Preying on invasives: the exotic New Zealand mudsnail in the diet of the endangered tidewater goby

This study documents predation by the endangered tidewater goby, Eucyclogobius newberryi, upon the invasive New Zealand mudsnail, Potamopyrgus antipodarum, in Big Lagoon, California, USA. To estimate the prevalence of NZ mudsnails in the diet of tidewater goby, the gastric contents of 411 individuals, collected monthly from April 2009 to August 2010, were examined. NZ mudsnails were found in the digestive tract of tidewater goby that ranged in size from 14 to 52 mm total length, corresponding to post-settlement and nearly maximal sizes of this species. Unlike other native species which are unable to extract nutrition from these snails, tidewater goby fully digest this hard-shelled prey, as evidenced by the presence of shell fragments and complete absence of intact shells in the hind gut. The number of ingested NZ mudsnail ranged from 1 to 27 (mean 4.4), and ranged in length from 0.39 to 4.0 mm. The average size of ingested snails increased with fish length (r ² = 0.42, P < 0.001). NZ mudsnails were found in over 80% of individuals during the summer and fall of 2009, when the estimated population size of tidewater goby in Big Lagoon was greater than three million. This study documents the first instance of a native and endangered species that preys upon and utilizes the NZ mudsnail as a food source, and suggests that tidewater goby can exert substantial predation pressure upon NZ mudsnails and take advantage of these readily available novel prey items.