Aetideus truncatus is an aetideid calanoid copepod known after both sexes (female 1.60-1.80 mm and male 1.45 mm in total length). Species found in the north-eastern Pacific.
Female. Total length 1.60-1.80 mm. Cephalothorax 3.5 times longer than abdomen. When looking laterally, cephalon more prominent near the level of Md and Mx1. Rostrum small with short wide spaced rami. Width/depth ratio of rostral excavation 1.46-2.00. Rostral rami seem to be very wide from lateral view. Points of posterior Th5 corners not exceeding posterior border of genital segment. Duct connecting dorsal and ventral parts of spermatheca rather thick (shorter than the same of A. divergens ). A1 not exceeding posterior border of anal segment, or of the same length as body. Oral parts and P1-P4 typical of the genus.
Male. (Description after Bradford, (1971a) with modification). Total length 1.45 mm. Body shape very similar to that of A. armatus , points of Th5 not exceeding Abd1. Line of fusion between Th4-Th5 visible in dorsal view. Caudal rami in 2.51-2.66 times longer than wide. All appendages of the same structure as in A. armatus. Terminal spines at Re3 P2 and P3 with 23 and 21 denticles respectively, these denticles at P3 more widely spread than in A. armatus , because this spine is longer than Re 3. Second-fourth P5 segments 2.32-2.86, 6.0-6.51, 5.4-6.55 tims as long as wide respectively.
Remarks. Females of this species are distinguished from all other Aetideus species by short widely spaced rostral rami and short Th5 points. Males hardly distinct of A. armatus males (ratio of segments and caudal rami overlapping in both species). Only noticeable distinction - widely spaced denticles at terminal spine of Re3 in A. truncatus , that has terminal spine Re3 P3 longer than segment itself.
The species is found in hauls from 121-238, 350-500 and 477-594 m and in total hauls (Bradford, 1971a; Markhaseva, 1996).
Pacific Ocean: the north-eastern part, to 32°N (Bradford, 1971a).
Type locality: the north-eastern Pacific, the northern limit for species distribution apparently 32°N (Bradford, 1971a), the Gulf of California : 22°59'N , 107°13' W (Bradford, 1971a).
3 females from samples 209, 214, 215. See examined samples module.