Littorellicola billhawkinsi n. gen., n. sp. (Digenea: Aporocotylidae) from the myocardial lacunae of Florida pompano, Trachinotus carolinus (Carangidae) in the Gulf of Mexico; with a comment on the interrelationships and functional morphology of intertrabecular aporocotylids.

Littorellicola billhawkinsi n. gen., n. sp. infects the myocardial lacunae of the ventricle and atrium of Florida pompano, Trachinotus carolinus in the northern Gulf of Mexico. It differs from other aporocotylid genera by the combination of having a body 10-30× longer than wide, a posterolateral body protuberance, lateral spine rows, an aspinous anterior sucker comprising a slightly muscular rim circumscribing the mouth, asymmetrical posterior ceca 14-20× length of the anterior ceca and lacking diverticula or secondary rami, tens of testes distributing in a cobblestone-like field anterior and posterior to the distal ends of the posterior ceca, an oviducal seminal receptacle comprising the distal portion of the oviduct, and a post-cecal ovary plus by lacking rosethorn-shaped spines, a pharynx, and a Laurer's canal. The new species appears host specific to Florida pompano because no conspecific infection was detected in 134 carangids of 8 species in 4 genera captured nearby the type locality. Psettarium sebastodorum Holmes, 1971 is transferred to the new genus, as Littorellicola sebastodorum (Holmes, 1971) n. comb., because it and the new species differ from species of Psettarium by the combination of having multiple testes plus 8 other features detailed herein. This report brings the number of nominal Gulf of Mexico aporocotylids to 12 species of 8 genera, represents only the second record of an aporocotylid from a carangid there, and supports the notion that elongated, "thread-like" aporocotylids with lateral spine rows are seemingly well-adapted for infecting myocardial lacunae or embedding in the myocardium of their definitive fish hosts.