Belgian mussels have developed stronger shells over the last hundred years. More calcareous shells protect them better from crabs’ claws and seagulls’ beaks. These predators have increased significantly in number during the last fifty years. ‘Belgian mussels adapt surprisingly well to new environmental conditions’, says biologist Thierry Backeljau (RBINS). ‘They might be more resilient to climate change than we think.’
An international team of biologists analyzed the calcareous structure of mussel shells that were collected along the Belgian coast this last century. You would expect the shells to become thinner because more acidic seawater - due to the increase in CO2 in the atmosphere - breaks down calcareous matter. But the team observed a marked increase in the calcification of mussel shells.
The main causes of the more calcareous mussel shells are changes in predators. The dog whelk (Nucella Lapillus) disappeared at the end of the seventies, after which the number of crabs and seagulls increased during the 1980s and 1990s respectively. This led to a pressure on mussels to develop thicker shells, protecting better against the crabs’ claws and the seagulls’ pecking beaks. According to the scientists, this might mean that our Belgian mussel populations can better cope with future climate changes than previously thought.
A special collection
The researchers evaluated a total of 268 mussels that were collected between 1904 and 2016 on the breakwaters between Nieuwpoort and Ostend. The specimens collected between 1904 and 1987 are part of the collections of the Royal Belgian Institute of Natural sciences (RBINS). This unique collection of one single species is composed of ‘wet’ specimens (shells and body tissue, preserved in ethanol), and ‘dry’ specimens (shells only). They were collected during monitoring programs over the past century. ‘This mussel collection is unique,’ says biologist Thierry Backeljau (RBINS), co-author of the study. ‘It may sound paradoxical, but to have such an extended collection of an animal that is so ubiquitous is rare. Researchers usually focus on exceptional species.’
Dog Whelks and acidification
The dog whelk is an important predator of mussels in the North Sea. Dog whelks make a small hole in the mussel shell, through which they suck the mussel empty. To do this, they must drill through the dark, organic outer layer of the mussel: the periostracum. Mussels with a thicker periostracum are better protected against this type of predator. This created a selective pressure on mussels, favoring a thicker periostracum. The acidification of the North Sea - which breaks down calcareous matter - led to additional pressure in favor of more periostracum, offering better protection to the underlying calcareous layer.
But as of the late 1970s, things changed. The dog whelk population suddenly declined sharply and even died out locally due to the use of tin based paint on ship hulls, particularly tributyltin hydride (TBT). The selection pressure on mussel populations in favor of more periostracum decreased.
Crabs and seagulls
In the meantime, average spring and summer temperatures of North Sea surface waters continued to rise, in line with global ocean trends. The input of minerals and nutrients from the land also increased steadily over the past sixty years due to the discharge of fertilizers and wastewater into rivers (eutrophication). The result: an increase in the amount of algae and thus a greater food supply for all kinds of organisms, including the larvae of decapods such as crabs and lobsters. As a result - helped by overfishing of cod, which feeds on those larvae - the number of crabs and lobsters skyrocketed from the 1980s.
Just like the dog whelk, crabs and lobsters are fond of mussels, which they crush with their claws. Protection by a periostracum makes little difference against this, but a stronger, more calcareous shell does. Moreover, depositing calcium requires less energy than producing a periostracum. Thus, a new selection pressure arose, in favor of more calcareous shells.
This selective pressure was reinforced by the exponential population growth of seagulls in the 1990s, due to the increased number of decapods. The breeding season of seagulls (May and June) coincides with the peak of decapods, which are an important food source for the chicks. But seagulls also eat mussels and increased the selection pressure in favor of a calcareous, solid shell.
Hope for the future?
This study shows that the global effects of climate change, such as ocean acidification, do not simply apply on a local scale. Complex, local changes in ecological conditions can lead to biological outcomes that appear to conflict with predictions on a global scale.
‘The Belgian mussel populations seem able to adapt their shell formation to a wide range of local selection pressures and perturbations’, says Backeljau. ‘This gives hope for the future: mussels may be better armed against climate change than we thought.’ This research also illustrates the importance of natural science collections, such as those of the RBINS, in the study of, and fight against, climate change. ‘Collections and archival specimens help us investigate long-term effects of changes in the environment, which is difficult with experimental studies. It is a powerful research method that, as shown here, can yield surprising results and help us get a clearer picture of historical ecological changes’, concludes Backeljau.
Consider this your o-fish-al welcome to Monterey Bay, Hoodwinker Sunfish! You certainly had us fooled
Divers in Monterey Bay have photographed two hoodwinker sunfish this year—the first confirmed sightings of this new species of sunfish in Central California!
A hoodwinker sunfish being cleaned by señorita wrasses off of Pacific Grove. Video: Joe Platko
The word “tecta” is Latin for hidden—a perfect moniker for a hoodwinker. Mola tecta were thought to live mainly in the cold waters around Australia, New Zealand, South Africa and Southern Chile.
This sighting of Mola tecta was tantalizing for sunfish researchers: Are hoodwinkers new arrivals to the area, carried by Chile’s cool Humboldt current and somehow punching their way through the equator and into our temperate waters due to some climatic abnormality? Or have hoodwinkers been around these parts for some time, hiding in plain sight until Marianne’s discovery gave attentive observers the right clues to look for? Maybe a little bit of both? Something else entirely?
Mola mola, known as the common sunfish, in the Open Sea display at the Aquarium
Mola tecta found in Monterey Bay just offshore of Pacific Grove. This was the first confirmed sighting of a Mola tecta in Monterey Bay. Photo: Jr Sosky
Key characteristics of Mola tecta for identification. Photo: Jr Sosky/Marianne Nyegaard
A blessing in disguise
Whatever the case, there are now at least two more Mola tecta confirmed here in California, and the first ever identified in Monterey Bay.
In early August, a merry band of underwater photographers came across a large ocean sunfish being cleaned by señorita wrasses at Eric’s Pinnacle, a rocky outcrop off Lover’s Point in Pacific Grove.
Our (erroneous) post on Twitter. More social media copy mistakes that lead to discoveries of new sunfish species in our backyard, please!
Mola mola is no stranger to the Monterey Bay—we see youngsters and heavyweights throughout the year just offshore of the Aquarium, and we’ve frequently had them on display in the Open Sea.
Weighing nearly 5,000 pounds and spanning over 11 feet from tip to tip, Mola mola is one of the heaviest bony fishes in the sea (its Western Pacific cousin, the bumphead sunfish Mola alexandrini is just a touch heavier in the record books.)
Something fishy about that fishy…
Content with our content, we looked to see what you all thought of this magical “Mola mola”—and that’s when things got exciting!
A comment right here on Tumblr by Drop Science mentioned that this fish looked more Mola tectathanMola mola. The two are remarkably similar in appearance, but there are a few tells. Most noticeably, a Mola tecta caudal fin is is divided by a smooth band projecting backwards to the fin’s edge.
Intrigued, we forwarded more images from Joe Platko and his dive buddy Jr Sosky to Senior Aquarist and resident mola expert Michael Howard.
Michael has been instrumental in our ocean sunfish program at the Monterey Bay Aquarium over several decades, pioneering training methods, specialized diets and tracking programs for these megafish. The Aquarium is the only one to successfully display Mola mola in North America.
Growing from just a few dozen pounds to several hundred, our resident sunfishes are released back to the wild. Once back in the bay, Michael’s satellite tags have revealed that Mola mola may migrate very far up and down the coast, and spend considerable time diving into the deep sea to feed on a varied diet of jellies, squid, crabs and other fare.
After reviewing the images, Michael thought there was certainly the chance that a hoodwinker had been found. He got us in touch with Marianne Nyegaard herself, and she confirmed that these were indeed the first images of a live Mola tecta in Monterey Bay!
Then, just three weeks later, diver Wei Wei Gao happened upon another Mola tecta off of Cannery Row!!
A hoodwinker sunfish filmed off Cannery Row. Video: Wei Wei Gao
A tecta-nic shift in our sunfish understanding!
In email exchanges that used up both of our yearly supplies of exclamation points, Dr. Marianne remarked that these sightings show just how little we know about one of the ocean’s most iconic fishes.
Michael is now diving into our records to see if there’s a chance we have had a Mola tecta hidden in our studies. And as for us, we’re buzzing with excitement at the discovery of this neighbor in our backyard, pleasantly deceived by a hoodwinker sunfish, and awestruck by the limitless wonder and mystery of our beloved Monterey Bay.
The first-ever confirmed Mola tecta in Monterey Bay being cleaned by señorita wrasses. Welcome to the neighborhoodwinker! Video: Joe Platko.
