Japanese seahorse (Hippocampus mohnikei)

Classification (WoRMS)

Full classification as recorded by WoRMS: Kingdom: Animalia · Phylum: Chordata · Class: Teleostei · Order: Syngnathiformes · Family: Syngnathidae · Genus: Hippocampus.

Accepted name: Hippocampus mohnikei · Authority: Bleeker, 1853 · Taxonomic status: accepted · AphiaID: 275202.

Synonyms and superseded combinations recorded for this taxon: Hippocampus fasciatus Kaup, 1853; Hippocampus japonicus Kaup, 1856; Hippocampus monckei Bleeker, 1853; Hippocampus monickei Bleeker, 1853; Hippocampus monikei Bleeker, 1853.

Source: World Register of Marine Species (WoRMS) — https://www.marinespecies.org/aphia.php?p=taxdetails&id=275202 (CC BY 4.0). Accessed 2026-09-23.

Biology and life history (FishBase)

Measured values published by FishBase for Hippocampus mohnikei. Only fields the database actually holds for this species are shown; these are database values, not aquarium guidance.

Maximum length: 8 cm · Common length: 5 cm · Depth range: 0–20 m · Habitat zone: demersal · Body shape: other · Vulnerability index: 10 of 100 · Aquarium trade: never/rarely.

FishBase notes: In Zostera seagrass beds; also in estuaries. Length type refers to height (= TL - head length). Ovoviviparous (Ref. 205). The male carries the eggs in a brood pouch which is found under the tail (Ref. 205). Minimum depth reported taken from Ref. 128812.

Source: FishBase species summary (SpecCode 53911) — https://www.fishbase.se/summary/Hippocampus-mohnikei.html (CC BY-NC 3.0 — non-commercial use only). Accessed 2026-09-23.

Distribution and occurrence records (GBIF)

296 georeferenced occurrence records for Hippocampus mohnikei are published through GBIF from 10 countries or marine areas. The map above plots 16 distinct locations from a sample of 296 records, omitting 25 inland records that carry an institution's coordinates rather than a collection site; it shows where the species has been observed and reported, not a modelled range.

Most records by country or area: China (258); Japan (18); Korea, Republic of (4); Chinese Taipei (4); Viet Nam (4); Hong Kong (3); Russian Federation (2); India (1); Malaysia (1); Singapore (1).

Source: GBIF Occurrence Search (taxonKey 5201096) — https://www.gbif.org/occurrence/search?taxon_key=5201096&has_coordinate=true (records CC0 / CC BY); WoRMS distribution records (MarineRegions) — https://www.marinespecies.org/aphia.php?p=taxdetails&id=275202 (CC BY 4.0); Natural Earth 110m basemap — https://www.naturalearthdata.com/ (public domain). Accessed 2026-09-23.

Conservation status (IUCN Red List)

The IUCN Red List assesses Hippocampus mohnikei Bleeker, 1853 as Vulnerable (code VU). No trend, population estimate or threat analysis is given here, because none is available from this source without the Red List API; follow the source link for the full assessment, including its date.

Source: IUCN Red List of Threatened Species (taxon 41005) — https://www.iucnredlist.org/species/41005/0; GBIF IUCN Red List category endpoint (taxonKey 5201096) — https://api.gbif.org/v1/species/5201096/iucnRedListCategory. Accessed 2026-09-23.

Description and natural history (Wikipedia)

H. mohnikei has a planktonic stage in the first six months of life, after which they settle, preferably in Zostera sea grass beds or estuaries, and begin to breed. They are ovoviviparous and the males carry the eggs in a brood pouch. While several seahorse species are used in traditional Chinese medicine, H. mohnikei is not one of them.

The mid-range of H. mohnikei distribution is around 28 N latitude. For many years the only confirmed distribution was around Japan. However, it was recently observed as far south as the Palk Bay coast of southeastern India. It has also recently been positively observed in Kampot, Cambodia. It is also suspected to be in the waters around China, Thailand and Vietnam.

They have a planktonic stage during the first six months of life, when they are between the lengths of 15.1–45.5 mm, in which they drift with other plankton in the water column. As they grow their food preferences shift from smaller items such as Oithona davisae and Penilia avirostris to larger food species. After the planktonic stage, they prefer to settle in Zostera sea grass beds and estuaries.

H. mohnikei reaches a maximum length of 8.0 cm and is usually dark brown all over, but may be mottled. The coronet of H. mohnikei has a low, ridge-like crest. Its spines are low and the body appears laterally flattened. It has double rounded cheek spines and double rounded spines below the eye. Its tail is long in proportion to its body. There is a slight enlargement of the 4th, 7th and sometimes the first trunk rings, as well as a slight enlargement of the 5th, 10th, 14th and sometimes 9th tail rings.

Many seahorse species look similar. If it is not immediately identifiable, specific measurements and counts are used to narrow the possibilities. Following is a table of measurements for H. mohnikei. (If there is a range, it is given in parentheses.)

Similar species to H. mohnikei include crowned seahorse (H. coronatus, which has 10 trunk rings, an extremely high coronet, and greatly expanded spines bordering a short dorsal fin), Shiho's seahorse (H. sindonis, which has 10 trunk rings, fewer tail rings, and a more prominent coronet), and Korean seahorse (H. haema, which has 10 trunk rings, fewer tail rings, a more prominent coronet, and greatly expanded spines bordering a short dorsal fin).

H. mohnikei have completely developed dorsal, anal, and pectoral fins even as juveniles of 6.0 mm long. Juveniles 6.0–26.4 mm total length also have a tiny caudal fin with just two rays. H. mohnikei reaches maturity at 55 mm and can reach a length of 80 mm. They start breeding in the season after birth, at six months to one year. H. mohnikei is ovoviviparous. The male carries the eggs in a brood pouch, which is found under the tail.

Source: Wikipedia: Japanese seahorse — https://en.wikipedia.org/wiki/Japanese_seahorse (CC BY-SA 4.0). Accessed 2026-09-23.