Advancing marine conservation
They are commonly also found in estuaries where the water may be quite brackish or even experience intermittent freshwater floods. Seahorses are sparsely distributed in most populations, found at 0.06 per m2 or one seahorse per 16.67 m2, for example. They do, however, occur in higher densities at particular times and places, especially in lagoons. The highest density of seahorses – up to 0.66 per m2 (one seahorse every 155 m2) – has been found in a tidal lake in the Bahamas that is isolated from the ocean.1 A lagoon in Portugal also had high densities of about 1.5 per m2 when first studied in the early 2002, although densities had declined by 94% by 2008.
Seahorses occupy a huge diversity of shallow water habitats: seagrasses, mangroves, corals, sponges, seaweeds, and shallow dips in mud and sandy bottoms.
Most species settle down onto the bottom, anchored to a holdfast of an emergent plant, animal or seaweed. Some pygmy seahorses are found only in association with gorgonian corals or sea fans and will spend their entire life on a single sea fan. In contrast, at least four species have been encountered several kilometres offshore and have been recorded at depths greater than 100 m. The discovery of seahorses in the stomachs of open ocean predators – e.g. yellowfin tuna, trevally and dolphinfish – confirms that some might even occur far offshore.
Most seahorses are very camouflaged, matching their habitats closely. They change colours to blend into their background – even including matching fluorescent survey tape – and grow skin filaments to hide even better. By using their prehensile tail to grip a holdfast tightly and staying immobile, they become almost invisible to the casual (and even the determined) seeker. Seahorses also brighten during elaborate pair bond and courtship displays but these short term colour changes serve to make the seahorses more visible. Speaking of brightening, recent work has been exploring how biofluorescence in seahorses can be used to spot them at night.2,3
Some seahorse species engage in migration, moving long distances, for at least three reasons: juveniles move to adult habitats; adults move on a seasonal basis; and seahorses go rafting. With respect to rafting, seahorses from a number of species have been found tucked into sargassum and other seaweed clumps, drifting long distances. Given that such travel creates the potential for long distance dispersal, much more needs to be learned about migrations and rafting.
[Updated 19 May 2021]