Specimen of the Week 236: The Seahorse Skeleton

1. Familiar when fleshless

Can you name some animals that look more or less the same with or without their skin and flesh on? Those which are instantly recognisable from their skeletons alone?
Crocodiles, penguins and seahorses spring to my mind. Can you think of any more?
What these unrelated animals have in common – and what sets them apart – is that their skin sits directly on bone, at least on the important bodyparts. But this comes about in different ways.

 

2. Aquatic armour

Crocodiles have solid skulls and lack facial muscles. It is the muscles that would normally cause a difference in appearance between the “complete” head and the skull. Penguin wing bones are dorso-laterally flattened and lack flight feathers. Alongside their familiar barrel-shaped torsos, this makes their flippers instantly recognisable. Seahorses go a different way – their bodies are covered in a series of protective bony plates, like an exoskeleton.

 

3. See horse?

Seahorses have these bony plates instead of scales. As well as being covered in skin which can change colour, the structural plates can aid in camouflage. In some species they even have spines, which make them less palatable for predators that manage to overcome the difficulty of actually locating these cryptic fish. It’s also the reason that they have such rigid tails, which are square in cross-section.

 

4. Fish fathers

Seahorse are remarkable in many other ways. Most famously, the males “give birth”. Below the rings of bony plates on his trunk, the male has a brood pouch, into which the female deposits eggs after a prolonged dance-filled courtship. He then fertilises them, and even nourishes them by excreting nutrients into the brood pouch. When they are suitably developed, the father “gives birth” to minute, perfect baby seahorses. In a previous job I once witnessed around 1200 3mm babies being born. It was amazing.

5. Field horse?

Many seahorses have the scientific name Hippocampus, which always confused me. “Hippos” is famously Greek for horse (Hippopotamus translates to “river horse”), which obviously makes sense given their equine faces. I was more confused by the “campus” bit, as campus means field in Latin, and seahorses are essentially the opposite of field horses. However, it turns out that “kampos” means sea monster in Greek, which makes more sense.

 

Jack Ashby is Manager of the Grant Museum of Zoology