Spatial distribution of ascidian sperm: two-dimensional patterns and short vs. time-integrated assays

Abstract

Understanding sperm dispersal patterns in free-spawning marine invertebrates is fundamental to assessing the likelihood of successful fertilization under a range of biological and physical conditions. Field estimates of sperm dispersal in egg broadcasters have relied on sperm assays that sample short-term sperm availability, and tended to focus on simple patterns of availability with distance along a 1-dimensional transect at a single site. However, sperm are diluted in multiple dimensions, and the decrease in concentration with increasing distance from a sperm source in any single dimension is dependent on the rate of dilution in other dimensions. We used short-term sperm availability assays to evaluate the 2-dimensional distribution of sperm downstream of 2 different populations of the egg-brooding colonial ascidian Botryllus schlosseri. Female colonies were exposed to water samples collected at various distances and 2 depths. Because many egg brooders that feed on phytoplankton appear to collect sperm throughout an extended period of time, short-term assays of sperm availability may underestimate absolute fertilization. Consequently, at one site we also compared fertilization in our short-term assays with time-integrated fertilization levels obtained by deploying colonies in the sampling locations for the entire period of egg viability. Our results indicate that vertical patterns of sperm availability with horizontal distance can vary between sites, suggesting that sperm transport patterns will be site-specific and dependent on local flow, and that vertical mixing may cause a more rapid decline in sperm concentration with increasing distance. Our comparison of instantaneous vs. time-integrated assays suggests that short-term assays may vastly underestimate actual field fertilization levels in some free-spawners. KEY WORDS: Fertilization ecology · Free-spawning invertebrate · Sperm dispersal · Vertical mixingResale or republication not permitted without written consent of the publisher

Fertilization efficiency and gamete viability in the ascidian Pyura praeputialis in Chile

“…The levels of fertilization success in our study decreased drastically with sperm concentrations below 10 4 ml -1 , which agrees with the data available for several broadcast spawning marine invertebrates (see Pemberton et al 2003). However, most of the above-cited studies have not considered an extended period of egg longevity (Meidel & Yund 2001, Yund et al 2007). Nevertheless, the confounding effect of differences in sperm fertilization efficiency due to increased sperm concentration cannot be ruled out as a potential cause of the decrease in fertilization at high sperm concentrations.…”

“…conducted with spermcast ascidians suggest that sperm availability declines with distance from the spawning point and is modified by depth and local hydrodynamic conditions (Yund et al 2007). In our study, naturally shed eggs of P. praeputialis were immersed at approximately equal depths.…”

“…This suggests that biofoam may not necessarily be selectively advantageous under certain circumstances. Although our field study only included short trials (see Yund et al 2007) with the obvious limitations imposed by this type of approach (e.g. lack of gamete manipulation, selection of optimal observation times, hydrodynamic conditions), we suggest that biofoam in P. praeputialis (1) is linked to spawning and might act as a mechanism that enhances fertilization in this species ; and (2) enhances settlement on top of or nearby P. praeputialis matrices.…”

Male reproductive success during spawning events of the octocoral Pseudopterogorgia elisabethae

“…However, Johnson & Yund (2007) found that the estimate of the minimum number of males fertilizing broods of the tunicate Botryllus schlosseri did not vary between high and low density populations. Ayre & Miller (2006) reported genetic diversity that is consistent with multiple paternity in broods of a scleractinian in which colonies were scattered over several meters, and Yund et al (2007) found that fertilization of B. schlosseri eggs could occur over distances as far as 207 m.…”

“…However, Johnson & Yund (2007) found that the estimate of the minimum number of males fertilizing broods of the tunicate Botryllus schlosseri did not vary between high and low density populations. Ayre & Miller (2006) reported genetic diversity that is consistent with multiple paternity in broods of a scleractinian in which colonies were scattered over several meters, and Yund et al (2007) found that fertilization of B. schlosseri eggs could occur over distances as far as 207 m.The reproductive characteristics of Pseudopterogorgia elisabethae and studies of fertilization in marine invertebrates in general lead to the following questions about male spawning, which we address in this study: How variable is male reproductive success? Are males close to females more successful in siring larvae than males farther away?…”