Distribution of Benthesicymus tanneri Faxon, 1893 (Dendrobranchiata, Benthesicymidae) off the west coast of Mexico and notes on its morphology

A large series of specimens of Benthesicymustanneri Faxon, 1893 (Dendrobranchiata; Benthesicymidae) was collected during an extensive survey of deep-water invertebrate fauna off western Mexico. In total, 61 males and 122 females (M:F ratio = 1:2) from 44 sampling stations were examined, considerably increasing the number of known specimens and sampling localities for this species which is widely distributed along the Pacific coast of Mexico. The collection is the largest available for this species to date and presents first records from off the west coast of the Baja California Peninsula and a slight increase of the northernmost record within the Gulf of California. On the whole, females grew larger than males. The petasma of males of different sizes and the female thelycum of Benthesicymustanneri are illustrated. The petasma of Benthesicymustanneri presents a ventrolateral crescent-shape process otherwide found only in Benthesicymustirmiziae Crosnier, 1978 and in Benthesicymusbartletti S.I. Smith, 1882. A key to the four species of Benthesicymus presently known from the eastern Pacific is presented.