Masanori Kohda

Social system and reproduction of helpers in a cooperatively breeding cichlid fish (Julidochromis ornatus) in Lake Tanganyika: field observations and parentage analyses

It is suggested that some fish of the genus Julidochromis, substrate-brooding Tanganyikan cichlids with biparental care, breed cooperatively with helpers. We studied the social system of J. ornatus in the wild and analysed genetic parentage using microsatellites. Within the studied population three patterns of social system were identified: monogamous pairs (61%, 44 of 72 groups), pairs with helpers (29%, 21), and polygamous harems with helpers (controlled by either a large female or large male owner; 10%, 7). In cooperatively breeding groups, the number of helpers at each nest ranged from 1 to 6 (median 1), and male helpers were more numerous than female helpers. In both sexes, the body size was different among individuals of different social status (harem owner > breeder > helper). Helpers and harem owners of both sexes exhibited brood defence although its frequency was low. The molecular analysis revealed that (1) the helpers were mostly unrelated to dominant breeders, (2) many helpers of both sexes contributed genetically to the next generations, (3) male helpers had high siring success (41% of young in total), and (4) large young unrelated to group members were detected at 30% of observed nests, which may be due to breeder (or helper) replacements and immigration of young. We discuss the implications of these results for understanding the complex social system of this species, especially the low reproductive skew in comparison with other cooperatively breeding cichlids.

A new hypothesis for species coexistence: male–male repulsion promotes coexistence of competing species

We propose a new hypothesis for species coexistence by considering behavioral interactions between individuals. The hypothesis states that repulsive behavior between conspecific males (male–male repulsion) creates space for competing species, which promotes their coexistence. This hypothesis can explain the coexistence of two competing species even when their ecological niches completely overlap in spatially homogeneous environments. In addition, the mechanisms underlying such behavior might play a role in enabling the coexistence of two species immediately after speciation, with little or no niche differentiation, as in the case of cichlid fish communities, for example. Although there is limited evidence supporting this hypothesis, it can nevertheless explain the occurrence of species coexistence and biodiversity, which cannot be explained by previous theories.

Courtship in fast water currents by a male stream goby (Rhinogobius brunneus) communicates the parental quality honestly

The stream goby Rhinogobius sp. DA (dark color type) shows exclusive paternal care of the eggs. Males court females in the stream current and previous field observations suggest that females favor males that perform their courtship display in faster water currents, and that such males may have high parental ability because of good physical condition. To validate these observations we examined female choice under controlled laboratory conditions. Mate choice experiments indicated clearly that females preferred males that courted in the faster currents, whereas neither sexually different morphological traits, such as body size, nor the water current alone were important. The experiments with food supply treatments indicated that only males of high physical condition are able to court in the fast currents. Furthermore, males that courted in the fast currents achieved good egg survival, whereas males that were unable to court in the currents did not, due to cannibalism of their egg clutch. Thus, the maximum speed of the water current in which a male courts should be indicative of his quality and of the subsequent survival of eggs under his care. We conclude that Rhinogobius sp. DA females utilize the male courtship display in the water current as an honest indicator of parental quality.

Inheritance pattern of lateral dimorphism in two cichlids (a scale eater, Perissodus microlepis, and an herbivore, Neolamprologus moorii) in Lake Tanganyika

Abstract Antisymmetry in the direction of the mouth opening, to either the right (“lefty”) or left (“righty”), was documented in the scale-eating cichlid Perissodus microlepis. This study revealed the presence of lefty and righty mouth morphs in the herbivorous cichlid Neolamprologus moorii, although the degree of deviation was not large. Both species are biparental brooders and guard their young. We examined the inheritance pattern of the dimorphism (laterality) using parents and broods of P. microlepis and N. moorii collected in the wild. In P. microlepis, lefty-lefty pairs had a 2:1 frequency of lefty:righty young, lefty-righty pairs a similar number of each type of young, and righty-righty pairs only righty young. Similar inheritance patterns were observed in N. moorii. We propose two hypotheses to explain the inheritance pattern: Mendelian genetics with the lefty allele dominant over the righty and the dominant allele homozygous lethal, and cross-incompatibility that is predominant in lefty homozygotes.

Geographical colour variation in cichlid fishes at the southern end of Lake Tanganyika

SynopsisGeographical colour variation and distribution of 48 common cichlid fish species were studied at 20 sites along an 85 km shoreline at the southern end of Lake Tanganyika, Africa. Sixteen species had two or more colour morphs and 11 species showed a limited distribution in the study area. They were all rock-dwellers. Distributional borders of the color morphs and species with a limited distribution mostly lay in two long sandy beaches, 7 and 13 km long. We suggest that the long sandy beaches can be effective barriers against dispersal of shallow-water rock-dwelling cichlid fishes.

Lateral bias of agonistic responses to mirror images and morphological asymmetry in the Siamese fighting fish (Betta splendens)

Behavioural laterality (e.g., during social interactions) is often observed at the individual level in lower vertebrates such as fish, whereas population-level laterality is observed in many higher vertebrates. Population-level laterality can be explained mainly by internal factors (e.g., cerebral lateralization), whereas little is known about the behavioural mechanisms underlying individual-level laterality. Recently, it was revealed that many fish have asymmetrical body morphology, but the relationship between asymmetric morphology and social behaviours has been rarely examined. Here we report the relationship between lateralized eye use during aggressive displays (e.g., body posture) of male Siamese fighting fish, Betta splendens, toward their own mirror image and morphological asymmetry. Of 25 males, five exhibited significantly more leftward eye use during left displays, and eight males exhibited predominantly rightward eye use during right displays. Morphological measurement results for the craniovertebral angle and opercular area showed that the craniovertebral angle and opercular area displayed antisymmetry and fluctuating asymmetry, respectively. We found that lateralized eye use during agonistic responses by each fish was associated with the craniovertebral angle, but not with operculum size; lefties (left-curved body) showed mainly left eye use (during left-side displays), and righties (right-curved body) demonstrated the opposite. We suggest that antisymmetric morphologies, such as head incline, are potentially useful for studying the association between cerebral lateralization and individual laterality of behavioural responses. Further, we propose that in fish, morphological asymmetry is related to laterality in various behaviours.

Male–male competition for large nests as a determinant of male mating success in a Japanese stream goby, Rhinogobius sp. DA

Abstract Males of the stream goby Rhinogobius sp. DA (dark type) court females in deep pools and care for the eggs under stones in shallow riffles. We studied male–male competition for access to females and nest sites to understand how male size influences the mating success of this species. In field observations, larger males won in fighting with other males. However, large males did not tend to monopolize courtship opportunities, and the frequency of successful courtships, after which males led the females to the nests, was not related to male body size. The fact that courted females always escaped from the fighting sites once males began fighting likely explains why male size was not positively related to courtship success. Large males occupied large nest stones, and the number of eggs received in the nest was correlated positively with nest size. In aquarium experiments with two tiles of different sizes provided as nesting materials, males always chose the larger nest and, when two males were introduced simultaneously, the larger one occupied the larger nest. These results suggested that male mating success of this goby is determined by male–male competition for large nests rather than for access to females.

Is the cichlid fishJulidochromis marlieri polyandrous

mong the vertebrates, the greatest diversity of mating systems is seen in the fishes, where the mating system is influenced greatly by the parental care pattern (Kuwamura, 1988; Barlow, 1991; Keenleyside, 1991). Within Lake Tanganyika, cichlid fishes of the tribe Lamprologini (ca. 60species) are substrate spawners which guard their young (Kuwamura, 1986). Many lamprologine fishes are monogamous and show biparental care, but several species are polygynous with maternal care (Kuwamura, 1986; Yanagisawa, 1987). In this tribe, females are usually smaller than males and are more closely tied to the fry (i.e., free-swimming young) (Kuwamura, 1986; Yanagisawa, 1987; Barlow, 1991; Gashagaza, 1991; Keenleyside, 1991). In some species of the genus Julidochromis (including 5 species), however, females are larger than males (M. Hori, unpubl.). Julidochromis marlieri is one of the small benthic fishes resident in the shallow rocky shores of Lake Tanganyika (Brichard, 1989). It forages mainly on sponges and algae on rock surfaces (Hori et al., 1993). While observing breeding in this fish, we found that monogamous pairs were most common, females being larger than their mates, and that males played a more important role in parental care than females. Our data also suggested that large females may mate with 2 males, although polyandry has not been documented in fishes. The reversed parental roles and mating system ofJ. marlieri are briefly reported herein.

Description of alternative male reproductive tactics in a shell-brooding cichlid, Telmatochromis vittatus, in Lake Tanganyika

Telmatochromis vittatus (Cichlidae) is a Tanganyikan substrate brooder which spawns in the gastropod-shell nests of a cichlid, Lamprologus callipterus. We describe male reproductive tactics of T. vittatus in and around the shell nests, where males of various sizes were found. Based on utilization patterns of the shell nests, interactions among males, and spawning behaviors, males could be categorized into four types based on reproductive tactics and in order of body size: sneaker males, satellite males, territorial males and piracy males. Size range of males in tactic groups rarely overlapped. Territorial males defended shell nests harboring multiple females, but during pair-spawning they were occasionally taken over by large piracy males that visited several nests repeatedly. Small sneaker males darted to pair-spawning territorial males and might ejaculate sperm. Satellite males did not perform parasitic spawning but pair-spawned in a single shell outside the nests. Spawning of satellite males was infrequently parasitized. The largest gonado-somatic index (GSI) was found in sneaker males followed by piracy males, territorial males and satellite males, suggesting that gonadal investment of males using the four tactics may be consistent with intensity or risk of sperm competition.

Asymmetry of Mouth Morph of a Freshwater Goby, Rhinogobius Flumineus

Abstract Morphological analysis of a small freshwater goby, Rhinogobius flumineus, showed a distinct dimorphic asymmetry in the lower jaw. The mouth of each fish opened with a slight but definite distortion toward the right or left side, depending on the individual. Right-opening mouth (dextral) fish had a right lower jaw that was more protruded than the left one, and left-opening (sinistral) fish had a more protruded left lower jaw. No fish had laterally symmetric lower jaws, indicating that the asymmetry was different from ‘fluctuating asymmetry’. These fish used either the right or left side of the mouth when picking up food from the bottom, but neither dextral nor sinistral individuals used one side more frequently than the other side. The mouth asymmetry, however, was related to the stationary posture: dextral fish stayed on the bottom with the line of their bodies curved to the right more frequently than to the left, and vice versa for the sinistral fish. Genetics of the morph phenotype were investigated by observing the frequencies of morphs in F1 under captive breeding. Mouth dimorphism seems to be determined by the Mendelian one-locus-two-alleles system, in which dextrality is dominant over sinistrality and the dominant gene acts as the lethal one when in a homozygote.