Species Abundance, Spatial and Vertical Distributions, and Eye-Size Trends of Large Heteropods (Pterotracheidae and Carinariidae) in the Northern Gulf of Mexico

Abstract: Heteropods are predatory planktonic gastropods that are important in pelagic ecosystems. However, distributions of large heteropod species are poorly known. Heteropod collections from two midwater sampling programs conducted after the 2010 Deepwater Horizon oil spill (DWHOS): the Offshore Nekton Sampling and Analysis Program (ONSAP) in 2011 and the Deep Pelagic Nekton Dynamics of the Gulf of Mexico (DEEPEND) in 2015-2018 were used for this study These programs collected over 3,495 heteropods in the two target families from 46 sampling stations in the northern Gulf of Mexico (GOM). The zone along the northeastern GOM continental slope had the greatest species richness and abundances. The family Pterotracheidae (Pterotrachea coronata (Forsskal 1775), Pterotrachea hippocampus(Philippi 1836) and Pterotrachea scutata (Gegenbaur 1855) was the most abundant and contained the largest specimens examined. Common carinariids included Carinaria lamarcki (Péron & Lesuer 1810) and Cardiapoda placenta (Lesson 1830). We found evidence of diel migration in P. coronata and P. scutata but not for C. lamarcki, C. placenta or P. hippocampus. We evaluated body and eye size at capture depth for each species. There was no evidence of eye size increasing relative to body size with depth among the five species and relative eye size is species-specific. However, it was observed that vertical migrators had a different eye-type than the non-migrators. We determined that pterotracheids have smaller eyes relative to their total body size than carinariids. The allometric pattern of eye growth differed in P. scutata from those of the other species which could indicate that a factor other than depth plays a part in heteropod eye development. This is the first comprehensive study of large heteropods in the northern GOM which provides an important baseline for continued study of this pelagic gastropod.