A small silvery fish hanging head-down between the spines of a sea urchin, and one of the very few animals here that carries its own light.
The organ runs along the gut and is packed with luminous bacteria the fish acquires from seawater as a juvenile - not inherited, acquired, and only one bacterial species out of everything in the water will do. It feeds them and houses them; they glow. The light shines downward through the muscle, and the working theory is counter-illumination: from below the fish is lit to match the surface glow, so its silhouette disappears.
The urchin is the other half of the arrangement. These fish spend their entire lives among the spines of Diadema, turning to keep the spines between themselves and anything approaching, and a group of them will rotate around the urchin as a diver circles it.
Males brood the eggs in the mouth and do not feed while they carry them - the family's rule. Seven centimeters (3 in), and easily overlooked as a reflection between the spines.
The genus. Siphamia Weber 1909 - síphōn (pipe or tube). ETYFish
The species. Siphamia tubifer Weber 1909 - tubus (pipe or tube) + -ifer (to have or bear). ETYFish
- small translucent silvery fish with a dark stripe from eye to tail
- always among the spines of a Diadema urchin, head down
- belly with a pale strip along the gut - the light organ
- length up to 7 cm
Why it's threatened
This species is a component of the marine ornamental trade, but the potential impact is unknown.
Threat classification from the IUCN Red List.
