Sexless Starfish Reveals Why Sex Is So Great
Single parenting takes on new extremes for certain starfish that are hermaphrodites -- male and female at the same time and, in some cases, self-fertilizing. The species faces high risk of extinction, according to new research.
The dire situation faced by the non-mating starfish, Parvulastra parvivipara and Parvulastra vivipara, helps to explain why so many organisms, including humans, have sex. Genetic diversity and the dispersal of youngsters support population growth.
The plight of the starfish, documented in the latest issue of the journal Biology Letters, reveals how a life without sex but with self-fertilization could result in eventual oblivion.
“There are quite a few reasons why these species are vulnerable,” senior author Michael Hart of Simon Fraser University’s Department of Biological Sciences told Discovery News. “The whole species could be wiped out.”
Hart and his team studied the starfish, which are restricted to high intertidal pools of South Australia and Tasmania. These starfish also go by the nickname "sea cushions," since they look a bit more like a cushion than a star when viewed from the side.
Most adult starfish of other species do reproduce via a separate male and female. Females usually produce eggs that males fertilize in the seawater. At that point, the fertilized eggs develop and grow before becoming little starfish that will attach themselves to the substrate and start the whole process over again.
The extreme self-parenting starfish often have no such male-female interaction.
As a hermaphrodite, the individual produces its own eggs, which are then fertilized right inside the body, likely with that same individual’s own sperm.
“The fertilized eggs are not sent out into the environment for development, and instead the parent keeps the developing embryos inside its body, where they complete their embryonic development and become juvenile sea stars (starfish),” Hart explained.
He added, “In some species, these juvenile sea stars eat each other, and grow to become relatively very large.”
A family with little or no sex and with kids that eat each other predictably has problems.
Loss of genetic diversity and low potential for dispersal keep the starfish stuck in their particular territories.
“Their habitats are in the highest intertidal zone, which is already a pretty stressful place to live,” Hart said.
Adults of these species are small, only about 1 centimeter in diameter. They have few offspring at a time and each only lives anywhere from a few years to about a decade.
“Because they lack genetic variation, these populations stuck in place probably are not very likely to evolve new adaptations for living in those rapidly changing environments,” Hart said.
The starfishes’ unusual style of reproduction probably evolved 1-2 million years ago “as a way to ensure that all eggs get fertilized,” he said. Mate choice must have been slim to non-existent in some cases, leading to the self-fertilizing.
Many plants do something similar, in situations where pollen is scarce. Certain snails, such as the species Physa acuta, will also opt for extreme single parenthood if they do not find a partner.
“They can’t wait for a mate indefinitely, especially if the risk of mortality is high,” said West Chester University biologist Josh Auld, an expert on what's known as “the evolution of selfing.”
Auld continued, “Self-fertilization is a last-ditch effort.”
Starfish that multiply in such a way can be raised in laboratories, but populations in their native habitats “seem to be marked for extinction,” Hart and his team conclude.