Tomoyuki Kokita

Latitudinal compensation in female reproductive rate of a geographically widespread reef fish

Latitudinal variation in fitness-related traits has often been attributed to local adaptation to climates. In poikilotherms including fishes, lower temperatures and shorter reproductive seasons at high latitudes would be expected to cause a reduction in annual reproductive output of an individual. Theories of latitudinal compensation predict that organisms at high latitudes should evolve compensatory responses for these climatic effects. Therefore, latitudinal compensation in female reproductive rate (egg production rate), that individuals from high latitudes produce eggs at higher rates than those from lower latitudes, is likely to occur. I tested this hypothesis with a latitudinally widespread reef fish Pomacentrus coelestis that is a multiple batch spawner, from three different localities, from temperate to subtropical waters, within Japan. I used common-environment experiments at three different temperatures to compare reproductive capacity among local populations. In the experiments, average inter-spawning intervals were the shortest and average size-specific clutch weight was the heaviest in fish from the most northern locality across all temperatures, showing clear latitudinal clines. Thus, the northern fish can achieve higher reproductive output per unit time both by shortening inter-spawning intervals and increasing size-specific clutch weight. Additionally, faster egg production rate of the northern fish did not result from increased food consumption. This finding suggests that gross egg production efficiency was higher in the northern fish and that northern fish had a superior capacity for reproduction within a season. These results support the prediction that latitudinal compensation occurs in the female reproductive rate of P. coelestis. As the reproductive season of this species decreases drastically with increasing latitude, the observed cline in the reproductive rate must be an adaptive response to the local selective regime, i.e., length of the reproductive season. Such latitudinal compensation in female reproductive rates may be a common pattern in latitudinally widespread fishes.

Pair territoriality in the longnose filefish, Oxymonacanthus longirostris

The longnose filefish,Oxymonacanthus longirostris, usually lives in heterosexual pairs, the male and female swimming together and sharing the same territory. Pair territoriality in the species was examined in detail in relation to sexual differences in territorial defense activities. Rigorous pair territoriality was maintained only during the breeding season, although pairs used their home ranges exclusively to a certain extent, during the non-breeding season. The frequency of aggression against other conspecific pairs in the breeding season was higher than in the non-breeding season. Agonistic interactions appear to be over both mates and food resources, the strict pair territoriality in the breeding season possibly being due to mutual mate guarding. In intraspecific aggressive interactions, males usually led their partner females when attacking intruders. The feeding frequency of males was much lower than that of females in the breeding season. Mate removal experiments indicated that females could not defend their original territories solitarily and their feeding frequency decreased. Conversely, males could defend territories solitarily without a decrease in feeding frequency. These results suggest that males contribute most to the defense of the pair territory, with females benefiting from territorial pair-swimming with their partner males.

Genetic comparison of two color-morphs of Apogon properuptus from southern Japan

The anterior half of the mitochondrial 16S rRNA gene (ca. 650 bp) was compared for two color-morphs (dotted and lined types) of the orange-lined cardinalfish, previously identified as Apogon properuptus. Twelve and nine specimens, respectively, of dotted and lined types were collected from four and three localities along the coasts of southern Japan and the Ryukyu Islands, specimens of both color-morphs being collected from one of these localities on the southern coast of the Shikoku Island. Phylogenetic analyses using maximum parsimony (MP) and neighbor-joining (NJ) methods, with haplotypes of A. semilineatus and A. cyanosoma as an outgroup and comparative OTU, respectively, showed that the haplotypes of each color-morph were reciprocally monophyletic with 100% bootstrap values. These results, together with their distinct coloration and partly overlapping geographical ranges, indicated that the two color-morphs of A. properuptus from Japanese waters represent two distinct species.

Male secondary sexual traits are hydrodynamic devices for enhancing swimming performance in a monogamous filefish Paramonacanthus japonicus

The Japanese filefish Paramonacanthus japonicus has extreme sexual dimorphism in its overall shape, even though its mating system is monogamy with biparental care. This sexual dimorphism is mainly due to the development of secondary sexual traits in males. Males become more slender in body with elevated soft dorsal and anal fins as they mature. We examined the function of such male secondary sexual traits by field research and fluid-dynamic analysis. Underwater observations showed that movement rate and steady swimming speed of males were higher than those of females. Male and female P. japonicus showed similar feeding habits and egg-tending behavior, although males attacked potential egg predators more frequently. A wind-tunnel experiment using the air bearing and spring system showed that the drag coefficient of males was significantly lower than that of females, indicating a lower male hydrodynamic drag performance. Also, male elevated soft dorsal and anal fins are considered to give rise to higher thrust performance in monacanthids. Thus, these results suggest that male secondary sexual traits are hydrodynamic devices for enhancing swimming performance that seem to be actually functional under natural conditions. We discuss the evolution of such conspicuous male sexual traits in P. japonicus.

Centroid sizes of the samples from natural populations

Centroid sizes of the samples from the natural populations in NTS format. The numbers and the population codes (see supplementary Table S1) of the analysed populations are: 11 E1T, 20 E1S, 34 E1D, 32 E1M, 8 E2H, 18 E2R, 10 E2K, 6 E2I, 14 E3K, 10 E3I, 3 S, 28 CM, and 12 CN, in this order.