Period Rock? You’re calling me Period Rock now? Guys, seriously, I might look like a stone, but that doesn’t mean I have the heart of one. Why doesn’t anyone ever just call me Michael?”


Despite appearances, this is not some kind of cruelly bisected alien stone organism or a tomato thunderegg. This is Pyura chilensis, a sea creature that lives on the rocky coast of Chile and Peru. And if (like me, very recently) you’ve never seen one of these before, you’ll probably be interested to know that in Chile, they are fished commercially, and the locals eat them raw or cooked with salad and rice because apparently they’re delicious.

P. chilensis belongs to the Ascidiacea class of non-moving, sac-like marine invertebrate filter feeders that are otherwise known as sea squirts. They belong to the Tunicata subphylum, so-called because they wear thick ‘tunics’ made of tunicin, which is a hardy matrix of molecules that help the animal attach itself to a hard surface on which it will carry out its days. The insides of this tunic are lined with an epidermis and a muscular band, and inside these layers lies the main part of the animal.

P. chilensis has two siphons that connect the animal to the surrounding ocean through its tunicin – one for exhaling and one for inhaling. It eats by inhaling the water and filtering out the edible microalgae using a moving layer of mucus in its enlarged pharynx, or branchial sac, before exhaling the water back out the other siphon. The pharynx is connected to the animal’s digestive tract, which basically acts like a mouth.

Their blood is clear and, strangely, can accumulte extremely high qualities of a mysterious and rare element called vanadium. The concentration of vanadium in the blood of P. chilensis and other tunicates can be up to 10 million times that of the surrounding seawater. Just why and how these creatures are able to accumulate vanadium in such huge quantities remains unknown.

P. chilensis can often be found in densely packed aggregations of thousands or small handfuls of just a few, or they can be found on their own - in which case they must reproduce on their own, as there is no way of them moving to find a mate. This means P. chilensis is hermaphroditic, with the gonads of both a male and a female that can release eggs and sperm simulataneouly to meet as a fertile cloud in the surrounding water. If the sperm-egg collisions are successful, they will produce tiny tadpole-like offspring that will eventually settle onto a rock to grow into the adult form.

In 2005, biologists Patricio H. Manríquez from the Universidad Austral de Chile and Juan Carlos Castilla from the Pontificia Universidad Católica de Chile published a paper in the Marine Ecology Progress Series revealing for the first time the particulars of this creature’s reproductive habits (They also use the verb ‘selfing’ often and with glorious earnestness). They collected 30 sexually mature P. chilensis from habits in central and northern Chile and set them up in lab tanks as isolated and paired individuals. They wanted to assess the occurrence and success of fertilisation via these two types of reproduction followed by the settlement of the resulting offspring to a hard surface and their subsequent metamorphosis into adulthood.

First, the isolated individuals were placed in plastic bottles, and were left alone for 90 days, free to do all the selfing they wanted (YOLO). After this period, their body size relative to the amount of sperm in the water was measured for each spawning episode. Next, the researchers combined pairs either from the same population, or from two different populations, to see how well they would breed in comparison to the selfers. A third experiment saw them keep P. chilensis individuals in isolation for one to 16 months, to see if an extended period alone would improve the success of selfing. Finally, the researchers conducted ‘manipulated ferilisation’, which involved removing eggs from the specimens and fertilising them with extracted sperm in Petri dishes.

The results showed that P. chilensis is born male, before becoming cosexual – having both male and female gonads – in its adolescence as it increased in size. The researchers also found that given the choice - that is, if situated around other individuals - these organisms prefer to breed via cross-fertilisation, writing, “Given that more events of natural egg spawning followed by successful settlement and metamorphosis were recorded in our paired specimens and in our manipulated cross trials … it appears that cross-fertilisation predominates in this species.”

Manríquez and Castilla also found that a greater number of fertilised eggs resulted from the paired specimens, which suggests that cross-fertilisation, or reproducing with another individual, predominates because it is more effective. This assumption was strengthened by the fact that individuals that had cross-fertilised before being put in isolation took at least two months before successfully producing offspring via selfing. However, they were careful to note that while cross-fertilisation was preferred, selfing did not produce inferior offspring. “No perceptible differences in fertilisation, settlement and metamorphosis success among self and outcross progeny were found,” they reported. This suggests that when stuck alone in the ocean, selfing provides an advantageous opportunity for loner P. chilensis individuals to still pass on their genes.

Here's a video of a German man knifing some P. chilensis with great aplomb to the tune of a handful of angry YouTube villagers.