Tetsumi Takahashi

Abstract The evolution of sexual dimorphism is widely acknowledged as a manifestation of sex-specific genetic architecture. Although empirical studies suggested that sexual dimorphism evolves as a joint consequence of constraints arising from genetic architecture and sexually divergent selection, it remains unclear whether and how these established microevolutionary processes scale up to the macroevolutionary patterns of sexual dimorphism among taxa. Here, we studied how sexual selection and parental care drive sexual dimorphism in cichlid fishes from Lake Tanganyika. We found that male–male competition, female choice, and maternal mouthbrooding are associated with sexual dimorphism in body length, body colour, and head length, respectively, despite strong allometric relationships between body length and head length. Within-species (static) allometry of head length on body length evolved as sex-specific responses to mouthbrooding, where females evolved higher intercepts while males evolved steeper slopes. Thus, selection to increase mouth size in mouthbrooders may have broken down and reorganized the pattern of allometric constraints that are inherently strong and concordant between sexes. Furthermore, sex-specific responses to mouthbrooding left a remarkably clear signature on the macroevolutionary pattern, resulting in a decoupling of co-evolution in parameters of static allometries between sexes observed exclusively within maternal mouthbrooders. Our study provides multiple lines of evidence that are consistent with the idea that macroevolutionary patterns of sexual dimorphism in Lake Tanganyika cichlids result from sexually divergent selection. Our approach illustrates that an examination of within-population phenotypic variance in the phylogenetic comparative framework may facilitate nuanced understandings of how macroevolutionary patterns are generated by underlying microevolutionary processes.

We conducted an investigation into ghost fishing, a phenomenon in which abandoned fishing equipment remains operational for extended durations, in the southern region of Lake Tanganyika, Republic of Zambia. The perils associated with ghost fishing have achieved international recognition; however, there exists a conspicuous dearth of scholarly investigations in particular regions, notably inland aquatic environments and developing countries. Employing scuba diving for field observations, we discovered that ghost fishing was solely evident in monofilament nylon nets, despite the presence of four different net types in the littoral area of the lake. Notably, endemic crabs Platythelphusa armata and P. maculata from Lake Tanganyika constituted 65.2% of the organisms captured by abandoned nets, with the remainder comprising various fish species. The number of captured organisms exhibited a dependence on the interplay between the length and mesh size of the abandoned nets, indicating that the risk of ghost fishing is heightened when both net and mesh size are substantial. Furthermore, we encountered predatory Spiny Eels Mastacembelus moorii ensnared in nets around their jaws and teeth. Intriguingly, the digestive tract of one of the Spiny Eels contained a partially digested cichlid species entangled in the same nets, strongly suggesting the occurrence of secondary ghost fishing.

A focus on pair bonds between males and females is fundamental to study the evolution of social organization. Because pair bonds are generally identified from direct observations of pairs that maintain physical proximity, pair bonds may have been overlooked in animals that do not exhibit such visible pairs. The Lake Tanganyika cichlid fish <italic>Xenotilapia rotundiventralis</italic> forms schools that consist of mouthbrooding and non-brooding adults in mid-water, and visible pairs are not recognized. A previous study suggested that mouthbrooding females transfer fractions of the young to males when the young become large. However, it remains a mystery whether the mating pairs maintain pair bonds so that the females can transfer the young to their mates. To answer this question, we conducted a parentage analysis using 10 microsatellite markers. The analysis showed that the mouthbrooding adults were most likely genetic fathers and mothers of the young in their mouths. This finding suggests that the female-to-male shift of young takes place between mating partners, and thus the mating pairs maintain pair bonds at least until the shift of young. The present study is the first to detect pair bonds in animals in which physical proximity has not been observed.

Although the cichlid fishes from Lake Tanganyika are treated as a textbook example of adaptive radiation, many taxonomic problems remain unresolved. <italic>Cyathopharynx furcifer</italic>, which belongs to the currently monospecific genus <italic>Cyathopharynx</italic>, contains two colour morphs at the southern end of the lake: one has a yellow anal fin, and the other has a black anal fin. Some books for hobbyists of ornamental fish treat these morphs as different species, but taxonomic studies have neither mentioned the existence nor addressed the status of these colour morphs. In the present paper, we analysed these two colour morphs using mitochondrial, microsatellite, morphometric, and meristic data sets. Both molecular and morphological data allowed clear discrimination between these morphs, suggesting the existence of two distinct sympatric species. Three taxonomic species have been described in this genus, and only <italic>C. furcifer</italic> is currently considered valid. Observations of type specimens of these three nominal species will be needed to determine the scientific names of these colour morphs.

Morphological dimorphism in the mouth-opening direction (‘lefty’ versus ‘righty’) has been documented in several fish species. It has been suggested that this deflection is heritable in a Mendelian one-locus, two-allele fashion. Several population models have demonstrated that lateral dimorphism is maintained by negative frequency-dependent selection, resulting from interactions between predator and prey species. However, other mechanisms for the maintenance of lateral dimorphism have not yet been tested. Here, we found that the scale-eating cichlid fish <italic>Perissodus microlepis</italic> exhibited disassortative mating, in which reproductive pairings between lefties and righties occurred at higher than expected frequency ( <italic>p</italic> &lt;0.001). A previous study reported that a lefty–righty pairing produces a 1 : 1 ratio of lefty : righty young, suggesting that disassortative mating contributes to the maintenance of lateral dimorphism. A combination of disassortative mating and negative frequency-dependent selection may stabilize lateral dimorphism more than would a single mechanism.

Lepidiolamprologus mimicus n. sp. is described from material collected along the Zambian coast of Lake Tanganyika. It is distinguished from congeners by its unique color pattern of bright yellow fins, a brownish-tan flank coloration and large, dark brown spots along the flanks, in addition to a series of meristic and morphometric characters. Lepidiolamprologus mimicus n. sp. exhibits an interesting feeding ecology, in which individuals blend into schools of their prey, yellow-finned cyprichromines, with the aid of similar coloration. This is the first instance of aggressive mimicry reported for lamprologines.