Life history variation and diet of the endangered tidewater goby, Eucyclogobius newberryi

The fitness consequences of low genetic diversity in wild animal populations are of great concern to species conservation. The endangered tidewater goby, Eucyclogobius newberryi, occurs in reproductively isolated populations along the California coast that exhibit tremendous variation in genetic diversity. Otolith microstructural analysis was conducted to evaluate the relationship between genetic diversity and life history variation in two focal populations exhibiting high and low genetic diversity (Ho = 0.58 and 0.08). Daily increment deposition in sagittal otolith of tidewater goby was validated and a predominantly annual life cycle was observed in both populations (annual survivorship Ŝ, 3%). Back-calculation of birthdates indicates year-round reproductive activity in the population with high genetic diversity, but reveals a very narrow, single annual reproductive period in the genetically depauperate population. Analysis including genetic and demographic data from ten additional populations reveals a correlation between genetic diversity and life history variation, as expressed in variation in the duration of the reproductive period within populations. The threat of reduced genetic diversity to isolated populations was dramatically illustrated through extinction of the genetically depauperate focal population following a drastic increase in salinity. Naturally, the presence of more resilient adult individuals allows tidewater goby populations to persist through these periodic environmental fluctuations with high juvenile mortality. In contrast, a narrow population age structure, associated with reduced genetic diversity, resulted in localized extinction. These findings support the assertion that genetic and life history variation can serve as a safeguard against environmental stochasticity. This study also documents predation by the tidewater goby upon the invasive New Zealand mudsnail, Potamopyrgus antipodarum, in Big Lagoon, California, USA. The gastric contents of 411 individuals, collected monthly from April 2009 to August 2010, were examined. New Zealand mudsnails were found in the digestive tract of tidewater goby that ranged in size from 14 mm to 52 mm total length, corresponding to post-settlement and nearly maximal size of this species. 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 mm to 4.0 mm. The average size of ingested snails increased with fish length (r² = 0.42, p 0.001). 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 over 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 this readily available, exotic prey item.