Sea horses – strange and fascinating

By Dr Malcolm Francis

Sea horses have long intrigued humans because of their odd shape, bony body and endearing features. Sea horses are fish, though you need to look closely to see the small transparent fins (one on either side of and behind the head, and one in the middle of the back) that are characteristic of fishes. The common New Zealand sea horse is scientifically known as Hippocampus abdominalis, which translates loosely as “horse-like sea monster with a prominent abdomen”. The allusion to a horse’s head with its long snout is clear, and the large abdomen is an obvious feature with an important function as we’ll see later. In Australia, our sea horse is called the big-bellied sea horse to distinguish it from more than 20 other species in their waters. In New Zealand, we have only one sea horse species so we don’t need a fancy name, and simply call it ‘the sea horse’! To Maori, it is manaia, presumably in reference to the supernatural being of that name who wards off enemies and dangers, and is often shown in Maori carvings in a figure-of-eight shape, which is similar to the usual sea horse posture.

Sea horses often face away from a diver, and probably also from potential predators. They are possibly trying to hide their face and snout in order to blend into their background more effectively, and avoid looking like something edible

New Zealand sea horses, like all other sea horse species, are ambush hunters that spend most of their time anchored to something (often seaweed) with their prehensile curled tails, waiting for food to swim by. They clearly aren’t built for speed, but can actually swim quite fast by leaning forward at a 45 degree angle and rapidly beating their dorsal fin (the one on their back). But they prefer to sit tight and blend in with their environment, a feat they achieve extremely well through their extraordinary camouflage. They can change their colour pattern by enlarging or contracting small pigment cells in their skin until they closely mimic their surroundings. But that’s not all – some even grow long tendrils on their head and body to make themselves look even more like their seaweed habitat. This of course makes it very difficult to find sea horses, and you can spend hours looking for them without having any luck. There are some tricks to finding them, but I won’t publicise them as dried and powdered sea horses are highly sought after for traditional Chinese medicine, and populations of sea horses in New Zealand are just too small to withstand any level of fishing.

Sea horses feed mainly on small crustaceans, particularly amphipod shrimps. That long snout functions like a drinking straw with powerful suction to vacuum up unsuspecting amphipods drifting past. They also target small shrimps that live attached to seaweed, and other small animals, including fishes, living in crevices and under cobbles. A single sea horse can ingest hundreds of amphipods per day. They are also capable of dealing with larger shrimps, which they break into smaller pieces by repeated powerful feeding strikes, before sucking them up.

When caught out in the open away from protective shelter, sea horses may curl up on the seabed with the tail protecting the soft abdomen.

Most fishes breed by spawning thousands or even millions of eggs which develop into drifting larvae that spend weeks to months in the open sea before settling back on to a reef. This strategy is high risk, with most of the tiny eggs and larvae being eaten before they settle. Sea horses have an alternative, very un-fish-like strategy: the female deposits her eggs into the male’s large abdominal pouch (yes, that’s why they are big-bellied! and it’s also how mature males and females can be distinguished) and he fertilises and broods them in his pouch for about 30 days. The eggs hatch into little (13-22 mm long), very skinny sea horses which he eventually ejects from his pouch with powerful contractions of his abdomen. The young sea horses spend up to several weeks floating near the sea surface, but by this time they have bypassed the most vulnerable and edible egg and larval stages: the bigger you are, the fewer animals there are which can eat you. Large males may produce broods of up to 720 young, but the average brood size is about 270. In some sea horse species, a male and female may produce a number of broods together over a breeding season that lasts most of the year, suggesting some sort of pair fidelity.

This male sea horse’s single dorsal fin is quite obvious here, though the small pectoral fin behind the gills (which are behind the eye!) is hard to see. The body is armoured with bony rings except for the male’s soft unprotected abdominal pouch. Females have bony rings around the abdomen

The sea horse is obviously not a horse, but it’s not much like a fish either. Altogether a very strange and fascinating animal.