Echiodon rendahli is a pearlfish of the family Carapidae, found in the south west Pacific oceans around Australia and New Zealand at depths down to 196.85 ft (60 m). Its length is between 3.94 and 4.72 in (10 and 12 cm).
Echiodon rendahli is similar in shape to the rattails with a long tapering eel-like body ending in a pointed tail. Both the dorsal and anal fins start about an eighth of the body length from the snout and end at the fused pointed tail fin. The pectoral fins are small and paddle-shaped, and there are no pelvic fins.
In front of the anal fin a short portion of the intestine protrudes from the belly just behind the head, ending in the anus, an arrangement unique among fishes. Adults of this species have been found living in cavities inside sponges and it is possible that the external portion of the intestine can be extended forwards beneath the head so that the fish can defecate outside its shelter without exposing itself to outside dangers.
The wide mouth contains large canine teeth along the front of each jaw, indicating a carnivorous diet.
Echiodon rendahli are pale yellow-brown with a few black spots on the head, have blue eyes, and the exposed intestine is black.
Larval specimens have a single long dorsal spine on top of the head, its position marked in adults by a small raised lump, and a much longer external intestine.
Illustration by Dr Tony Ayling