Background. The longnose seahorse, Hippocampus trimaculatus Leach, 1814, included in the Red List of Threatened Species is the most common and least studied seahorse species along the south-west coast of India. Practically no information is available on numerical taxonomy and sexual dimorphism of H. trimaculatus from India. The purpose of the reported study is to describe the morphometry and meristics of H. trimaculatus occurring along Kerala, south-west coast of India. Materials and methods. A total of 45 specimens of H. trimaculatus, represented by 25 males and 20 females, obtained from trawl by-catch, were studied following standard morphometric and meristics procedures. Statistical data analyses include Student’s t-test, Mann–Whitney U test, and Principal Component Analysis to describe the sexual dimorphism of the species. Results. Male H. trimaculatus are longer than females, with relatively longer tails. The mean values of head depth, snout depth, trunk depth, trunk width, dorsal fin base length, and pectoral fin base length are higher in males, while head length, snout length, coronet height, and trunk length are higher in females. Trunk length and snout depth are the two principal components that determine sexual dimorphism in H. trimaculatus. Conclusion. The numerical taxonomy of H. trimaculatus from Indian coastal waters is reported for the first time. The study shows that sexual dimorphism in H. trimaculatus is reflected in differing morphometric characters between the sexes.

Syngnathidae, longnose seahorse, three-spotted seahorse, low-crowned seahorse, morphology, conservation, sexual dimorphism