Australian scientists have found that seahorses evolved their upright posture about 25 million years ago, in response to an expansion of seagrass habitats.
Peter Teske and Luciano Beheregaray report the results of their study in the latest issue of the journal Biology Letters.
The authors studied the molecular phylogeny of the seahorse family (Syngnathidae) using five nuclear-gene fragments.
Among the species they studied were representatives of a group known as pygmy pipehorses (containing the genera Acentronura, Amphelikturus and Idiotropiscis).
Pygmy pipehorses look very similar to seahorses, but lack the upright posture. According to the authors, This suggests that they could be a surviving evolutionary link between seahorses and the remaining members of the family Syngnathidae, all of which have a horizontal posture.
The phylogeny obtained by the authors confirmed that the pygmy pipehorses are the closest living relatives of seahorses. Using a relaxed molecular clock technique, the authors date this split to about 25"28 million years ago (the late Oligocene).
This coincided with the formation of vast areas of shallow water and expansion of seagrass habitats in Australasia (which is where seahorses first evolved as Teske had earlier shown).
An upright posture in seahorses enables them to manoeuver exceptionally well in seagrass beds. The seagrass beds also provided good camouflage for the seahorses upright bodies, and in that way afforded them both protection from predators and an improved ability to ambush prey. Pygmy pipehorses would not have benefited from the seagrass radiation, and for that reason probably remained restricted to the macroalgal reefs in which they still occur today.
For more information, see the paper: Teske, PR and LB Beheregaray (2009) Evolution of seahorses upright posture was linked to Oligocene expansion of seagrass habitats. Biology Letters 5, pp. 521"523.