Sperm Diffusion Models and In Situ Confirmation of Long-Distance Fertilization in the Free-Spawning Asteroid Acanthaster planci

Egg longevity and time-integrated fertilization in a temperate sea urchin (Strongylocentrotus droebachiensis)

Recent field experiments have suggested that fertilization levels in sea urchins (and other broadcast spawners that release their gametes into the water column) may often be far below 100%. However, past experiments have not considered the potentially positive combined effects of an extended period of egg longevity and the release of gametes in viscous fluids (which reduces dilution rates). In a laboratory experiment, we found that eggs of the sea urchin Strongylocentrotus droebachiensis had high viability for 2 to 3 d. Fertilization levels of eggs held in sperm-permeable egg baskets in the field and exposed to sperm slowly diffusing off a spawning male increased significantly with exposure from 15 min to 3 h. In a field survey of time-integrated fertilizations (over 24, 48, and 72 h) during natural sperm release events, eggs held in baskets accrued fertilizations over as much as 48 h and attained fairly high fertilization levels. Our results suggest that an extended period of egg longevity and the release of gametes in viscous fluids may result in higher natural fertilization levels than currently expected from short-term field experiments.

How severe is sperm limitation in natural populations of marine free-spawners?

Successful fertilization in marine organisms that release sperm into seawater is potentially limited by the rapid dilution of gametes; cases of severe sperm limitation have been demonstrated in nature. However, recent surveys of naturally spawning populations indicate fairly high fertilization levels in many taxa. The extreme selection exerted by sperm limitation has resulted in numerous adaptations to reduce sperm limitation and enhance fertilization. Thus, most taxa show indications of the evolutionary consequences of sperm limitation even when population level, ecological effects are minimal.

Fertilization in the sea: are the hazards of broadcast spawning avoided when free-spawned sperm fertilize retained eggs?

In broadcast-spawning marine animals, rapid dilution and short lifespan of sperm following release may impose severely localized patterns of mating. Partial or total failure of external fertilization due to sperm limitation appears commonplace. However, it is not clear to what extent the restrictive kinetics of fertilization in water also constrain mating in animals that release sperm but retain their eggs for fertilization.
The compound ascidian Diplosoma listerianum liberates sperm that are dispersed to other colonies and taken in prior to internal cross-fertilization. The fertile lifespan of sperm was found to be long (half-life ca . 8 hours), and a substantial number of fertilizations occurred with 24-hour-old sperm. Fertilizations were obtained from sperm concentrations that would typically produce little or no external fertilization. In a separate experiment, a very small piece of D. listerianum (dry weight less than 2 mg) sired abundant progeny throughout a 3840 l tank. Paternity of progeny in these experiments was confirmed by molecular markers. The same markers were used to extend, to over seven weeks, the known maximum period of storage of exogenous sperm prior to fertilization in this species.
The production of only a few thousand sperm at a time by each zooid, poor synchronization of release between zooids, and the existence of many well-spaced exhalant openings in large colonies suggest that D. listerianum is incapable of generating a dense plume of sperm, even close to the source. It is suggested that, unlike external fertilization, successful internal cross-fertilization in D. listerianum is not dependent upon the interception of a dense cloud of gametes just released by a near neighbour. It seems instead that dilute, long-lived sperm can be extracted efficiently from seawater by this suspension feeder, potentially over a period of time. This capability, and other features of the life history, make it unlikely that sperm limitation is an acute problem in this species and comparable taxa, a conclusion with potential significance for expected patterns of mating, sex allocation and gamete attributes in sessile aquatic invertebrates. Variance in reproductive success between individuals due to differences in fertilization rate may be much lower than in broadcast spawners exhibiting external fertilization.

Beakers versus breakers: how fertilisation in the laboratory differs from fertilisation in nature

The fertilisation of free-spawning invertebrates, mainly sea urchins, has been studied extensively during the last hundred years. However, results obtained from in vitro experiments do not always reflect what happens in the real world. Organisms in their natural habitats have a complex set of challenges, cues and behaviours to contend with during fertilisation and early development, factors that are normally not considered in the laboratory setting. This review examines recent work on fertilisation ecology and discusses the relevance of these results to the findings gleaned from laboratory research. Emphasis is placed on stresses associated with fertilisation in situ, and how responses to environmental stresses (such as from turbulence, oxidative stress, ultraviolet radiation and pathogens) might affect the fertilisation process.