Gyrocerviceanseris passamaquoddyensis n. gen., n. sp. is described from the gills of silver hake, Merluccius bilinearis, in Passamaquoddy Bay, southwestern New Brunswick, Canada. Diagnostic features include an oval haptor with asymmetrically distributed marginal hooks (3 pairs anterior, 5 pairs posterior) and a prominent muscular flange on the lateral margins, between the anterior and posterior groups of hooks. The hamuli are well developed, with a straight point, a short shaft, and an elongate, blade-like ventral root having a ventrally directed, gooseneck-shaped tip. The ventral bar is rectangular, with a small, hourglass-shaped posterior membrane. There is no dorsal bar. The marginal hook sickle is sharply curved, with a long fine point, an upturned toe, a small heel, and a straight handle. A partial DNA sequence from 18S rDNA was generated and found to be over 11% divergent from its closest matches in GenBank. However, it was less divergent (7.8%) from a previously unpublished sequence of Gyrodactylus emembranatus Malmberg, 1970, suggesting this species is its closest known relative. Interestingly, both species have almost identical marginal hook sickles and a male copulatory organ (MCO) with small spines in multiple arched rows. Gyrocerviceanseris n. gen. is the sixth genus of viviparous monogeneans to be described from marine and estuarine fishes of the northwestern Atlantic Ocean.

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