Lives its whole life among urchin spines, and carries a light organ lit by bacteria.
A small, deep-bodied cardinalfish to about 7 cm, with a large eye and a dark, subdued pattern that gives nothing away in the shade of an urchin. The identification is the association rather than the colour. Several cardinalfish shelter among urchin spines as juveniles and leave as they grow; this one is there for its whole life, stays small, and occurs in groups of several to a few dozen deep inside a single Diadema. Much of the published work on the fish appeared under the name Siphamia versicolor, and the same animal is now generally referred to S. tubifer, so both names turn up for it.
The light organ runs along the gut in the abdomen and the light is made not by the fish but by a bacterium, Photobacterium mandapamensis, which the fish takes up from seawater after the organ has formed rather than inheriting it from its parents. Output is controlled with a shutter. The best-supported function is feeding: the light appears to draw zooplankton within reach in the low light around dusk, close to the bottom. Individuals show strong fidelity to one urchin and will find their way back to it after being displaced. Males mouthbrood the eggs.
Getting a look means going down to the urchin and waiting, because the fish move to the far side of the spines as a diver arrives and come back round when nothing further happens. The urchin is the reason to keep hands out of it.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
