Marine invertebrate larvae swim by using cilia or muscles, or a combination of these. The effectiveness of cilia as locomotory organelles diminishes with increasing body size above 1 mm. Thus, larvae propelled by cilia are small and, owing to the small Reynold's numbers that operate in this size range, their movements are governed by viscous forces rather than inertial ones. Cilia may be distributed uniformly over the surface of the larva and (or) localized on rings, bands, arms, or lobes. During development the pattern of ciliation may change; this often increases the swimming ability of the larva, particularly its manoeuverability. In many cases, redistribution of cilia coincides with the onset of feeding behavior. The locomotory currents produced by ciliary beating or the action of swimming appendages may simultaneously convey food particles to the mouth. Muscles may have enabled some larvae to exceed the size limit imposed by ciliary propulsion and also have enabled greater swimming speeds. Invertebrate larvae that use muscular locomotion possess some form of skeleton (hydrostatic, exoskeleton. or notochord) to provide the necessary resistance for muscular contraction. The density of most marine invertebrate larvae exceeds that of seawater, therefore, they must swim to stay suspended. A wide variety of parachute structures, density-reducing devices, and passive hydrodynamic mechanisms counteract the sedimenting effects of gravity. The timing of development in some larvae is such that when the tendency to sink exceeds the ability to swim, the larva is preparing for settlement and metamorphosis.

Many organisms reproduce by releasing gametes into the environment. However, very little is known about what proportion of released eggs become fertilized. We examined the influence of spawning group size, degree of aggregation, position within an aggregation, and water flow, on in situ fertilization in the sea urchin Strongylocentrotus franciscanus. This study was conducted at a depth of 9 m on the west coast of Vancouver Island, British Columbia, Canada. Males were simulated by syringes filled with sperm; females were simulated by sperm—permeable containers filled with eggs. Individuals were placed 0.5 or 2.0 m apart within a 2 x 2 or 4 x 4 (group size of 4 or 16 individuals) experimental array. The results indicate that group size, degree of aggregation, position within a spawning group, and water flow all affect fertilization success. Fertilization success. Fertilization success ranged from 0 to 82%. Increases in group size and aggregation, decreases in flow velocity, and central and downstream positions within an aggregation all lead to increase in fertilization success. Thus, individual reproductive performance is dependent on, and highly sensitive to, population parameters and environmental conditions.

Determining fertilization success of free spawning organisms in the field requires knowledge of how eggs and sperm interact under varying encounter frequencies and durations. In the laboratory, we investigated the relative influence of sperm concentration, egg concentration, sperm-egg contact time, and sperm age on fertilization in the sea urchin Strongylocentrotus franciscanus. Our results indicated that sperm concentration, sperm-egg contact time, sperm age, and individual variability were sequentially the most important factors influencing fertilization success. Egg concentration was not significant over the range tested. A theoretical model of fertilization (Vogel-Czihak-Chang-Wolf model) was used to estimate the two rate constants of fertilization kinetics: the rate constant of sperm-egg encounter and rate constant of fertilization. This model explained 91% of the variation in fertilization success, provided estimates of the rate constants involved in fertilization, and indicated the proportion (3%) of sperm-egg contacts that result in fertilization. Estimates of sperm swimming velocity and egg diameter were used to independently calculate the rate of sperm-egg encounter and confirm the predictions of the model. This model also predicts the non-significant effect of egg concentration on fertilization success found empirically.

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