Off Japan’s coast: An ancient animal symbiosis comes back to light
Here’s something new under the sea. Well, not new exactly, more like returned to the light. Scientists in Poland said this month that they’ve found non-skeletal corals growing on marine animals called sea lilies, or crinoids, on the floor of the Pacific Ocean, off the coast of Japan. It’s surprising because the symbiotic relationship between these two sea animals hasn’t been seen in the fossil record for 270 million years. Geologist Mikolaj Zapalski of University of Warsaw led the study, which is published in the peer-reviewed Palaeogeography, Palaeoclimatology, Palaeoecology for June 15, 2021.
Michelle Star explained in ScienceAlert on May 10:
During the Paleozoic era, crinoids and corals seem to have gotten along very well indeed. The seafloor fossil record is full of it, yielding countless examples of corals overgrowing crinoid stems to climb above the seafloor into the water column, to stronger ocean currents for filter-feeding.
Yet these benthic besties disappeared from the fossil record around 273 million years ago, after the specific crinoids and corals in question went extinct. Other species of crinoids and corals emerged in the Mesozoic, following the Permian-Triassic extinction, but never again have we seen them together in a symbiotic relationship.
Never again … until now. In 2015 and 2019, the Polish and Japanese research team collected specimens collected the crinoids and corals from depths exceeding 100 meters (300 feet) near the Pacific coasts of the islands of Honshu and Shikoku in Japan. The specimens were found in waters as deep as 146 meters (479 feet). Two different species of coral were found – a very rare hexacoral of the genus Abyssoanthus, plus Metridioidea, a type of sea anemone – growing from the stems of living Japanese sea lilies (Metacrinus rotundus). According to the website Paleoaerie.org, hexacorals are:
… noted for having six main septae [radiating vertical plates] and six main compartments, all of which are further subdivided with minor septae, thus having six-fold symmetry (you can draw six lines through the center of the organism in which one side is a mirror image of the other).
The scientists analyzed them using micro-CT scanning, a way of using X-rays to create cross-sections of physical objects that can be used to recreate a 3D model without destroying the object being studied.
… unlike their Palaeozoic counterparts, recent corals do not modify the host’s skeleton. Despite such differences in the skeletal record, the newly discovered coral-crinoid associations may serve as a good model of relevant Palaeozoic interactions.
Science Alert explained that this difference could actually explain the huge gap in the fossil record. Soft corals don’t usually leave fossils, so if corals were growing on sea lilies without altering their structure, it would likely be lost to time.
This also means that the new find can help researchers better understand the relationship in Paleazoic times. The study authors wrote:
Understanding of the ecology of past ecosystems is impossible without a deep knowledge of their modern analogs.
Now, they have their analog, a modern-day counterpart to an ancient symbiosis. The study authors continued:
These specimens represent the first detailed records and examinations of a recent syn vivo [living together] association of a crinoid (host) and a hexacoral (epibiont), and therefore analyses of these associations can shed new light on our understanding of these common Palaeozoic associations.
Bottom line: Scientists said in May 2021 that they’ve found a surprising modern-day symbiosis between two undersea animals – sea lilies and corals – that had been gone from the fossil record for 270 million years.