Relative Effects of Gamete Compatibility and Hydrodynamics on Fertilization in the Green Sea Urchin Strongylocentrotus droebachiensis LOUISE T. KREGTING 1,2, * , †, FLORENCE I. M. THOMAS 2 , ANNA L. BASS 1 , AND PHILIP O. YUND 1, ‡ 1 Marine Science Center, University of New England, Biddeford, Maine 04005; and 2 Hawai’i Institute of Marine Biology, University of Hawai’i, Kane’ohe, Hawai’i 96744 Abstract. Intraspecific variation in gamete compatibility among male/female pairs causes variation in the concentra- tion of sperm required to achieve equivalent fertilization levels. Gamete compatibility is therefore potentially an im- portant factor controlling mating success. Many broadcast- spawning marine invertebrates, however, also live in a dynamic environment where hydrodynamic conditions can affect the concentration of sperm reaching eggs during spawning. Thus flow conditions may moderate the effects of gamete compatibility on fertilization. Using the green sea urchin Strongylocentrotus droebachiensis as a model sys- tem, we assessed the relative effects of gamete compatibility (the concentration of sperm required to fertilize 50% of the eggs in specific male/female pairs; F 50 ) and the root-mean- square of total velocity (urms; 0.01– 0.11 m s -1 ) on fertil- ization in four locations near a spawning female (water column, wake eddy, substratum, and aboral surface) in both unidirectional and oscillatory flows. Percent fertilization decreased significantly with increasing urms at all locations and both flow regimes. However, although gamete compat- ibility varied by almost 1.5 orders of magnitude, it was not a significant predictor of fertilization for most combinations of position and flow. The notable exception was a signifi- cant effect of gamete compatibility on fertilization on the aboral surface under unidirectional flow. Our results suggest that selection on variation in gamete compatibility may be strongest in eggs fertilized on the aboral surface of sea urchins and that hydrodynamic conditions may add envi- ronmental noise to selection outcomes. Introduction Incompatibility between gametes occurs both within and among species of broadcast-spawning invertebrates and is often mediated by variation in gamete recognition proteins (Palumbi and Metz, 1991; Palumbi, 1994). The selective forces operating on this variation are thought to include a mix of conspecific processes and reinforcement to avoid hybridization (Palumbi, 1994; Levitan and Ferrell, 2006; Evans and Sherman, 2013). Within a species, variation in compatibility among male/female pairs potentially results in differential success in fertilization because less compatible individuals can achieve equivalent fertilization levels only at higher sperm concentrations (McCartney and Lessios, 2002; Rawson et al., 2003; Levitan and Ferrell, 2006; Slaughter et al., 2008). Specific male/female pairs should thus exhibit different levels of fertilization when environ- mental conditions are held constant. Many broadcast-spawning invertebrates, however, live in energetic coastal environments, where hydrodynamic con- ditions create spatial and temporal variation in the concen- tration of sperm reaching eggs. In general, fertilization is expected to decline in higher velocity and more-turbulent flows (Pennington, 1985; Levitan et al., 1992; Yund and Meidel, 2003; Thomas et al., 2013). In addition, small-scale turbulent processes may result in regions of high concen- trations of sperm and eggs within the water column Received 22 January 2014; accepted 23 June 2014. * To whom correspondence should be addressed. E-mail: l.kregting@ qub.ac.uk † Current address: School of Planning, Architecture and Civil Engineer- ing, Queens University of Belfast, Northern Ireland, BT22 1PF, UK. ‡ Current address: The Downeast Institute, P.O. Box 83, Beals, Maine 04611. Abbreviations: urms, root-mean-square of total velocity; F 50 , concen- tration of sperm required to fertilize 50% of the eggs in specific male/ female pairs; OWT, oscillatory water tunnel; UF, unidirectional flume. Reference: Biol. Bull. 227: 33–39. (August 2014) © 2014 Marine Biological Laboratory 33