(Fig. 7)
Synonyms. Pennella cervicornis Heegaard, 1943, P. plumosa (Dekay, 1844) Type host and locality. Diodon sexmaculatus, Philippines.
Morphology. Size: 24–28 mm. Papillae: partial coverage, clavate and club-like, consistent in size and shape, not found in organized groups. Holdfasts: two only, short. First antenna with three segments, second with two segments. Plumes: simple, single.
Remarks. A valid species. Original description from porcupine fish, Diodon sexmaculatus (= D. holacanthus) (Oken 1816); redescription from the same host species, Ceylon (Kirtisinghe 1932; Fig. 7). Considered incorrectly as infesting two host genera: Lophius and Diodon (Wilson 1917). The history of P. diodontis was often confused with that of P. sagitta (Wilson 1917), however, P. diodontis reported from Lophius was an error of Blainville (1822) based on a poor description and incorrect host identification; P. diodontis is in fact a parasite of porcupine fish (Diodontidae: Diodon) and P. sagitta is found so far only on frogfish (Antennaridae: Histrio, possibly Antennarius)). Lasarus and Sreenivasan (1977) reported P. diodontis from the moorish idol (Zanculus canascens (= Z. cornuta) in the Indian Ocean. The single habitus figure and brief description are not sufficient to confidently place their specimens (eight) in P. diodontis. However, the size of the parasite (26mm) and the cephalothoracic papillae (few small at center, and larger branched papillae around the cephalothorax periphery) are reminiscent of the species. As a parasite of Diodon, P. diodontis is a small species with uniformity in most structures between specimens (Hogans 1988a), however, its cephalothoracic papillae may vary slightly in arrangement and number depending on the individual parasite (Hogans 1988a; Heegard 1943).