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Thomas, F.I.M., Kregting, L.T., Badgley, B.D., Donahue, M.J., Yund, P.O. (2013) Fertilization in a sea urchin is not only a water column process: effects of water flow on fertilization near a spawing female. Marine Ecology Progress Series 494: 231-240.

 ABSTRACT

Fertilization efficiency in free spawning invertebrates depends on a complex interaction between biological and physical factors.  In some species, a considerable amount of fertilization may take place on the substrate and within flow structures in close proximity to a spawning female. Gametes can be retained on a spawning animal, resulting in a slow release over relatively long periods of time, and retained eggs can be fertilized before they are released into the water column.  Hydrodynamic conditions are likely to influence the retention of gametes and their subsequent mixing in the water column.  Thus, we expected a relationship between hydrodynamic regime and the relative importance of fertilization in the water column and in other locations near spawning animals.  Here, fertilization in the broadcast-spawning sea urchin, Strongylocentrotus droebachiensis, was explored over a range of velocities (2 – 15 cm s-1) to determine the effect of velocity on fertilization and the relative contribution of different locations (aboral surface of the female, water column, eddy, and the substrate behind the female) to overall fertilization. As velocity increased, the percentage of eggs fertilized declined in all locations.  At all velocities more eggs were fertilized on the aboral surface and the substrate surrounding the female than in the water column. Further, as velocity increased, the relative contribution of the aboral surface to overall fertilization was enhanced.  These results highlight the importance of considering the interaction of hydrodynamics, organism morphology, and gamete properties in studies of fertilization in broadcast-spawning invertebrates.

 

In unidirectional flow, piles of sea urchin eggs form on top of and behind a spawning female.  Eggs are released in a viscous fluid that clings to the aboral surface of a female.  Eggs that are swept from this pile may fall to the substrate or be temporarily entrained in an eddy that forms behind a female.  Research in laboratory flumes indicates that under a range of unidirectional flow conditions, most of the eggs are fertilized on the aboral surface or substrate, not in the water column.  The interaction of viscous gametes with flow structures around females may represent an adaptation to enhance fertilization in a system where sperm might otherwise be limiting.

 

Kregting, L.T., Bass, A.L., Guadayol, O., Yund, P.O., Thomas, F.I.M. (2013) Effects of oscillatory flow on fertilization in the green sea urchin Strongylocentrotus droebachiensis. PLoS ONE 8(9): e76082.

ABSTRACT

Broadcast spawning invertebrates that live in shallow, high energy coastal habitats are subject to oscillatory water motion which creates unsteady flow fields above the surface of animals. The frequency of the oscillatory fluctuations is driven by the wave period, which will influence the stability of local flow structures and may affect fertilization processes. Using an oscillatory water tunnel, we quantified the percentage of eggs fertilized on or near spawning green sea urchins (Strongylocentrotus droebachiensis; sampling locations included water column, wake eddy, substratum and aboral surface) that were subjected to a range of different periods (4.5 – 12.7 s) and velocities of oscillatory flow. The root-mean-square wave velocity (rms(uw)) was a good predictor of fertilization in oscillatory flow, although the root-mean-square of total velocity (rms(u)), which incorporates all the components of flow (current, wave and turbulence), also provided significant predictions. The percentage of eggs fertilized varied between 50 – 85% at low flows (rms(uw) < 0.02 m s-1), depending on the location sampled, but declined to below 10% for most locations at higher rms(uw). The water column was an important location for fertilization with a relative contribution greater than that of the aboral surface, especially at medium and high rms(uw) categories. We conclude that gametes can be successfully fertilized on or near the parent under a range of oscillatory flow conditions.

 

Wave-driven oscillatory flows present additional challenges for external fertilization.  Flow reversals create increased shear on the aboral surface, pulling eggs away from the animal more rapidly than in unidirectional flow.  Eddies repeatedly form and break down on two sides of the urchin.  Under these conditions, overall fertilization levels are lower than in unidirectional flow, with more of the fertilization occurring in the water column.  Nevertheless, many of the eggs are still fertilized on the aboral surface.

 

Kregting, L.T., Thomas, F.I.M., Bass, A.L., Yund, P.O. (2014) Relative effects of gamete compatibility and hydrodynamics on fertilization in the green sea urchin Strongylocentrotus droebachiensis. Biological Bulletin 227: 33-39.

ABSTRACT

Intraspecific variation in gamete compatibility among male/female pairs causes variation in the concentration of sperm required to achieve equivalent fertilization levels. Gamete compatibility is therefore potentially an important factor controlling mating success. Many broadcastspawning 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 system, we assessed the relative effects of gamete compatibility (the concentration of sperm required to fertilize 50% of the eggs in specific male/female pairs; F50) and the root-mean square of total velocity (urms; 0.01– 0.11 m s1) on fertilization 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 compatibility 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 significant  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 environmental noise to selection outcomes.

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