) are given with the standard deviation in paren- theses below. These distances are significantly different from those expected given a random distribution as noted: *** P < 0.00 I; **P < 0.0 I; * P < 0.05; C = clumped, 0 = overdispersed, R = random. Duration = the number of days that an individual fish was present in the study area

Study Density Duration Species area (fishper IsINN 2ndNN mode Sile Reef (m') 100 m') mean(SD) mean(SD) Spatial pattern (range) fidelity O. maxillosus A (west) 545 1.65 2.24 C** 12.72 0*** heterosexual 130 high (1.11) (13.82) pairs (48-130) A (east) 15,600 0.11 4.45 C*** 25.740*** heterosexual 130 high (9.93) (18.78) pairs (33-130) O. aurifrons K 59 29 0.46 C** 1.590*** heterosexual 138 high (0.40) (1.13) paIrs (8-138) G 69 56 0.52 C** 1.11 0* heterosexual 72 high (0.28) (0.38) pairs (28-72) O. macrognathus W29 202 4.9 2.14 C* 5.01 R heterosexual > 100 high (1.99) (2.51) pairs (29-127) W30 (south) 204 4.4 4.140** 5.790*** overdispersed 61 high (3.09) (4.34) (21-61) W30 (north) 72 9.7 1.94 R 4.600*** overdispersed 61 high (0.87) (2.18) (23-61) O. whitehursti K 477 6.5 1.34 C*** 2.39 C* clumped 10-14 low (0.79) (1.22) (9-75) censused on Korbiski, Tiantupo, and Wichubhuala #27. The number of individuals of each species seen at least twice during regular censusing are as follows: 109 O. maxillosus, 216 O. aurifrons, 40 O. macrognathus. and 128 O. whitehursti. Patterns of lunar periodicity of spawning were tested in all four species using a Rayleigh test (Batchelet, 1981). Sequential months with minimal fluctuation in population size were selected for analysis of spawning periodicity for each species. The data on spatial patterns of individuals within populations, behavioral interactions, and spawning synchrony were assessed to determine the mating system of each jawfish species. Although mating was never directly observed in this study, a monogamous mating system can be inferred from several types of patterns. Long-term pairing, evidenced by spatial or behavioral patterns, can be indicative of a monogamous mating system, although other mating systems cannot be absolutely ruled out (Thresher, 1984). If pair members can be shown to be consistently heterosexual, this lends strength to the conclusion that they are mating partners. In addition, if only paired individuals are seen tending eggs while isolated individuals never are, the implication is that mating occurs only between members of a pair, indicating monogamy. A high degree of spawning synchrony can also be taken as evidence of a monogamous mating system (Knowlton, 1982). In mouthbrooders that are able to care for only one clutch at a time, additional mating opportunities are limited if most spawning takes place within a narrow window of time, before the current clutch a male is brooding can complete development. Strict spawning synchrony provides unambiguous evidence of at least short-term monogamy. While the arrangement of individuals in obvious long-term pairs may suggest monogamy, in the absence of direct observation of mating, a temporal pattern of clutch deposition that prohibits sequential mating by males guarantees they are limited to a single mate. RESULTS Opistognathus maxi/losus Opistognathus maxi/losus was found in shallow (<2 m) water on the protected sides of reefs on substrates usually composed of sand, coral rubble and turtle grass 810 BULLETIN OF MARINE SCIENCE, VOL. 52, NO.2, 1993

Table 2. Sexual dimorphism in four jawfish species. Standard length and mouth volume for males and females of each species were compared using a student's I-test; P-values for one-tailed tests. s = standard deviation; N = sample size. For all species, males are significantly larger for both traits except O. maxillosus, in which there is no significant difference between male and female standard length

Standard length Mouth volume

Species il(mm) N P il(ml) N P O. maxillosus Male 109 8.4 13 0.68 0.25 4.0 1.19 13 4.33 <0.001 Female 107 9.7 19 2.1 1.24 19 O. aurifrons Male 69 2.7 10 2.44 0.01 0.49 0.10 10 2.39 0.01 Female 64 6.2 II 0.39 0.11 II O. macrognalhus Male 92 14.7 18 2.33 0.01 2.23 1.30 18 1.90 0.03 Female 76 19.5 8 1.25 0.94 8 O. whitehursli Male 47 5.7 36 1.74 0.04 0.33 0.13 36 2.87 0.003 Female 44 5.3 21 0.24 0.09 21

(Thallassia). Populations were very sparse, and individuals were spatially clumped in heterosexual pairs (Table 1).Nearest neighbors were significantly closer together than expected, while second-nearest neighbors were overdispersed, giving a paired spatial pattern. In 25 of 26 cases, the nearest neighbor was a member of the opposite sex (G = 27.71, d.f. = 1, P < 0.001), and most of these pairs remained intact during the entire study. Individuals were extremely site-attached and only rarely moved to build a new burrow elsewhere. Of 29 fishes censused at Agua- dargana in 1988, the main study population, 21 (72%) were present at the end of the study, which lasted 130 days (Table 1). Pairing was size-assortative (correlation between male and female standard length within a pair, r = 0.79, N = II pairs, P < 0.01). The male in a pair was always larger than the female although there was no overall sexual size dimorphism in O. maxillosus (Table 2). However, for the same individual length, males had significantly larger mouth volumes than did females (ANCOV A, F = 17.80, P < 0.001; Table 2). Male mating success (number of clutches received per unit time) was not significantly correlated with male size, mouth volume, maxilla length, or residence time in the population (P > 0.20 for all correlations). Clutches were produced synchronously, with the most clutches being laid on the day before full moon in all populations, resulting in a significant lunar periodicity of spawning (Rayleigh test; z = 10.6, N = 40, P < 0.001; Fig. 1). Repro- ductive activity was high during November and December of 1987 and April and May of 1988; only one clutch was observed during June-September, 1988. During the months in which breeding occurred, males typically brooded a single clutch per month, although during the last month of spawning activity (May, 1988), four males tended a second clutch. There are several lines of evidence suggesting that O. maxillosus has a monogamous mating system. Individuals were arranged in heterosexual pairs, and most of these pairs were stable over the course of the study due to the high degree of site fidelity exhibited by this species (Table 1). Only males that were a member of a pair were ever seen tending eggs, while isolated males (not the nearest neighbor of any conspecific) never had eggs. There was also a high degree of synchrony of HESS: JAWFISH MATING SYSTEMS 811

0 maxillosus 0 aurifrons

12 n = 40 12 n = 112 III III CIl CIl J:. 10 J:. 10 ~ U ::::I 8 '5 8 U .•.u '0 6 0 6 •.. •.. .aCIl 4 .aCIl 4 E E ::::I 2 ::::I 2 Z Z 0 0 F N F N

0 macrognathus 0 whitehursti 8 n - 37 10 n - 119 III III CIl CIl J:. J:. () () 8 6 ; ~ .•.() .•.() 6 0 4 0 •.. •.. 4 CIl CIl .a .a 2 E E 2 ::::I ::::I Z Z 0 0 F N F N Figure 1. Patterns of spawning periodicity during the lunar month for four jawfish species. Each new clutch observed during April-September, 1988 as well as those observed during November-December, 1987 for o. rnaxillosus have been collapsed onto a single lunar month for each species. F = full moon; N = new moon.

clutch deposition, precluding the care of the eggs of more than a single female. By the time the current clutch being brooded by a male had completed development freeing him to care for another clutch, egg laying was largely over in the population for that month.

Opistognathus aurifrons Opistognathus aurifrons was typically found on the sandy slopes that border coral reefs. Opistognathus aurifrons formed relatively dense aggregations and had the highest population density of the four species studied (Table I). In both O. aurifrons populations the individuals were spatially paired (Table 1). Nearest neighbors were significantly closer than expected while second-nearest neighbors were significantly further apart than expected given a random spatial distribution of individuals (Table 1). Pairing was also evident in the fishes' behavior, and certain behaviors between nearest neighbors were never observed between other nearby individuals. For example, it was not unusual for pair members to exchange burrows or occupy the same burrow for short periods, even while the male was brooding eggs. This behavior was never seen other than between members of the same pair. Members of other pairs were largely ignored. Pair members appeared to be of the opposite sex. Within a pair, only one individual was ever seen brooding at one time, and in cases where the pair 812 BULLETIN OF MARINE SCIENCE, VOL. 52, NO.2, 1993

members could be distinguished, it was always the same individual that held the eggs. All collected pairs were heterosexual (N = 6 pairs). All males that were members of a pair were seen brooding eggs at some time during the study, and individuals without partners (not the nearest neighbor of any conspecific) were never seen brooding. Opistognathus aurifrons frequently built new burrows, but these burrows were typically within a meter of the old burrow. Individuals occasionally disappeared from the study population, but many were present during the entire censusing period (Table 1). At Korbiski, the sparser population, 33/50 (66%) of the individuals censused were present at the end of the study. At the denser Guigalatupo subpopulation, 39/41 (95%) of the individuals censused were present at the end of the 70-day censusing period. Because individual identification was not possible for many of the O. aurifrons in the study, these estimates of residence times for individuals may be high. There were unambiguous cases of one individual being replaced by another from outside the study population between censuses. How- ever, additional replacements could have gone unnoticed. Based on collections outside the study area, males were larger than females (Table 2), but pairing was not size-assortative (r = 0.31, N = 7 pairs, P > 0.5). However, this sample was small, encompassed a narrow range of sizes and would be unlikely to detect size assortativity in pairing. Estimates of individual sizes in the large study populations suggested that pairs were size-assortative. These individuals are part ofa long-term monitoring project and were not collected. There was no sexual dimorphism for mouth volume after taking standard length into account (ANCOVA; F = 0.42, P = 0.57). Spawning occurred during the entire study period (April-September, 1988). More clutches were laid between full and new moon than at other times (65/94, 69% of the clutches; Fig. 1), but there was no statistically significant lunar periodicity in reproductive activity of this species (Rayleigh test; z = 2.79, N = 94, P > 0.05), nor was reproduction temporally clumped on specific days as in O. maxillosus or O. macrognathus (see below) (Poisson test; G = 5.98, d.f. = 3, N = 59 days, P > 0.1). Pairs typically produced from 0-2 clutches per month; two different pairs each produced three clutches in a single month. All 49 pairs observed produced at least one clutch during the study period, and unpaired individuals were never seen tending eggs. The fact that individuals were found in heterosexual pairs which remained intact for long periods of time and that only individuals in pairs were seen brooding eggs is suggestive of a monogamous mating system.

Opistognathus macrognathus Opistognathus macrognathus occurred on the sand surrounding submerged reefs in 8-10 m of water. Hovering gobies (Ioglossus sp.) were frequently found in the same habitat. Opistognathus macrognathus burrows were built in fine sand, and pieces of coral rubble and shells were used for structural support. On the two main study reefs near Wichubhuala, adults were arranged in pairs and small clumps plus isolated individuals. On Wichubhuala #29 the spatial pattern was significantly paired, while on Wichubhuala #30 there were enough isolated individuals in the population for the spatial pattern to be significantly overdispersed (Table 1). The sex of the nearest neighbor was random on both reefs (G = 3.16, 0.18; d.f. = 1; N = 10, 16; P > 0.05 for both tests), and the sex ratio did not differ significantly from 1:1 on either reef. Although individuals sometimes built new burrows several meters away from HESS: JAWFISH MATING SYSTEMS 813 an old burrow, they rarely left the study area. The reefs where O. macrognathus occurs are small and isolated, and movement between adjacent reefs was never seen during this study. Twenty-eight of33 (85%) individuals censused were present at the end of the study. Male O. macrognathus are larger than female O. macrognathus (Table 2). Ab- solute mouth volume was larger in males because of their larger size (Table 2), although males and females of similar size had similar mouth volumes (ANCOYA; F = 0.62, P = 0.51). Jaw morphology is dramatically different between the sexes. Males have an elongated maxilla which extends well past the eye and is significantly longer than that in similarly sized females (ANCOY A; F = 32.61, P < 0.00 I). This elongated flange occurs posteriorly to the articulation of the upper and lower jaws and does not appear to affect mouth volume. Male mating success may have been dependent on the sex of the nearest neighbor; only males whose nearest neighbor was a female received clutches (t = 6.46; N = 6 males with a female nearest neighbor, 6 males with a male nearest neighbor; P < 0.00 I). In three cases, males moved during the study from sites neighboring another male to sites neighboring a female. In all cases, the males had not received clutches before the move, but received clutches after moving. Male mating success was not correlated with size, maxilla length, mouth volume, or residence time in the populations (P > 0.15 for all correlations). Spawning occurred in O. macrognathus in all the months they were observed (April-September, 1988). The pattern of spawning periodicity had a single peak centered on the day of full moon (Rayleigh test; z = 4.00, N = 28, P < 0.05; Fig. I). Spawning peaks on the day of full moon occurred in both populations on the Wichubhuala reefs as well as in isolated pairs on other reefs. The single spawning peak in O. macrognathus populations was preserved even though some males received more than one clutch per month because most females laid a clutch on the day offull moon, while the additional clutches acquired by some ofthe males appeared less synchronously. In some cases mating groups consisted of a single heterosexual pair that was rather isolated from other conspecifics (second-nearest neighbor distances were at least one standard deviation greater than the mean in the population) and exhibiting a high degree of site fidelity, with males obtaining one or two clutches per month. These presumably monogamous pairs were found on reefs that had no other O. macrognathus (N = 2) as well as on more populated reefs (N = 2). More often, one or two males and several females formed a local cluster of burrows in which the male or males sequentially received clutches from one to three females (N = 3). In these cases mate fidelity appeared variable; the temporal pattern of clutch acquisition by males within groups and the relative sizes of the clutches suggested that some females spawned exclusively with a single male while others laid alternate clutches with two different males. In these mating groups, males received two to four clutches per month. There were also isolated males in these populations (the nearest neighbor either of no conspecific or of another male), and they never received eggs (N = 6).

Opistognathus whitehursti Opistognathus whitehursti was found in the same type of habitat as O. maxi!- losus, but O. whitehursti occurred at much higher density, had a clumped spatial distribution (Table 1), and had nearest neighbors that were random with respect to sex (G = 7.23, d.f. = 4, N = 36, P > 0.1). Opistognathus whitehursti frequently rebuilt burrows, and some individuals moved considerable distances (as much as 814 BULLETIN OF MARINE SCIENCE, VOL. 52, NO.2, 1993

Table 3. Reproductive biology offour jawfish species. Average female reproductive effort (R.E.) was estimated as the mean clutch volume divided by the mean weight of a female for each species. cv/ mv = mean ratio between the volume of a clutch and the volume of the brooding male's mouth. SD = standard deviation of cv/mv; N = sample size

Egg vol Clutch Female Devel Species (mm') vol (ml) R.E. time cv/mv SD N O. maxil/osus 0.81 3.8 0.14 7-8 1.04 0.43 3 O. aurifrons * 5-7 O. macrognathus 0.57 1.1 0.19 5 0.62 0.22 6 O. whitehursti 0.59 0.35 0.24 8-9 0.99 0.43 II • Egg volume of O. auriJrons is 0.33 mm" based on measurements of the axes of 10 eggs from a single clutch (P. L. Colin, pers. comm.).

10m) to a new burrow site. Individuals often disappeared from the study area. In the population at Korbiski, only 16/65 (25%) of the adult individuals censused were present at the end of the study. The temporal pattern of clutch aquisition for individual males was irregular, and, combined with the unstable spatial patterns of individuals, suggested that the clutches were produced by different females which did not exhibit a high degree of mate fidelity. Male mating success was not significantly correlated with standard length, maxilla length, mouth volume, residence time in the population, or the sex of the nearest neighbor (P > 0.20 for all correlations). Mating did not appear to be size-assortative. The correlation between clutch size, which probably reflects female size, and standard length of II parental males who were collected was not significant (r = 0.28, P > 0.4). Males were slightly larger than females (Table 2) and had significantly larger mouth volumes for their size (ANCOVA; F = 4.10, P = 0.02; Table 2). Opistognathus whitehursti did not breed in November and December, 1987, but bred throughout the entire study period in 1988 (April-September). There was no lunar periodicity in spawning (Rayleigh test; z = 0.26, N = 84, P > 0.1; Fig. 1). Males received 1.7 clutches per month on average (range 0-4 clutches per month); the number of clutches acquired by males in the Korbiski population that were observed during the entire study period ranged from two to seven. The low site fidelity and relatively short residence times of individuals within populations suggest a low degree of mate fidelity in O. whitehursti and a promiscuous mating system. Reproductive Biology. -Egg size, clutch size, and the size ofthe clutch relative to the brood space varied among the three species where data were collected (Table 3). Development time of the eggs also varied, although late-stage embryos of all three species looked similar (Table 3). Egg size in O. whitehursti and O. macrognathus did not differ significantly from each other (t = 1.03, P > 0.3). O. maxillosus eggs were significantly larger than eggs of both O. whitehursti (t = 14.1, P < 0.001) and O. macrognathus (t = 12.8, P < 0.00 I). These differences in egg size were not correlated with differences in development time across species; there is nearly a two-fold difference in development rate between the two species with similar egg sizes (Table 3). The species with a substantially smaller brood size relative to brood space (0. macrognathus) had the most rapid development, while the other species, whose broods completely filled the male's mouth had slower rates of development (Ta- ble 3). Development was synchronous within a clutch for all species, and with one exception, collected clutches contained eggs of only one age, suggesting they were HESS: JAWFISH MATING SYSTEMS 815

laid at one time, probably by a single female. One clutch of O. maxil/osus had two different early stages of eggs. The male brooding this clutch had two neighboring females. When collected, the clutch was composed of a region of 2-day- old eggs and a region of I-day-old eggs. The only other clutch observed containing eggs at two developmental stages was not collected, and the clutch disappeared 2 days after it appeared. Ninety-five to 100% of the eggs in each clutch collected appeared to be developing normally. Occasionally a few opaque eggs that were not developing could be seen throughout the mass. Complete mortality of clutches during development appeared to be very low. However, these estimates are probably lower than actual values; some clutches may have disappeared before they could be censused the first time. Mortality of O. aurifrons clutches was never observed (112 clutches seen). A single clutch disappeared prematurely in O. whitehursti (1/ 119 clutches, 1%) and in O. macrognathus (1/37 clutches, 3%). The only clutch loss observed in O. maxil/osus involved a rare case, mentioned above, where a male appeared to receive clutches from two females, who were each neighboring the male (1/40 clutches, 2.5%). No other males in the study populations were ever seen to tend more than one clutch at a time or to receive eggs from more than one female. Parental Care and Feeding. - Males hold the eggs in their mouths during development, and although the eggs can be briefly laid down inside the burrow, mouthbrooding hampers feeding of parental males. O. aurifrons fed at reduced rates while tending eggs. Brooding caused an 86% decrease in feeding rates: parental males averaged 0.40 bites per minute (s = 0.97, N = 10), while non-parental individuals averaged 2.93 bites per minute (s = 2.67, N = 70). Also, brooding individuals were much more likely to take no bites during a three-minute observation period (8/10) than were non-brooders (4170) (X2 = 43.95, P < 0.00l). Parental O. aurifrons spent significantly more time resting at the burrow entrance and less time in the water column when brooding eggs. In a 3-min time period, non-parental males averaged only 6 sec (N = 70) in their burrows; parental males averaged 24 sec (N = 10, t = 3.55, P < 0.01). In addition to spending less time hovering, parental males were often holding eggs while they hovered and were unable to feed. No jawfish was observed taking bites while the egg mass was in the mouth; instead, the eggs were put down inside the burrow for short periods while the male fed. Females were never observed brooding eggs in any of the specIes.

DISCUSSION Monogamous Mating Systems in Opistognathus.-Mouthbrooding appears to constrain male mating success and limit the extent of polygyny to some degree in all four jawfish species studied. In two species, O. maxil/osus and O. aurifrons, male mating success appears limited to a single mate, resulting in a monogamous mating system. Several factors appear to restrict additional male mating opportunities in monogamous Opistognathus. In O. maxil/osus population density is low, and additional mates may not be close at hand. More importantly, in O. maxillosus egg laying is synchronous, with most spawning occurring within a 7-day interval, which is the minimum time required for the development of a clutch. Thus, the opportunity for additional, sequential matings by males is virtually non-existent. Spawning periodicity has been proposed by Knowlton (1982) as a possible strategy by females to enforce monogamy, but spawning synchrony may also result from selection during larval or juvenile stages of the life history (reviewed in Robertson et a1., 1990). 816 BULLETIN OF MARINE SCIENCE. VOL. 52. NO.2. 1993

Factors restricting polygyny are less clear in the other strongly paired species, O. aurifrons. Opistognathus aunfrons has the highest population density of the four jawfishes studied, and spawning is not markedly synchronized as in O. maxillosus. It seems that the opportunityfor males to participate in extra-pair matings exists. Despite their higher density, allowing males potentially to interact with several females, polygyny in O. aunfrons may be prohibited or greatly restricted due to the the time required to brood the eggs produced by a single female. Feeding rates are dramatically reduced in parental males, and in addition to the development time of the current brood, a recovery period may be required after the current clutch has hatched before the male is able to undertake another relative fast while brooding another clutch. In other fishes, parental males have reduced feeding rates and are in poorer condition after completing a bout of parental duty (DeMartini, 1987; Cote and Hunte, 1989; Petersen, 1990; Robertson et aI., 1990; Petersen and Hess, 1991). There may not be enough time for a male to successfully brood the clutches produced by more than one female. Barlow (1984) included as conditions that may lead to the evolution of monogamy in fishes the need for biparental care offree-swimming fry and ecological factors such as advantages arising from the joint defense of resources or low population density. Egg-bearing by the male parent may represent an additional factor favoring the evolution of monogamy in fishes (Gronell, 1984; this study). Non-monogamous Mating Systems in Opistognathus.- The most common mating group in O. macrognathus consists of one or two males and two to four females. Thus, spawning synchrony in this species does not enforce monogamy as in O. maxillosus. Spawning in O. macrognathus is less synchronous than in O. maxillosus, and development time of the eggs is shorter in O. macrognathus (5 days vs. 7-8 days), making sequential care of clutches possible, in spite of a significant spawning peak. If male O. macrognathus are able to mate with several females, it is unclear why monogamous pairs exist on populated reefs where males may have additional opportunities to mate. When pairs are isolated, monogamy is the only possibility, but monogamous pairs also exist amid larger mating groups. Proximity to females seems to be the most important factor affecting male mating success in this species. Males that do not neighbor a female are never seen brooding eggs; males near a single female receive eggs at a rate that suggest they have one mate; males near several females receive more clutches. Behavioral interactions between individuals were never seen, and it is impossible to know whether female mate choice, male aggressive interactions, or both determine mating partners within populations. The mating system of O. whitehursti appears to be polygynous or promiscuous. However, the long development time of the eggs limits the rate at which males can receive sequential clutches. A male that received four clutches in a lunar month spent 32 of 36 consecutive days providing care for those clutches. There was not enough time in this period for an additional clutch to have completed development; thus, the mating success ofthis male was at the upper limit imposed by the developmental period of the clutches he was brooding. Compared with fishes where the male guards demersal eggs, male mouthbrooding probably sets a more restrictive upper limit on the reproductive success of the most successful males in all of the jawfishes in this study. Sexual Dimorphism and Egg Development Rate. -In addition to constraining male mating success and influencing the mating systems in Opistognathus, male HESS: JAW FISH MATING SYSTEMS 817 mouthbrooding may also affect the evolution of sexual dimorphism and developmental rates of embryos of these species. An egg mass developing in a confined space such as the male's mouth probably experiences reduced water flow, and embryos, especially in the center of the egg mass, may be subject to hypoxia, which may result in higher mortality or retarded development (Strathmann and Chaffee, 1984; Giorgi and Congleton, 1984). Males are able to agitate the egg mass by partially spitting out the ball of eggs(Thresher, 1984), which presumably flushes water through the mass, but the egg mass lies quietly within the male's mouth for the vast majority of the time and may be subject to limited ventilation. A larger male mouth may alleviate some of these problems of ventilation. If the eggs are not packed so tightly in the male's mouth, flow through and around the egg mass will be less restricted and ventilation of the embryos more thorough. Because of the advantages of enhanced ventilation of embryos, there should be both natural and sexual selection favoring male mouths that are large relative to the egg mass being brooded. There is some degree of sexual dimorphism in all four jawfish species studied that results in larger male mouths and may enhance ventilation of the brood (Table 2). The ability of the male to adequately ventilate the eggs, which will depend in part on the size of the male's mouth relative to the size of the clutch, may have a role in determining development time of mouthbrooded eggs. The factors that usually explain differences in egg development time, egg size and temperature during development, are not adequate to explain the pattern found in these jawfish species, which opposes the pattern predicted from eggsize and temperature alone. Species with smaller eggstypically have shorter development times (Steele, 1977), but O. whitehursti had the longest development time of the species in this study in spite ofa small egg. Eggstypically develop more quickly at higher temperatures (Hempel, 1980; however, O. macrognathus, which occurred in the deepest water and experienced the lowest temperatures, had the most rapid development. Rapid development may occur in O. macrognathus due to male mouths that are large relative to the clutches they hold, and the differences in developmental period among jawfishes may reflect different amounts of ventilation provided by males in each species.

ACKNOWLEDGMENTS

This work was supponed by a Smithsonian Graduate Fellowship and a National Science Foundation Predoctoral Fellowship. I thank the Smithsonian Tropical Research Institute for logistical suppon, D. R. Robenson and C. W. Petersen for advice and assistance in the field, and the Kuna Indians and the Republic of Panama for permission to work in the San BIas. C. W. Petersen, P. L. Colin, and an anonymous reviewer made suggestions that improved this manuscript.

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DATEACCEPTED: August 6, 1992.

ADDRESS: Department of Zoology. NJ-15. University of Washington. Seattle. Washington 98195; PREsENTADDRESS: College of the At/antic. 105 Eden Street. Bar Harbor, Maine 04609.