Fish
Spinetail Devil Ray
Mobula mobular · also called spinetail devil ray, giant devil ray, flying mobula
The spinetail devil ray is a manta in miniature — a diamond-winged plankton strainer that travels in schools sometimes thousands strong. Its signature behaviour is spectacular and unexplained: it hurls itself several metres into the air and belly-flops back down with a crack that carries across the water.
Overview
Spinetail devil rays live in warm seas worldwide, in the open ocean and along coasts, from the surface to deep water. They are filter feeders, funnelling small fish, krill, and plankton into the mouth with a pair of flexible cephalic fins — the rolled “horns” at the front of the head — and straining it through gill plates. They are large (up to ~3.2 m across) but smaller than the two manta species.
The leaping
Devil rays breach constantly and dramatically — up to 2 metres or more into the air, often somersaulting, and slapping down flat, sometimes a whole school in succession. Proposed reasons include parasite removal, communication or courtship, predator escape, and prey herding; none is confirmed, and it is likely several at once.
Deep divers
Tags show spinetail devil rays diving to 700 metres and beyond, into near-freezing water, apparently to feed in the deep plankton layer, then returning to the warm surface. A network of blood vessels keeps the brain warm during these plunges.
Why it is Endangered
The spinetail devil ray has one of the lowest reproductive rates of any fish — a single pup every 2–3 years — and high mortality. It is caught for its gill plates (dried and sold as a tonic in southern China, the same trade that threatens mantas) and taken in huge numbers as tuna-net bycatch, usually dead on arrival. Populations have dropped 50–80% in parts of its range.
Status
Endangered. All mobulid rays were placed on CITES Appendix II (mantas 2013, devil rays 2016), and the species is protected in the Mediterranean and elsewhere, but bycatch remains the dominant threat.
Spinetail Devil Ray: common questions
What is the difference between a devil ray and a manta ray?
They are close relatives in the same family (Mobulidae) and look similar — flat, diamond-shaped, wing-flapping filter feeders with a pair of "horns" at the front. Mantas are the two largest species, reaching disc widths of 5 to 7 metres, and they have a terminal mouth at the very front of the head. Devil rays (the other Mobula species) are generally smaller, up to around 3 to 5.5 metres depending on species, with a mouth positioned slightly underneath the head, and they tend to be found in larger schools. Both feed on plankton and small fish, both are slow to reproduce, and all mobulid rays are now threatened.
Why do devil rays jump out of the water?
This is one of the open questions in ray biology. Devil rays breach frequently and dramatically, sometimes an entire school taking turns, launching up to a couple of metres into the air, often with a spin, and slamming down flat. Several explanations have been suggested and none is fully proven — the impact may knock off skin parasites and clinging remoras; the noise and splash may be a form of communication or a courtship display (breaching often peaks in mating aggregations); it may startle predators; and it may help concentrate schooling prey. It probably serves more than one purpose. The behaviour is energetically expensive, so it is presumably worth something.
How deep can a devil ray go?
Surprisingly deep for a fish that is usually seen at the surface. Satellite and archival tags have recorded spinetail devil rays diving to 700 metres and occasionally deeper, into water only a few degrees above freezing, before returning to the warm surface layer. These dives are thought to be feeding excursions to the deep scattering layer, a dense band of plankton and small fish that rises and falls each day. The devil ray has an unusual network of blood vessels (a rete) that keeps its brain warm during these cold plunges.
Why is the spinetail devil ray endangered?
It has one of the lowest reproductive rates of any fish — a female produces a single large pup only once every two to three years — so any added mortality has a severe effect, and mortality is high. Devil rays are caught deliberately for their gill plates, which are dried and sold in southern China as a purported health tonic (the same trade that threatens mantas), and they are taken in enormous numbers as bycatch in tuna purse-seine nets and other fisheries, where they are usually dead or dying when hauled aboard. Populations in the Mediterranean and elsewhere have fallen by well over half. All mobulid rays were listed on CITES Appendix II in 2016 and 2013.