Tetsu Sato

Size‐Dependent Male Alternative Reproductive Tactics in the Shell‐Brooding Cichlid Fish Lamprologus callipterus in Lake Tanganyika

Reproductive parasitism among males is prevalent in fishes. Typically, small ripe males parasitize the reproductive effort of large bourgeois males by using various behavioural tactics. We examined the size-dependent advantages of parasitic behavioural tactics in a shell-brooding cichlid fish of Lake Tanganyika with three male types (large bourgeois males and medium–dwarf parasitic males). The extremely small dwarf males weighing only 2.5% on average of large males, perform a specialized tactic in which they avoid attacks by the resident large males by entering the inner part of the whorl of the shell where a female is spawning. Field observations and a manipulation experiment revealed that the very small size of dwarf males is essential for utilizing this positional advantage. Larger dwarf males and medium males opportunistically adopt darting and sneaking which likely result in very low reproductive outcomes. The size associated advantage and disadvantage of parasitic tactics are the major factors shaping the size distribution of ripe males in this species. The success of parasitic spawning by dwarf males is determined not only by body size of the males, but also by the relative sizes of females and shells in which they spawn. These factors would affect the choice of different tactics among dwarf males. The analyses of body condition among ripe and unripe males across a wide range of body sizes suggested that onset of sexual activities at very small body sizes in dwarf males may be associated with higher condition factors that is a pre-requisite for maintaining investment in testes and intense reproductive activities.

Population structure of two closely related pelagic cichlids in Lake Victoria, Haplochromis pyrrhocephalus and H. laparogramma.

Cichlid fishes in Lake Victoria show spectacular diversification that is thought to be recent. Therefore, by investigating those fishes, we may be able to elucidate recently completed or ongoing speciation processes. We studied the population structures of two closely related pelagic cichlid species, Haplochromis pyrrhocephalus and H. laparogramma, using a mitochondrial DNA locus and 12 nuclear microsatellite loci as putative neutral markers. Ten and two populations of H. pyrrhocephalus and H. laparogramma, respectively, were sampled from the southern part of Lake Victoria. We grouped those 12 populations into four mutually differentiated regional populations, one of which consisted of the two H. laparogramma populations. The levels of differentiation were substantial at the mitochondrial locus (F(ST) = 0.03-0.54), but very low at microsatellite loci (R(ST) = 0.008-0.116). The data from both types of loci indicated that the regional population of H. laparogramma was first separated from those of H. pyrrhocephalus if we set aside one erratic population of H. pyrrhocephalus. The data also suggested recent population expansions of the two species, the time scales for which were estimated to be on the order of 10(4)-10(5) years. These data suggested that dynamic speciation processes accompanied occasional spawning of new species and population size changes in this lake.

Landmark-based morphometric analysis of the body shape of two sympatric species, Ctenopharynx pictus and Otopharynx sp. “heterodon nankhumba” (Teleostei: Cichlidae), from Lake Malawi

Abstract Morphological differences in body shape of two sympatric benthophagous cichlid species from Lake Malawi, Ctenopharynx pictus and an undescribed species, Otopharynx sp. “heterodon nankhumba,” were investigated using geometric morphometric methods. From digitized data of landmark points on lateral profiles of fishes, the shape of each species was compared by the thin-plate spline method. Statistical analyses revealed significant variation in both uniform and nonuniform components of shape between the two species. From the splines generated, it was revealed that most of the significant variation between the two species occurs in the head region. Specifically, C. pictus has a longer and deeper head than Otopharynx sp. In addition, the mouth of C. pictus is larger than that of Otopharynx sp. In the trunk region, C. pictus has a shorter abdominal cavity, which may indicate possession of shorter intestines than Otopharynx sp. The variation in gross head morphology and intestinal length may reflect interspecific differences in trophic ecology, possibly facilitating the coexistence of the two species through resource partitioning.

Unusual allometry for sexual size dimorphism in a cichlid where males are extremely larger than females

When males are the larger sex, a positive allometric relationship between male and female sizes is often found across populations of a single species (i.e. Rensch’s rule). This pattern is typically explained by a sexual selection pressure on males. Here, we report that the allometric relationship was negative across populations of a shell-brooding cichlid fish Lamprologus callipterus, although males are extremely larger than females. Male L. callipterus collect and defend empty snail shells in each of which a female breeds. We found that, across six populations, male and female sizes are positively correlated with not only sexual and fecundity selection indices, but also with shell sizes. Given their different reproductive behaviours, these correlations mean that males are required to be more powerful, and thus larger, to transport larger shells, while female bodies are reduced to the shell size to enable them to enter the shells. Among the three size selections (sexual selection, fecundity selection and shell size), shell size explained the allometry, suggesting that females are more strongly subject to size selection associated with shell size availability than males. However, the allometry was violated when considering an additional population where size-selection regimes of males differed from that of other populations. Therefore, sexual size allometry will be violated by body size divergence induced by multiple selection regimes.

Prominent ornaments and rapid color change: use of horns as a social and reproductive signal in unicornfish (Acanthuridae: Naso )

The behavior of two unicornfishes, Naso unicornis with a horn-shaped protuberance on the forehead and Naso vlamingii with a round protuberance, was observed in social and reproductive contexts in an aquarium. Males of both species performed displays that were associated with quick changes in the colors of the protuberance and other body parts, highlighting the protuberance by color contrast. The displays with color changes of the protuberance took place when a male courted a female in the evening or in the night. The same displays were occasionally performed by males toward females throughout the daylight hours. In N. vlamingii, dominant males displayed the protuberance toward subordinate smaller males. Although the sizes and shapes of the protuberance were sexually monomorphic, females of both species rarely made displays of the protuberance. A hypothesis about the function of the protuberance in unicornfishes is proposed: that males use the protuberance as a conspicuous signal in courtship and contests among males by emphasizing it with quick changes of its color contrasts. We argue that the combination of morphologically distinct protuberances and quick changes of their color is a sexually selected trait among unicornfishes, because these characters play important roles in intersexual and intrasexual interactions.

Why are reproductively parasitic fish males so small?—influence of tactic-specific selection

Despite the wide prevalence of alternative reproductive tactics, little attention has been paid to why reproductively parasitic males are so small. In this study, we tackled this issue in a shell-brooding fish Lamprologus callipterus. Sneaky ‘dwarf males’ of this fish remain much smaller than bourgeois conspecifics throughout their life and employ a unique parasitic tactic, i.e. entering into a gastropod shell where a female is spawning, passing through the space between the female and shell wall and staying behind her to ejaculate throughout the spawning event. Here, we tested the prediction that they remain small to get past her through the shell spaces by interpopulation comparison. We showed, across populations, a negative allometry for sexual size dimorphism, an exponential increase of female size with an increase in shell size and a negative correlation between the magnitude of sexual size dimorphism and shell size. These results suggest that the inner spaces strongly regulate dwarf male size. We conclude that the small bodies of dwarf males arise from adaptation to their unique reproductive behaviour.

Genetic Structure of Pelagic and Littoral Cichlid Fishes from Lake Victoria

The approximately 700 species of cichlids found in Lake Victoria in East Africa are thought to have evolved over a short period of time, and they represent one of the largest known examples of adaptive radiation. To understand the processes that are driving this spectacular radiation, we must determine the present genetic structure of these species and elucidate how this structure relates to the ecological conditions that caused their adaptation. We analyzed the genetic structure of two pelagic and seven littoral species sampled from the southeast area of Lake Victoria using sequences from the mtDNA control region and 12 microsatellite loci as markers. Using a Bayesian model-based clustering method to analyze the microsatellite data, we separated these nine species into four groups: one group composed of pelagic species and another three groups composed mainly of rocky-shore species. Furthermore, we found significant levels of genetic variation between species within each group at both marker loci using analysis of molecular variance (AMOVA), although the nine species often shared mtDNA haplotypes. We also found significant levels of genetic variation between populations within species. These results suggest that initial groupings, some of which appear to have been related to habitat differences, as well as divergence between species within groups took place among the cichlid species of Lake Victoria.

Parker’s sneak-guard model revisited: why do reproductively parasitic males heavily invest in testes?

Alternative reproductive tactics are widespread in males and may cause intraspecific differences in testes investment. Parker’s sneak-guard model predicts that sneaker males, who mate under sperm competition risk, invest in testes relatively more than bourgeois conspecifics that have lower risk. Given that sneakers are much smaller than bourgeois males, sneakers may increase testes investment to overcome their limited sperm productivity because of their small body sizes. In this study, we examined the mechanism that mediates differential testes investment across tactics in the Lake Tanganyika cichlid fish Lamprologus callipterus. In the Rumonge population of Burundi, bourgeois males are small compared with those in other populations and have a body size close to sneaky dwarf males. Therefore, if differences in relative testis investment depend on sperm competition, the rank order of relative testis investment should be dwarf males > bourgeois males in Rumonge = bourgeois males in the other populations. If differences in relative testis investment depend on body size, the rank order of relative testes investment should be dwarf males > bourgeois males in Rumonge > bourgeois males in the other populations. Comparisons of relative testis investment among the three male groups supported the role of sperm competition, as predicted by the sneak-guard model. Nevertheless, the effects of absolute body size on testes investment should be considered to understand the mechanisms underlying intraspecific variation in testes investment caused by alternative reproductive tactics.

Comparative Morphometrics and Associated Growth Trends of Two Benthophagous Cichlid Species from Lake Malawi (Pisces, Perciformes)

Abstract Body shape differences between two benthophagous, sympatric cichlid species, Ctenopharynx pictus and Otopharynx sp. “heterodon nankhumba” were examined for 14 morphometric characters using measurements obtained from 11 morphological landmarks and additional 6 characters from the head region using principal component and regression analyses. The two species, though not closely related, superficially resemble each other and, being benthic feeders, directly compete for food resources unless morphological differences lead to trophic specialization. Hence, the present study explores the presence of such morphological variations. Principal component analysis (PCA), revealed that only four characters loaded highly on PC II and, therefore, account for most of the variations in the external morphology between the two species. C. pictus has longer lower jaws reflecting longer gape, and a shorter abdominal cavity than O. sp. “heterodon nakhumba”. The regression analyses showed significant differences in the allometric coefficients of the lower jaw and abdominal cavity, which might have contributed to differences in overall shape detected between the two species. The difference in gape size between the two species may be related to different feeding habits employed by each species, and such specializations are suggested to reduce interspecific competition for food resources. We suggest that the ensuing ecological separation promotes sympatry of these two species.