Seahorse Anatomy

Written and illustrated by Clare Driscoll


In order to determine the sex and species of a seahorse, it's important to know the something about seahorse anatomy. The following illustrations and descriptions will help you learn the basics of the external anatomy of seahorses.


Part 1 Orientation

Fig. 1Figure 1 shows the areas of a seahorse and the terms used to describe them. The front view of a seahorse, looking into its face or belly, is the ventral orientation. The view from behind the seahorse is its dorsal orientation. The side view, used here in the illustrations, is referred to as the lateral orientation. The lower parts, or the lower section of any parts, are called the posterior, and the upper parts, the anterior.



Part 2 Sections and Measurements

Fig. 2The sections of a seahorse are the head, trunk and tail. The length of a specimen is measured from the first trunk ring to the tip of the tail, or the combined trunk and tail lengths. The head is measured from the tip of the snout to just before the first trunk ring. Snout measurement is made from the tip of the snout to the gill opening.

The depth of the chest is measured from the superior trunk ridge to the keel.



Part 3 Parts of the Body

Fig. 3 Trunk rings, one of the features used to determine the species of the seahorse, are counted from the first (the uppermost ring seen from the dorsal view) to the ring immediately above the anal fin.

Trunk ridges are the vertical spines running down the back of the seahorse from trunk to tip of tail (the superior trunk ridge), the spine running down each side of the seahorse trunk (the lateral trunk ridge), and the spine running along each side of the keel from neck to anal fin (the inferior trunk ridge).

The seahorse's means of propulsion are its pectoral fin, located just behind the gill opening, and its dorsal fin, which joins the trunk at the tail.

Tail rings are counted from the ring just below the anal fin to the ring before the tip of the tail.


Part 4 Parts of the Head

Fig. 4 Features of the head are illustrated at the right. The coronet can be low and fairly smooth, on some species, to tall with pronounced points on others. Eye, nose, and cheek spines also differ in length from species to species, and within a species, from specimen to specimen.

All seahorses have independently orbital eyes, and a pair of pectoral fins immediately behind the gill opening.


Fig. 5Some species of seahorses have spindly appendages, called cirri, in the area of the facial spines and trunk ridges.



Part 5 Sexual Characteristics

Fig. 6 The external anatomy of female and male seahorses differ, a characteristic that is called sexual dimorphism. On the female seahorse, the lower abdomen joins the tail at a sharp angle and her anal fin is often higher and slightly larger.

On males, a brood pouch is found beneath the anal fin, and when empty, tapers gradually to the tail. During courting or when pregnant, the pouch is very pronounced and protruding. It features a vertical opening into which the female deposits her eggs, and from which fry emerge after gestation.

(See also Sexing Seahorses)

Most recent revision: January 12, 2004

Copyright 2004
Clare Driscoll & Seahorse.org
All Rights Reserved


Seahorse fact: Seahorses are masters at camouflage.
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