Bradyidius arnoldi is an aetideid calanoid copepod known after both sexes (female 1.67 mm and male 1.19-1.23 mm in total length). Species is known from the Gulf of Mexico only.
Female. (Description after Fleminger (1957) with modifications) Total length 1.67 mm. Cephalothorax 4 times longer than abdomen. Rostrum with not divergent rami. Points of Th5 posterior corners exceeding the posterior third of genital segment (dorsal view), directed towards the back side of specimen (lateral view). A1 nearly as long as cephalothorax. External spine at Re1 P1 very short, spine at Re2 P1 thick, reaching the base of following spine. Ri P2-P4 with spinules on posterior surface.
Male. Total length 1.19-1.23 mm. Cephalothorax 2.8 times longer than abdomen. Points of Th5 posterior corners not exceeding posterior border of Abd1. A1 reaching nearly the end of the body, right 20- left 21-jointed. Ri P2-P4 with surface spinules as in female. P5 nearly twice longer than abdomen, biramous. Right Ri P5 one-jointed, left 2-jointed; Right Re P5 2-jointed, left 3-jointed.
The species was found in the surface or near surface hauls, in night samples, between T 16.7°C and 23,5°C. (Fleminger, 1957).
The Gulf of Mexico (Fleminger, 1957).
Type locality: the Gulf of Mexico (Fleminger, 1957).
Species absent in the collections of the Zoological Institute, St. Petersburg.