Several types of parasites are found on or in wahoo.  Perhaps the most abundant is the trematode Hirudinella ventricosa, which inhabits the stomachs of wahoo. They are medium to dark brown and resemble large slugs in texture and appearance. They are usually about the size and shape of a human thumb. There are usually 2 or 3 in each wahoo stomach.  It is very rare to find none at all, but occasionally there are many, up to 12 or so.

Another parasite frequently encountered is the copepod Branchiella thynni. This parasite is about 3/4" in length including tentacles, and attaches to the socket located at the base of the pectoral fins. They attach their head region deep into the fin socket, leaving several tentacle-like structures exposed. These tentacles can be seen by pulling back the pectoral fins and examining the base. It is an unusual looking animal which can be removed and observed more closely by lightly grasping the tentacles with fingers or tweezers and gently pulling them out. About 70% of the fish we have inspected from the western Atlantic and Caribbean have one parasite at the base of each fin.  We have never observed these parasites on one fin only; it is either one on each fin, or both fins clean.  An angler in south Africa has reported to us that wahoo from that area also frequently contain the B. thynni parasite.

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