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Invertebrate encounters of a Sexual Kind
The production and survival of viable offspring, which in time reproduce themselves, is one measure of animal success, and animals spend a great deal of time trying to achieve this goal. Interoperates, into which category most molluscs fall, reproduce more than once.
This was said by Professor Alan Hodgson during his Distinguished Senior Research Award lecture which was dedicated to the reproductive habits of invertebrates, particularly those marine molluscs which are broadcast spawners, and later discussed sperm conjugation.
Speaking to a packed audience, Prof Hodgson said he initially looked at the factors affecting fertilization success in broadcast spawners. Marine molluscs often achieve gamete encounters by broadcasting their sperm and eggs into the surrounding water. This is a chancy manoeuvre, as gametes are tiny and extremely short-lived once released.
A relatively small gamete volume released into a large and dynamic ocean volume results in rapid dispersion of the sperm, which leads to decreased fertilisation.
Species which use this method of reproduction increase their chances by high fecundity (oysters release up to 50 million eggs per spawning) and by a high gonad index – up to 35% of body weight in limpets. Synchronised development and release of gametes, and high densities or spawning aggregations, increase the chance of successful fertilisations. External fertilisations also tend to occur rapidly.
A vital question is whether human activity can affect the fertilization success of marine invertebrates and Prof Hodgson outlined three possibilities: over-exploitation, climate change (temperature and ocean acidification) and pollution (both organic and inorganic, and light pollution).
Sea urchins and abalone is an example of the over-exploitation of natural resources. Exploited populations reach an Allee threshold where the per capita population growth becomes negative. This reduces the reproduction facilitation effect among neighbouring populations, and increases vulnerability to predators and harsh environments as insufficient adults remain to protect the young animals. Additionally, as a population is exploited it becomes more sought after.
The second way in which humans can influence fertility is through climate change, more specifically, through ocean acidification. Sadly there is very little research being carried out in South Africa, despite the fact that we know little about the impact global warming will have on our coastlines.
The effects of organic and inorganic pollution have been well-documented, and the effect of light pollution has now been noted and is being explored on areas such as Australia’s Great Barrier Reef.
The second part of Prof Hodgson’s lecture focused on sperm conjugates. While most sperm act as individuals, some males, from a wide variety of species, release sperm in groups called conjugates.
Prof Hodgson has been working on cicadas, where he has found that groups of spermatozoa form in the testis in a central elongated mass, while in spittle bugs, the groups of sperm are more circular in shape. More recently he has been working with a German colleague on carabid beetles, which are easier to keep alive in a lab setting than other insects.
In this beetle the male forms hundreds of ribbon-like structures to which sperm are attached in bundles, in the vas deferens, which are transferred to the female on mating.
No-one really knows what the function of sperm conjugates is. Theories include the facilitation of sperm transport through increased motility, and the possible enhancement of competitive fertilisation success by preventing the female from mating again.
Prof Hodgson’s carabid beetle study is currently engaged in studying the fate of sperm in the females, with the aim of increasing the understanding we have of the evolution of sex characteristics in animals.
Prof Hodgson ended with a fun facts quiz on February, inspired by his lecture taking place on the 29th which only occurs in a Leap Year.
Story by Jeannie McKeown
Picture by Judith Doubell