Why do seahorses swim standing up?

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The fact that seahorses swim in a vertical position is well known, but why do they do it? Rupert Collins has the answer.

Hippocampus spp. are believed to have evolved from their horizontal ancestors, the pipefishes, 25-28 million years ago in Australasia. They have since spread to most temperate and tropical seas.

The familiar vertical posture and prehensile tail is understood to have been adopted in response to increased seagrass habitat available at that time.

This could also be interpreted as better behavioural camouflage from predators and/or to help them become more efficient predators themselves.

Seahorses are capable of rapid colour changes, especially when interacting. They lead a largely sedentary lifestyle as ambush predators of zooplankton, but limited locomotion is provided exclusively by the dorsal and pectoral fins, as the tail and pelvic fins have been greatly reduced.

Densities of H. guttulatus studied in the wild have been shown as low at one seahorse per 14m2, although no territorial defence behaviour has been observed in the genus.

Individual seahorses are reported to have home ranges as small as 4.4m2 for the Australian H. breviceps, with the female’s territory larger than the male’s.

Most seahorses live only a few years.

This item was first published in the December 2009 issue of Practical Fishkeeping. It may not be reproduced without written permission.