California needlefish

Strongylura exilis, described by Girard in 1854, is a needlefish with an elongate body and a beak-like snout that lives in coastal waters of the eastern Pacific, from San Francisco, in California, to Peru, including the Galápagos Islands ^[1],[2]. It can reach 91 cm in total length and lives in small schools near the surface ^[2],[3].

Globally the IUCN rates it Least Concern, according to the 2018 assessment ^[4]. Much of what is known about its early development comes from an aquarium rearing study published in 1988 ^[3].

The genus name, Strongylura, comes from the Greek strongylos, meaning round, and oura, meaning tail, according to the etymology FishBase gives ^[2].

It is an elongate fish. The dorsal fin has 13 to 17 soft rays and the anal fin 16 to 19, with no spines in either; the tail fin is slightly notched and the skeleton has 67 to 74 vertebrae ^[2].

Young fish change shape a great deal as they grow. At hatching they measure between 6 and 9 mm and the mouth is slightly subterminal to nearly equal in both jaws; the lower jaw then lengthens faster than the upper one, producing a phase with a half-grown beak that peaks at about 24 mm of body length, when the difference between the two jaws equals 27 % of that length ^[3]. Colour changes too: the larva, covered at hatching in yellow and black pigment, comes to have a green back on a black ground and silvery flanks ^[3].

The species occupies the eastern Pacific between San Francisco, in California, and Peru, and includes the Galápagos Islands ^[2],[3]. It has been recorded in sea, brackish and fresh water, from the surface down to 100 m depth ^[2].

It is usually seen in bays and harbours, near the surface and in small groups ^[3]; it is also common in coastal areas and mangrove lagoons ^[2]. The adults used for the rearing study were caught in 1979 with a beach seine over sandy bottom, north of the Scripps Institution pier, in California ^[3].

It is oviparous. The eggs are spherical, measure between 2.4 and 2.7 mm and carry 28 to 40 long filaments spread over the shell; the filaments of neighbouring eggs tangle into sticky masses that stay attached to vegetation or floating objects ^[3],[2]. The mature females in the study measured between 500 and 636 mm standard length, and the males 435 and 480 mm ^[3].

In water at 19 to 21 °C, hatching took place between 10 and 19 days after fertilisation. At hatching the larvae already had working eyes, jaws and digestive tract, absorbed the yolk sac by about day 3 and, fed brine shrimp nauplii (Artemia) in captivity, grew about 1 mm per day ^[3].

Teeth begin to form on day 2 of life, and by about day 10 there is a row of large, widely spaced teeth with small ones between them. The pelvic fins are the last to complete their 6 rays, between days 18 and 39, later than in any other known needlefish (belonid) ^[3].

Adults form small schools and feed mainly on small fishes; their estimated trophic level is 4.5 ^[2].

The larvae are neustonic, that is, they live right at the water surface ^[2]. In the aquarium, newly hatched larvae took brine shrimp nauplii at once and larger ones took chopped squid, which does not show what they eat at sea ^[3].

Globally the IUCN classifies it as Least Concern, with an assessment dated 2018 ^[4]. FishBase lists it as not evaluated by CITES and of moderate vulnerability to fishing, scoring 37 out of 100 ^[2].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

FishBase describes it as harmless to people ^[2].

In California it is regarded as an excellent food fish that sport anglers occasionally take on hook and line, but it is not fished commercially ^[3]. FishBase, by contrast, records a minor commercial fishery in which it is caught under artificial lights and sold fresh ^[2].