About Entelurus aequoreus (Linnaeus, 1758)
The snake pipefish, Entelurus aequoreus, has an extremely long, elongated, slender body with smooth skin and a rounded cross-section. It differs from other co-occurring pipefish species by the near absence of prominent bony rings. It has a long head marked by a thin dark stripe along its sides, a long concave snout, and a very small protractile mouth. Its long-based dorsal fin has 37–47 short rays, the caudal fin is very small, and this species has no pectoral or anal fins. Body coloration is pale brown or yellowish-green; the 28–31 faint body rings are each outlined by pale blue rings with dark margins. The gill opening is reduced to a single pore in the membrane above the opercle, and the gill membranes are fused to the body and isthmus. Males grow to a maximum total length of 40 centimetres (16 in), while females reach up to 60 centimetres (24 in). Common lengths are around 32 centimetres (13 in) for males and 45 centimetres (18 in) for females. Juveniles under 70 mm long have membranous pectoral fins that disappear as they mature.
This species is found in the northeastern Atlantic Ocean, ranging from Iceland and Norway south to the Azores, and also occurs within the Baltic Sea. It is not present in the Mediterranean Sea. In the early 2000s, its range expanded northward to reach Svalbard and the Barents Sea.
Compared to other pipefish species, the snake pipefish lives in more open, deeper water, with a depth range of 10–100 metres (33–328 ft). It inhabits areas among kelp, other deep-water seaweeds, and seagrass such as Zostera marina, though both young individuals and large adults have been caught in pelagic waters. The species' color and patterning provide effective camouflage in these habitats. Colonization of Waddensee sand flats by the invasive Japanese seaweed Sargassum muticum has allowed snake pipefish numbers to increase in this area. Breeding occurs in mid-summer, when males and females pair up. This species is ovoviviparous: the female attaches more than 1,000 fertilized eggs (each around 1.2 millimetres (0.047 in) in diameter) to a layer of sticky mucus in a groove on the male's belly, where the eggs stay until they hatch. After hatching, fry remain pelagic until they reach a length of 12 millimetres (0.47 in). Adult snake pipefish feed on small crustaceans and larval fish, which they suck into their mouths. When populations of this species increased in the 2000s, some seabird species began preying on snake pipefish, but found the fish difficult to digest due to their bony structure. Auks and terns began feeding snake pipefish to their chicks after populations of their typical prey, sand-eels, declined. However, snake pipefish have much lower nutritional value than oily sand-eels, and many seabird chicks choked on the fish's hard, indigestible bodies.
