Morphological and molecular analyses were conducted on 182 specimens belonging to the Hippocampus
coronatus complex (H.
coronatus sensu lato), collected in Korea and Japan 1933–2015, in order to clarify the taxonomic status of the species within this complex. Three species are recognized based on the shape of the coronet, the number of trunk rings (TrR) and tail rings (TaR), and presence or absence of a wing-tip spine (WS) at the dorsal fin base. Hippocampus
coronatus Temminck & Schlegel, 1850 (H.
coronatus sensu stricto), is diagnosed by 10 TrR, 37–40 TaR, an extremely high coronet (55.7–79.0 % head length) with four tips on the corona flat (CoT), and one WS. Hippocampus
sindonis Jordan & Snyder, 1901 is diagnosed by 10 TrR, 35–38 TaR, a moderately high coronet (36.3–55.4 % HL) with five CoT, and no WS. A new species, H.
haema is described on the basis of 140 specimens, characterized by 10 TrR, 35–38 TaR, a moderately high coronet (34.1–54.9 % head length) with four CoT, and two WS. Hippocampus
haema is only known from the Korea Strait, western Kyushu, and East/Japan Sea. Recognition of the three species is supported by differences in mitochondrial DNA fragments (cytochrome b, 16S rRNA, and 12S rRNA).
Interspecific hybridization experiments were conducted between the common seahorse Hippocampus kuda (male) and the slender seahorse H. reidi (female) during artificial rearing to develop a new aquarium fish with unique polyandrous mating. Molecular analysis via mitochondrial DNA (mtDNA) cytochrome b and nuclear DNA (ncDNA) ribosomal protein S7 gene supported the hybridization between the two species, and the hybrid also showed morphological characteristics of both species. Juveniles of H. kuda have dense melanophores on the whole body or only on the trunk and tail, whereas juveniles of H. reidi have thin melanophores on the whole body or present in stripes only along their prominent trunk and tail rings. However, all the hybrid juveniles had dense melanophores only on the tail, with the striped trunk rings, thus showing an intermediate pattern, and these patterns were limited to the fairly early stage of development (1-10 days old). In contrast, the two eye spines in the hybrid were apparent after 9 days old, which were not inherited from H. kuda (one eye spine), but from H. reidi (two eye spines). According to LOESS (local regression) analysis, the growth rate increased between 20 and 25 days, and the hybrids grew faster than H. kuda when they entered the explosive second phase of growth between 25 and 45 days for all the seahorses. This study highlights the hybridization between H. kuda and H. reidi may contribute to the improved taxonomic information of young seahorses.
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