Rearing seahorses trigger powerful gulps

If you're not such a great swimmer, then it's best just to hang on and sit tight in the current, which is exactly what seahorses do. The enigmatic creatures, with only a rippling dorsal fin for propulsion, twist their prehensile tails around coral polyps and sea grass for security. And Corrine Avidan from Tel Aviv University, Israel, explains that having lost their powerful tail fins, the ethereal fish no longer retain the forceful swimming muscles that also drive the explosive slurps that allow other fish to gulp down morsels. Yet long-snouted seahorses still manage to swallow strongly, tucking their heads down to store energy in elastic tendons, before rearing their heads back, releasing the energy to generate the powerful suction required to vacuum up dinner. ‘I wanted to understand how these unique fish with tiny mouths at the tip of skinny snouts could feed on some of the fastest prey in the oceans’, says Avidan, who teamed up with Roi Holzman, working at the Inter-University Institute for Marine Sciences in Eilat, Israel, to find out how the fish's unconventionally shaped heads impact their dining style.

‘I chose three species that are common to the Red Sea, easily accessible and differ significantly in nose length’, says Avidan, who trained Jayakar's seahorses (Hippocampus jayakari), sea ponies (Hippocampus fuscus) and short-snouted seahorses (Hippocampus hippocampus) to peck at tiny live shrimp suspended in a sheet of laser light while she filmed the animals feeding. ‘These seahorses have very different snout lengths ranging from 5 to 9 mm, more than twice the size of their heads’, she says. Then Avidan compared how the suction of the three seahorse species performed relative to other fish that depend on powerful swimming muscles to drive their strong sucks.

Impressively, the tiny seahorses were able to suck eight times more strongly (for their size) than the conventional muscle powered fish, expanding their mouths in just 2.5 ms; the fish took up to 17 times longer (41 ms) to fling their jaws wide. And the seahorses’ sucks reached maximum strength within 2.1 ms, compared with the other fish, whose gulps only reached top speed within 41 ms. ‘The ability of seahorses to produce fast suction is truly extraordinary’, says Avidan. However, when she compared the strength and speed of the seahorses’ sucks, the Jayakar's seahorses with the longest (∼9 mm) snouts produced the slowest sucks (8 cm s^–1) while the short-snouted seahorses (∼5 mm) produced a slurp that was twice as fast, even though they only threw their heads back at ∼6560 deg s^–1. Their long-snouted seahorse cousins, in contrast, reared their heads at a blisteringly fast ∼10,200 deg s^–1.

‘Seahorses feed in a way that allows them to move their heads and suck in their food eight times faster than most other fish their size’, says Avidan, adding that the seahorses’ alternative elastic-powered mechanism for generating a powerful suck is a definite advantage for the dainty creatures. She and Holzman also suspect that the shorter-snouted seahorses with the most powerful suck depend on camouflage to disguise their presence, as they can't move their snouts to ambush shrimp prey as fast as the long-snouted Jayakar's seahorses. ‘The challenges of this feeding style favour the extremes’, says Avidan, adding, ‘this is exciting because this variation – long snouts for ambushing from a distance and short snouts for ambushing from camouflage – is what we see in nature’.