Aetideus mexicanus is an aetideid calanoid copepod known after both sexes (female 1.66-1.84 mm and male 1.58-1.68 mm in total length). Species found in the Gulf of Mexico and the north-eastern part of the Pacific Ocean in the upper 500 m.
Female. Total length 1.66-1.84 mm. Cephalothorax 3.6 times longer than abdomen. Rostrum of A. armatus type. Width/depth ratio of rostral excavation (1.07-1.16):1. Length of Th5 points highly variable - usually points reaching posterior border of genital segment, sometimes exceeding it. A1 exceeding posterior border of caudal rami by two segments or as long as body. All other appendages like in A. pacificus. Most specimens with asymmetrical P1 - with tooth-like protrusion at external edge of right basipodite, as for the rest P1-P3 like in A. pacificus. There are no spinules near the base of internal seta at P4 coxopodite. Re3 P2-P4 terminal spine with 15-17 teeth.
Male. (Described after Park (1974) with modifications). Total length 1.58-1.68 mm. Cephalothorax 3 times longer than abdomen. Points of Th5 posterior corners reaching posterior border of Abd1. Width length Abd2-Abd4 ratio: 1:(0.98-1.05), 1:(1.08-1.15), 1:(1.12-1.19) and same of caudal rami: 1:(1.82-2.00). A1 reaching about distal end of Abd4. Re3 P2-P3 terminal spines longer than their segments, with 23-29 teeth respectively. Second - fourth P5 segments with depth/width ratio: 1:(3.7-4.2), 1:(8.3-8.7) and 1:(9.0-9.7).
Remarks. A. mexicanus differs from A. armatus in slender body, longer A1 exceeding caudal rami by 2 segments and specially by lacking teeth near the base of seta at P4 coxopodite. A. mexicanus male very close to A. armatus but differs in width/length ratio of the caudal rami and Abd2-Abd4. Very close to A. divergens , differs in shorter Th5 points (although some specimens with points exceeding posterior border of genital segment (Park, 1974)).
In the Gulf of Mexico in the upper 500 m (Park, 1974), in the Pacific Ocean in 500-1000 m (Markhaseva, 1996).
Atlantic Ocean: The Gulf of Mexico (Park, 1974), tropical and the north-eastern part of the Pacific Ocean (13°N, 133°W) (Markhaseva, 1996).
Type locality: the Gulf of Mexico - 25°15' N, 81°11' W (Park, 1974).
1 female from sample 207. See examined samples module.