A transparent dart with side fins and a crown of grasping spines; a fast predator on copepods.
A straight, stiff, glass-clear rod with a pair of lateral fins and a small tail fin, and a head carrying a fan of curved spines either side of the mouth. It holds still, then crosses the beam in a single flick and stops again. Larval fish and small transparent worms are confused with it; the combination of paired fins on a rigid unsegmented body and the grasping spines is diagnostic for the phylum, but not for anything below it, and this entry is pitched at Chaetognatha because a photograph will not settle the genus.
Arrow worms are a small phylum of their own, with no close living relatives, and they are among the most abundant predators in the plankton. The spines close on copepods, other arrow worms and small fish larvae; many species deliver tetrodotoxin, produced by bacteria they carry, to subdue prey. They detect a target by the water movement it makes, using rings of sensory bristles along the body, and the strike is fast enough that it is not seen so much as inferred from the animal having moved.
Photographing one is largely a matter of finding it at all. The body returns very little light and the usual result is a pair of dark eye spots and a faint edge; side lighting and a dark background give the outline something to read against.
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.
