Antarctic bivalves have surprised scientists who have discovered that the molluscs switch sex.
The reproduction of Lissarca miliaris was studied in the 1970s and the species was first described in 1845.
But their hermaphrodite nature had remained unknown until they were studied by scientists from the National Oceanography Centre, Southampton, UK.
Researchers suggest the molluscs could switch between the sexes to efficiently reproduce in the extremely cold ocean.
The results are published in the journal Polar Biology.
“The previous reproductive study only looked at the large eggs and broods,” said PhD student and lead author Adam Reed.
Antarctic bivalves have surprised scientists who have discovered that the molluscs switch sex
This earlier work showed how females brood their young for up to 18 months, from “large yolky eggs” to “fully shelled young”, and found that females can support as many as 70 young inside their hinged shell.
But concentrating on reproduction at a cellular level, Adam Reed and colleagues discovered that the eggs were actually present in males.
“Curiously, we found huge numbers of very small eggs in functional males, which appear to be far higher in number than an individual could brood throughout the life of the animal,” he said.
The team suggested that the bivalves reproduce as males while they are still in the “small” stages of development, switching to female organs once they are large enough to brood a significant number of eggs.
“At present the traits we describe are unusual for Antarctic bivalves, but in 10 years perhaps this will be common too,” said Adam Reed.
“Hermaphroditism is not necessarily uncommon in Antarctic bivalves, and with many species still to study there may be many more to describe.”
Brooding meanwhile is a relatively common reproductive trait in Antarctic invertebrates and has been linked to the extreme conditions.
“Brooding is common for small bivalves and has been discussed for many years in Antarctic biology,” said Adam Reed.
“Large yolky eggs that are brooded have much lower mortality than small planktonic larvae, but fewer are produced.”
He explained that in extremely cold environments, development is slowed down so feeding larvae becomes a more exhaustive task.
“Brooding reduces the need for long periods of feeding”, according to Adam Reed, making it a more efficient strategy for many Antarctic invertebrates including bivalves and echinoids.
The researcher suggested that the bivalves may be further maximizing their efficiency when it comes to reproduction.
“We also found that after males become female, the male reproductive tissue persists for a long time,” he said.
But for now, the bivalves can maintain their mystery because scientists are restricted to studying them during the months that staff is based at the British Antarctic Survey’s remote research station.
“Perhaps they may alternate their sex so they can continue to reproduce as males while brooding their young for 18 months?” Adam Reed theorized.
“The study highlights how much we do not know about some of the common invertebrates living in the Antarctic, and how much research there is still to do.”
One of the strongest lines of evidence that dinosaurs were cold-blooded, like modern reptiles, has been knocked down.
Prior studies of dinosaur bones uncovered what are known as “lines of arrested growth”.
The creatures were presumed to be cold-blooded because modern cold-blooded animals show these same lines.
But scientists reporting in Nature have studied the bones of 41 modern mammal species from around the world, finding everyone had these lines as well.
A number of discoveries in recent years have challenged the 40-year-old notion that dinosaurs were cold-blooded.
But because soft tissues such as organs and skin are not preserved, much of what is known about dinosaurs must be inferred from their bones, and comparisons made with modern animals that can be studied in greater detail.
Lines of arrested growth, or Lags, occur because organisms tend to suspend their growth and rally their resources during seasonal periods of environmental stress such as cold or dry conditions.
One of the strongest lines of evidence that dinosaurs were cold-blooded, like modern reptiles, has been knocked down
This forms a boundary from one season to the next as growth resumes when conditions are more favorable.
They are familiar in creatures such as molluscs, whose slow annual accumulations can be seen as ridges in their shells.
Lags have also been found in the bones of reptiles and amphibians and have until now been assumed to be limited to ectotherms – cold-blooded animals – that are more subject to the whims of harsh environments.
Meike Koehler of the Catalan Institute of Palaeontology in Barcelona and her colleagues were therefore surprised by what they found.
“Originally this was not a paper that we aimed to do,” Dr. Meike Koehler.
“We were very curious to know how environmental conditions and changes affect bone growth in fossil and extant mammals, to get a good idea about… how they may have coped with these changes in the past.”
As the team studied the thigh bones of animals from all over the world – ranging from the Svalbard reindeer in the Arctic to muntjac deer species from South Asia – Lags showed up in every one.
“These lines of arrested growth have been used a lot in dinosaurs, but nobody has ever had a really deep look at mammals,” Dr. Meike Koehler explained.
David Weishampel, a palaeontologist at the Center for Functional Anatomy and Evolution at Johns Hopkins University School of Medicine in Maryland called the new work “a wonderful paper” and said it was a welcome addition to the debate.
“I think most (palaeontologists) regard dinosaurs as being [warm-blooded] but there’s a lot of waffling in the data that appeared before that wasn’t conclusive,” he said.
“It’s about time we have a connection between the modern bone histology and fossil bone histology, through a very nice ecological and metabolic comparison.”
While Prof. David Weishampel considers it a closed case, Dr. Meike Koehler herself is more reserved about the result.
“I don’t think that this debate is really settled,” she said.
“But this is the first time that you can say that Lags do not say anything about warm- or cold-bloodedness.”
She and her team will go on and put the Lags to use in studies of modern animals instead.
“It’s like dendrochronology – the rings in trees. You can do skeletal chronology in bones and infer things like longevity, age at maturity, juvenile states – traits which are very, very important to get an idea about the health of a population and whether it is vulnerable.
“It is very good to know now that mammals do show these Lags and we can use them in the same way that we do in amphibians and reptiles to understand the situation of a population.”
