Some unusual morphological features of Amazonian fish parasites (Cymothoidae)

Amazonian freshwater cymothoids have been found to possess certain morphological characters that are unreported for marine forms. In the genus Anphira, for example, the first coxal plates are free and moveable. In all other known Cymothoidae, the first coxal plates are fused to the pereion and the following six are free. In the genus Braga, as redefined by Thatcher, 1996, the pleopods are multilaminate. Pleopod 1 consists of two lamellae while pleopod 2 has four and pleopods 3 to 5 have three lamellae each. The mouth parts of South American freshwater cymothoids also differ from those of their marine cousins. The mandibles of the freshwater forms lack cutting shearing blades and incisors. The maxillules of freshwater species usually have five recurved spines (3–4 terminal and 1–2 subterminal) whereas the marine typically have only four, all terminal. The mouth parts of Amazonian fish parasitic isopods differ from one species to another but they do show a closer relationship with each other than any do to the marine species. These morphological features are illustrated and explained.