Abstract
The pygmy seahorse Hippocampusjapapigusp. n. is described based on three specimens, 13.9–16.3 mm SL, collected from a mixed soft coral and algae reef at 11 m depth at Hachijo-jima Island, Izu Islands, Japan. The new taxon shares morphological synapomorphies with the previously described central Indo-Pacific pygmy seahorses, H.colemani, H.pontohi, H.satomiae, and H.waleananus, including extremely small size, 12 trunk rings, strongly raised continuous cleithral ring, snout spine, large spine on the eighth lateral and fifth and 12 superior trunk ridges, respectively, and unusual wing-like-protrusions immediately posterior to the head. Hippocampusjapapigusp. n. can be distinguished from all congeners by the following combination of features in the anterodorsal area of the trunk: bilaterally paired wing-like protrusions formed by a single pair of large, truncate spines projecting dorsolaterad on the first superior trunk ridge, followed by a unique elevated dorsal ridge formed by triangular bony mounds dorsally on the second to fourth superior trunk ridges. In contrast, H.pontohi possesses a pair of large truncate spines projecting strongly laterad on both the first and second superior trunk ridges followed by flat surfaces dorsally on the third and fourth superior trunk rings. The new species can be further differentiated by genetic divergence from H.pontohi (an uncorrected p-distance of 10.1% in the mitochondrial COI gene) and a striking reticulated white and brown lattice pattern on the head, trunk, and tail. Hippocampusjapapigusp. n. represents the fifth species of pygmy seahorse recorded in Japan.
Highlights
The family Syngnathidae contains 57 valid genera and 300 described predominantly small-bodied and cryptic marine species (Dawson 1985; Froese and Pauly 2018), including the seahorses, pipefishes, pipehorses, and seadragons
We consider Hippocampus japapigu as a valid species due to its morphological uniqueness; a more detailed phylogenetic and systematic study is necessary to understand its evolutionary relationship to other pygmy seahorses
Using μCT, we have identified key diagnostic characters in the anterodorsal area of the trunk that differentiate H. japapigu from the morphologically similar H. pontohi
Summary
The family Syngnathidae contains 57 valid genera and 300 described predominantly small-bodied and cryptic marine species (Dawson 1985; Froese and Pauly 2018), including the seahorses, pipefishes, pipehorses, and seadragons. We recognize the current taxonomic status of H. waleananus as valid based on differences in diagnostic morphological characters, including tail ring counts, coronet profile, and body ornamentation (Tables 1, 3), and host association and diurnal versus nocturnal behavior. Another described pygmy species, Hippocampus severnsi (Lourie & Kuiter, 2008), was distinguished from H. pontohi primarily by features of coloration in life, the invalidity of this species was subsequently recognised as coloration is not a reliable morphological diagnostic character in seahorses (Lourie et al 2004, 2008, 2016). We can confirm the presence of a 7th species of pygmy seahorse that we hereby describe as the new species Hippocampus japapigu, so far only found in subtropical southeast Japan
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