The body of the slender suckerfish is elongate, with long dorsal and anal fins.

Slender suckerfish, P. lineatus .

The slender suckerfish feeds mostly on scraps of fish and plankton, as opposed to more host-dependent remoras that feed on host parasites.

With the smallest adhesive disk amongst the remoras, the slender suckerfish is not strongly host-dependent and can be encountered free-swimming or attached to inanimate objects.

The slender suckerfish or lousefish, Phtheirichthys lineatus, is a rare species of remora, family Echeneidae, and the only member of the genus Phtheirichthys.

The slender suckerfish or lousefish, Phtheirichthys lineatus, is a rare species of remora, family Echeneidae, and the only member of the genus Phtheirichthys.