Aspects of Fertilization Ecology in Evechinus chloroticus and Coscinasterias muricata

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Photo (by Elke S. Franke) shows fertilized 1-cell stage egg of E. chloroticus Egg diameter approx. 80 micrometers

Aspects of Fertilization Ecology in

Evechinus chloroticus and

Coscinasterias muricata

Elke Sabine Franke

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Abstract

Broadcast spawning marine invertebrates have been widely used as model organisms to study processes of evolution. One of these is the study of various life history stages associated with reproduction. Fertilization ecology in broadcast spawning marine organisms, i.e. the process by which sperm and egg fusion occurs once released into the ocean, has been the subject of intensive study for roughly the last 20 years, and represents thus a recent field in ecological sciences. This growth in interest was sparked by studies that showed spawning events may take place predominantly under sperm limiting conditions. More recent findings however, suggest that the occurrence of sperm competition, and the risk of polyspermy (multiple sperm entry in to the egg, which is lethal in echinoderms) can also occur during reproductive events in marine invertebrates.

This has also been predicted on theoretical grounds. Even though polyspermy has been observed previously, particularly during aquaculture studies, evidence to assess the occurrence of polyspermy in situ, and the conditions under which it occurs is lacking.

Simulated field studies in Evechinus chloroticus as well as laboratory studies in E.

chloroticus and Coscinasterias muricata found high levels of polyspermy, even under the sperm limiting conditions that are naturally found in the field in other marine broadcast spawners. Furthermore, laboratory results in both Evechinus and Coscinasterias showed that polyspermy is most likely to increase when sperm concentrations are increased. In addition, even though increasing sperm concentrations increases monospermy to a certain extent, an increase in gamete contact times generates similar levels of monospermy.

Results from these studies confirmed the mathematical model predicting some of these events.

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The timing of gamete release to achieve maximum fertilization success (monospermy) in a broadcast spawner is thought to be under strong selective pressures, because reproductive success is directly related to fitness. The spawning patterns in the sea urchin E. chloroticus observed in this study demonstrated that E. chloroticus spawns under varying environmental and ecological conditions. Interestingly, it appeared that highly synchronous and widespread spawning was found to occur when highly turbulent conditions existed. This may reduce the potential for polyspermy that may exist during mass spawnings under shallow and calm conditions.

The demonstration of polyspermy and the confirmation of the mathematical model, suggest that polyspermy is a common and frequent occurrence during broadcast spawning events. Thus polyspermy represents an evolutionary force that may shape the evolution of reproductive phenomena at levels ranging from the gamete to the population.

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Table of Figures, Illustrations and Tables

Chapter 1

Picture of Evechinus chloroticus………..……….14

Picture of Coscinasterias muricata………..………..17

Chapter 2 Fig.1……….………..……69

Fig. 2 ………...………..70

Fig. 3………..………71

Fig. 4………..72

Table 1………..…74-76 Chapter 3 Fig. 1………...……….114

Fig. 2………115

Fig. 3………...…….116

Fig. 4………..…..117

Fig. 5………118

Fig. 6………..………..119

Fig. 7………120

Fig.8……….121

Table 1……….123

Table 2………...124

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Chapter 4

Fig. 1………..………..166

Fig. 2………167

Fig. 3………...……….168

Fig. 4………..…..169

Fig. 5………...…….170

Fig. 6………171

Fig. 7………172

Fig. 8………..……...173-174 Fig. 9………...…….175

Table 1……….179

Table 2……….179

Table 3………...……..179

Table 4………...…..180

Table 5………...………..181

Table 6………...…………..182

Chapter 5 Fig. 1………215

Fig. 2 A………...…….216

Fig. 2 B………...….217

Fig. 3………218

Fig. 4………219

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Fig. 5……….…….220 Fig. 6………..….…..221-225 Table 1……….227 Table 2……….228 Table 3……….…229