Deep beneath the ocean lie oddities beyond imagining, and among them is Xenoturbella, a deflated, crawling, mollusk-eating sack. A pinky-purple one.
Two species of this curious animal have long been known, but in February scientists revealed four new species encountered over the course of 12 years of exploring the eastern Pacific Ocean. Together, these new specimens have helped them to figure out what the heck Xenoturbella might be.
Whatever it is, it is very simple. Like a jellyfish, it is basically a bag, although it lacks the stinging tentacles, pulsating body, and rave lighting commonly found along with jellyfish.
If you are an upstanding member of Xenoturbella, you will also have furrows. One goes around the head-ish portion of your body, and several extend from stem to stern, giving the creatures their Latin-American pastry look.
The entrance to Xenoturbella is small compared to a jellyfish, whose formal entry is famously large. Xenoturbella’s mouth (and anus) is a small hole found on its underside. It has no organs other than a gravity-sensing statocyst that works a bit like an inverse bubble level (I wrote about the awesomeness of statocysts before here). Since Xenoturbella does in fact have a top and a bottom, knowing up from down is probably important enough to warrant a device for measuring it.
There are neurons in Xenoturbella, but they are not organized into higher level networks like ganglia or brains. Since it has no proper ovary, Xenoturbella simply makes eggs and sperm all over its body anywhere it feels like it. ‘Cause hey – why not? Xenoturbella does not need your “rules”.
Other than that, Xenoturbella's just a sheet of cells with two layers: epidermis, and gastrodermis. The first is for protectin’, and the second is for digestin’. What they’re digestin’ is bivalve mollusks, which seems to be their food of choice. Xenoturbella was initially mistaken for a shell-less mollusk because they were so contaminated with their food’s DNA. How flacid Xenoturbella might go about liberating those mollusks from their extensively fortified shells remains a mystery to me.
Two species of Xenoturbella were already known from the waters off Sweden, discovered decades ago. But a new paper published in Nature in February by scientists at the Scripps Institution of Oceanography at the University of California, the Western Australian Museum, and the Monterey Bay Aquarium and Research Institute describes four new species, all encountered in the Pacific Ocean.
When scientists who discovered the four new species compared the sequences of the gene cytochrome oxidase I from the two Swedish species, they concluded they are probably one in the same. Thus, the discovery of four new species in a relatively short time -- compared to only one discovered in the entire 20th century -- is both surprising and exciting.
The new species were discovered in Monterey Canyon off the coast of California, and in the Gulf of California. The Gulf of California, is not, in fact, located in California, but instead is the long skinny body of water between the Baja Peninsula and the rest of Mexico also called, more poetically, the Sea of Cortez.
Two species were found near deep-sea cold-water hydrocarbon seeps (where methane and other hydrocarbons seep to the surface, creating brine pools that support life) around 5,650 feet and 9,500 feet. Another species was found near a deep-sea hot water vent at around 12,000 feet, while the fourth was found near a whale carcass at much shallower 2,070 feet.
The largest species, found marauding a clam field near the cold seep, was collected by the colorfully named “slurp system” beautifully demonstrated in this raw exploration footage documenting their discovery, which is worth watching for many other reasons as well. It was dubbed Xenoturbella monstrosa, and measured almost 8 inches (20 cm) long. That's enormous for a Xenoturbella – at least among the species we’ve encountered so far.
The vesicomyid clam field (such clams are often found near cold seeps) they were found in was full of them on discovery. Yet when the scientists returned multiple times, they had vanished. When X. monstrosa was found, the clams were making enormous clouds of mucus, leading the authors to speculate they were dying or spawning, although what relationship that might bear to Xenoturbella’s presence is unknown.
Xenoturbella churro, four inches long and named after the fried dessert of similar fluted appearance, is known only from a single specimen collected within 12 inches of an X. monstrosa in the Gulf of California. X. profunda, up to about six inches long, was on sediments with bacterial mats and vesicomyid clams near a hydrothermal vent in the Gulf of California, as you can see in this footage of the discovery. The video also shows the way in which Xenoturbella trundles along the seafloor, although the video has been speeded up to six times normal speed so you can notice it.
The much smaller (1 inch long) and shallower X. hollandorum was found on sediment near the bones of a grey whale in the Monterey Submarine Canyon. The three larger species all seem to be related to each other, while the smaller species, X. hollandorum seems to be more closely related to the single small species found in Swedish waters, X. bocki.
The animals’ bodies held some surprises. All four of the new species had a feature that had not been previously recognized: a mysterious branching network on the belly.
If the authors have ideas about what it might be for, they did not discuss them in the paper. Some sort of system for increasing the absorptive surface area of the digestive tract (and therefore the amount of food that can be absorbed) seems most likely to me, but there are many other possibilities.
The scientists also compared the genes that were actually transcribed to make proteins (as opposed to all the genes simply present in the Xenoturbella genome) to check the evolutionary relatedness of these species to the universe of other animals. This analysis supported earlier studies that found that Xenoturbella is a sister group to the acoelomorphs, extremely simple animals that do not even have a body cavity and share Xenoturbella's habit of making gametes anywhere they feel like it.
More importantly, the new analysis suggested the two groups are the earliest bilaterally symmetrical animals to have evolved that survive today. It is remarkable to think that these flabby, colorful bags -- or something possibly similar to them – marked the beginning of the march toward Left and Right that has proved so popular -- and dominant -- among animals throughout Earth's history.
Rouse, Greg W., Nerida G. Wilson, Jose I. Carvajal, and Robert C. Vrijenhoek. "New deep-sea species of Xenoturbella and the position of Xenacoelomorpha." Nature 530, no. 7588 (2016): 94-97.