Underwater mysteries: Genes reveal how the seahorse got its snout and why males get pregnant

Seahorses remain an enigma. These mysterious-looking creatures appear to float around coastal seas with a grace and level of perfection that sucks in observers and admirers from all quarters of society. In ancient Greek and Roman times, seahorses were revered. They were once seen as symbols of power and of authority. Even the Latin taxonomic family name stirs the imagination, as Hippocampus translates as the “horse sea monster”.

The reality is that seahorses are part of the teleost group – the most common type of fish – and so are far more closely related to an Atlantic cod than to any horses, monsters or imaginary creatures. But seahorses also have some very unusual features, from their equine appearance and body armour to the fact that it is the males that carry and give birth to offspring. Now a new analysis of all the genes in the seahorse’s DNA (known as its genome) could, for the first time, help us fully understand these rare characteristics and uncover some of the creature’s mysteries.

Seahorses belong to the syngnathids family of fish, which includes approximately 300 species. Analysing the seahorse’s genes confirmed that it is in a sister group of fishes to that of the stickleback. What makes the particular group, which includes seahorses, unique among vertebrates, however, is their “male pregnancy”. The males nourish developing embryos in a brood pouch until hatching and parturition occurs. The male provides all the parental care and so have adaptations to provide the embryos with the right balance of fluids, oxygen and nourishment.

We think this oddity of evolution came about as a way for male seahorses to ensure they were the fathers of their mate’s children. In some species of seahorse, this phenomenon of male “pregnancy” reverses the usual roles of attracting a mate, with females competing for the males’ attention and evolving their own secondary sexual characteristics to do so.

The new research, from the ASTAR institute in Singapore and published in the journal Nature, shows that this unique feature of seahorses is clearly coded into the male genome. The researchers found that five of the genes responsible for hatching embryos that are usually active in most female fishes are very active in male seahorses and may be involved in male pregnancy.

Beacons of hope

Of course, it is not just the paternal role of seahorses that attracts the attention of observers. Seahorses also have a distinct snout structure that leads many to compare them with other larger animals. This snout in fact evolved through the loss of mineralised teeth that led the seahorse’s jaws to fuse into a tube-like structure with a small mouth.

This unique tube is extremely efficient in sucking small food items that are abundant in the water above the seabed. The adaptation is critical in ensuring seahorses maximise their opportunities to catch and gather food. The new research sheds light on the genetic origins of this fusion of the teeth. In particular, the analysis suggests the fused jaw may be due to the loss of the genes responsible for the proteins in tooth enamel.

While this kind of research helps confirm our fascination with seahorses, they are sadly threatened by our over-exploitation of the ocean. As the seas become increasingly altered by human activities, we need unique and novel animal life such as seahorses more than ever to help communicate to the world the importance and value of this watery environment.

In particular, many seahorse species live in seagrass meadows, which are threatened the world over. Communicating the value of these habitats for other less enigmatic species, such as Atlantic Cod, is difficult at best, so seahorses provide a beacon of hope for explaining to the world why we need seagrasses. Being able to truly understand the genetic basis for many of the interesting characteristics of seahorses can only help us explain more fully the seahorse story.

Richard K.F. Unsworth, Research Officer (Marine Ecology), Swansea University.

This article first appeared on The Conversation.