Sea Squirts' Complex Sex Lives
Ascidians’ Sexual Development Offers Clues to Human Sexuality
Sep 11, 2008
Sea squirts are sedentary marine invertebrates, attached to rocks for most of their life cycle. Despite being unable to move, they can control their fertility to take advantage of good conditions or avoid bad conditions for reproduction.
Australian Angela Crean, researcher and PhD student with University of Queensland’s School of Integrative Biology said the organisms could tailor their reproductive cells depending on the level of competition in the sea.
She explained the common sea squirt studied in Queensland’s Moreton Bay area, Styela plicata, is hermaphroditic.
“Initially they function as males, but gradually increase investment in female gonads as they age and increase in size.
“When there are lots of competing males trying to fertilise the eggs of females, males produce larger, more competitive sperm that live for longer,” Ms Crean said.
“Similarly, when females detect that there are too many males competing for her eggs (too many sperm can kill the eggs of some organisms), the females ‘play hard to get’ producing smaller eggs that are harder for searching sperm to find.
Stuck on a Rock? Change Your Sexual Attributes
“These changes make sense if you’re stuck to a rock like sea-squirts are," she said.
“If you can’t run away from a highly competitive environment, you must therefore make your gametes more competitive in order to get a higher chance of successfully reproducing.”
Sea squirts ‘mate’ by releasing their eggs and sperm into the water. Very little is known of their natural spawning cycle.
“However, it seems that they spawn late in the afternoon, with free-swimming 'tadpole' larvae hatching out the next morning and settling throughout the day," she said.
"They appear to be capable of reproducing throughout most of the year in Moreton Bay with a short break over the winter months.”
Population Density Changes Behaviour
Because they are attached to rocks, the animals were easily studied in the ocean. About 400 were observed in situ. Another 50 were taken to the laboratory, where densities of adults in a given area could be varied.
Over five months the density of the animals was studied in various concentrations. The development of sperm and eggs in size and quantity was recorded.
Densities varied from one to 15 adults in restricted area.
The eggs and sperm were collected from the adults and examined by microscope for variations in size.
The Link to Human Reproduction
The results of her experiments gave clues why male animals produce millions of sperm while females generally produce fewer large eggs, Ms Crean said.
“Because the sea squirts reproduce by the ancestral mode of reproduction, it gives us an indication as to how competition between males led to all sorts of reproductive strategies including the one used by humans: internal fertilisation.”
She agreed that one cannot extrapolate directly from sea squirts to the very different reproductive strategies of humans.
“However, what these animals can tell us a little about is how sex evolved in the first place, “ she said.
“It is traditionally thought that sperm competition led to the evolution of 'males' producing many tiny sperm, and 'females' producing far fewer large eggs.
“However, my research suggests that sperm limitation (when there are not enough sperm to fertilize all the eggs) may have also been a strong selection pressure for small sperm and large eggs.
“Sperm limitation may have also been a strong selection pressure leading to internal fertilization,” she concluded.
Her study, Gamete plasticity in a broadcast spawning marine invertebrate, which is part of her doctorate, was published in the September 9, 2008 edition of Proceedings of the National Academy of Sciences.
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