Seagrass and seahorses
WHILST snorkelling off a South Devon beach in England this summer, I came across a vast meadow of eelgrass. This plant (Zostera marina) is found in swathes in warmer waters and is an angiosperm, or flowering plant, that produces pollen. It is the only pollinating plant that can survive in seawater. Eelgrass tends to prefer sheltered, sandy coves shallower than 10 metres so that it may reach up to sunlight to photosynthesise. Amongst the strands of eelgrass, I glimpsed several small seahorses.
This grass is common in the tropical waters of the Caribbean Sea and in tropical Pacific and Indian Ocean waters. In summertime, because of the effects of the warm ocean current, known as the Gulf Stream or North Atlantic Drift (which emanates from the Gulf of Mexico), sweeping across the western shores of the UK from the Isles of Scilly in the south to the Shetland Islands in the north and around Ireland, ideal conditions exist for eelgrass to spread. A branch of this current sweeps up the English Channel, washing beaches in Southern England.
Eelgrass provides a sanctuary for many fish species and an ‘ecosystem service’ to 75 per cent of the world’s commercial fisheries. With a complex rooting system, it binds sand particles together, thus reducing coastal erosion and helping to slow down the longshore drift of sand. Its swaying canopy also acts as a buffer to wave action but in severe storms it can get uprooted and may be seen stranded on beaches at the high tide mark. It is, in fact, capable of most efficiently storing 35 times more atmospheric carbon than tropical rainforests! Thus, it is an effective natural reduction agent of the amount of carbon dioxide in our atmosphere. Moreover, it is a veritable paddock for seahorses and pipefish. Seahorses The taxonomic name for this species of fish is ‘Hippocampus’, coming from the Greek ‘hippo’ (horse) and ‘campus’ meaning monster or caterpillar. In Greek mythology they were considered to be the progeny of giant creatures with horses’ heads and fish tails that pulled the chariot of the underwater god, Poseidon. Mainly located in shallow tropical and temperate waters from about 45 degrees south to 45 degrees north of the Equator, seahorses are found not only in seagrass meadows but also in estuaries, on coral reefs and in mangrove swamps.
Generally, male seahorses are very territorial and protect their 0.5 square metre territory, whilst the females roam in an area of 1.4 square metres. In Malaysia, the species Hippocampus kuda, also known as kuda laut, may be found in mangrove swamps.
To date, worldwide, 54 species of seahorse have been identified and it is thought that there may be as many subspecies. Around the British Isles, two species are found – the Spiny seahorse (Hippocampus guttulatus) and the Short Snouted seahorse (Hippocampus hippocampus). The latter may also be found in abundance in the Canary Islands and around the Italian coastline. Since 2008, these fish have been observed in the River Thames estuary in England, suggesting that the quality of this river’s discharge water has dramatically improved. All seahorses in UK waters are protected by the Wildlife and Countryside Act 1981. Sizes and shapes The largest seahorse species are 36 centimetres in length, whilst the smallest measures only two centimetres. With horse-like heads and prehensile snake-like tails, they breathe through gills and possess a swim bladder. Distinctive features are their long necks and thin snouts which allow them to probe into crevices for food. They suck up food from their snouts in a vacuum cleaner-like fashion, having first disintegrated the food for they have no teeth and thus cannot chew. Feeding mainly on small crustaceans and especially on Mysid shrimps, an adult seahorse eats about 30 to 50 times a day and each of their fry (babies) may eat up to 3,000 pieces of food each day.
With excellent eyesight, their eyes work independently, enabling them to look forwards and backwards at the same time, thus making them formidable hunters. To communicate with each other, they make a clicking sound. Somewhat chameleonlike, they change readily change their colouration to blend in with their surroundings or also when in courtship.
Distinct from most other fish species, because of their exoskeleton, their bodies are composed of hard, bony, ring-like plates which are fused together with a fleshy covering. Their prehensile tails make a firm grip onto eelgrass that prevents them being washed away by strong currents. Alas, they are poor swimmers, relying on their dorsal fin beating at 30 to 70 times a second for propulsion and by using their pectoral fins on either side of their heads as stabilisers, as well as for steering. Courtship and procreation Some species pair for life, renewing their bond early in the morning in the male’s territory. They then set out on a courtship display by changing colours. The male circles around the female and they interlock their tails in unison before wheeling around for up to an hour before the female retreats to her territory. The male seahorse is the only creature to have a reversed pregnancy, for he opens a bodily pouch in which the female transfers her eggs and then he self-fertilises them. The eggs are embodied in the pouch wall and are surrounded by a spongelike tissue which allows the male to supply them with prolactin. This hormone is responsible for milk production in mammals.
With gestation times in most species of seahorse varying from two to four weeks, the number of small fry released, when the male gives birth at night, varies from 100 to 1,000 or even more. Fewer than 0.5 per cent of these will reach adulthood for they are susceptible to both predators and ocean currents sweeping them away from the feeding grounds in which they were born. Threats There is a distinct lack of data on the size of the various seahorse species, yet some of these may be extinct. Commercial overfishing by trawl nets and ships dragging their anchors over coral reefs and seagrass meadows are constant threats to these peculiar creatures in tropical seas. In both temperate and tropical climes, seagrass beds are threatened by agricultural runoff from the land containing pesticides, herbicides and phosphates.
Many Indonesians and Central Filipinos eat seahorses as a delicacy, where, on hawker stalls, cooked seahorses are sold on sticks like kebabs. In Asia, dried seahorse fetches a ridiculous price per kilogramme, more than silver and just below gold.
The traditional Chinese medicine trade takes over 150 million seahorses a year for manufacturing into pills alleging to combat impotence, wheezing and all sorts of pain. The worldwide pet trade consumes a million seahorses a year for aquaria with fewer than a thousand seahorses surviving longer than six weeks.
In Asian and Mediterranean countries, I have seen dried seahorses sold as souvenirs to tourists – this trade takes another million of these fascinating fish each year. I pray that these delicate, vertically swimming fish which have graced our oceans for 23 million years remain undisturbed. They offer no threats to mankind but we increasingly continue to threaten them and their seagrass, mangrove and coral reef habitats.