Hamelin Cockle

Domain: Eukarya
Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Order: Cardiida
Family: Cardiidae
Genus: Fragum
Species: Fragum erugatum

When I wrote about Shell Beach, Australia, I mentioned the Hamelin cockle, Fragum erugatum. Today, I want to expand on what I wrote.

The Hamelin cockle is a bivalve that belongs to the phylum Mollusca, along with oysters, snails, and squids, to name a few. It’s native to the shallow shores of Western Australia, though it is prevalent in Shark Bay and Shell Beach.

Shark Bay is a hypersaline marine environment. Its seagrass beds restrict tidal movement, and the rate of evaporation is higher than the rate of precipitation, which makes the water really salty. In fact, the water is plankton-deficient because the high salinity makes it hard for plankton to survive.

So what does the cockle do for food? Isn’t it a filter feeder like many of its bivalve brethren?

Hamelin cockles are not strict filter feeders. Instead, they have a partnership with our favorite oceanic BFFs, zooxanthellae. Like coral, the cockle receives leftover food from the zooxanthellae in exchange for protection in well-lit waters. Fragum erugatum will siphon plankton from the water when they can, but it’s never enough to sustain them.

The soft body of the cockle is brown, and the photosynthetic algae live in the soft tissue. The shells are white and appear translucent in the light. Fun fact, zooxanthellae also help to collect calcium carbonate that the cockle uses to make its shell. The entire organism is less than 20 millimeters, which is a little smaller than an inch.

Hamelin cockles are hermaphrodites, meaning they have both male and female sex organs; however, they still need other individuals to reproduce. Between winter and spring, F. erugatum will release their gametes, or eggs, into the water to be fertilized by other Hamelin cockles. The fertilized eggs develop into zooplankton that float around in the water before they settle to the ground and further develop into cockles.

I find these bivalves to be every interesting. They entered Shark Bay over 4000 years ago and really put forth the effort to make the bay and Shell Beach their home. Most living things do not prosper in extreme conditions, especially in areas of high salinity. However, the Hamelin cockle not only adapted to the hypersaline water, but they prospered so beautifully that they left a noticeable mark in the local geology.

Four thousand years’ worth of cockle shells replaced the sandy beach of Shell Beach. Building material was made from the dense accumulation of these shells that, over time, became cemented together. It just blows my mind to think how successful these tiny little organisms are, and that makes them special!

Sources and links:
Ocean the Definitive Visual Guide made by the American Museum of Natural History
https://www.sharkbay.org/publications/fact-sheets-guides/hamelin-cockle/

Zebra Mussels

Domain: Eukarya

A colony of zebra mussels (Dreissena polymorpha), living in freshwater. Photo by Dr. Alex Mustard, find more at www.amustard.com

Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Order: Myida
Family: Dreissenidea
Genus: Dreissena
Species: D. polymorpha

Today, we’re going to talk about zebra mussels. We’re not going to talk about zebra muscles like I had originally written down on my blog schedule. Honestly, why would I talk about the muscles of a zebra? They’re not even aquatic!

I know that was a lame introduction. It just doesn’t have enough strength to land a clever opening—maybe it needs more mussels…

Okay, I’ll stop!

Zebra mussels, D. polymorha, are freshwater bivalves native to Eurasia. Bivalves are shelled creatures; specifically mollusks with two shells that close together, like clams and oysters. Zebra mussels are about an inch long and are shaped liked a stretched out “D”. They are named from the black, zigzag patterning on their shells.

Humans can be so creative with their naming schemes.

Zebra mussels have a relatively short life span, between 2‒5 years, reaching reproductive maturity at 2 years of age. Each female can produce up to a million eggs per year, spewing them into the surrounding water and using the currents to transport the eggs.

The reason I’m bring up D. polymorpha is because it is an invasive species in the United States and Canada. The mussels were first discovered in the early 1980s near the Great Lakes and are believed to have been transported by accident in the ballast water of a ship. Since then they have been found in the Great Lakes, the Mississippi and St. Croix rivers, and the Chesapeake Bay.

Why are the zebra mussels bad for these environments? Don’t they help filter the water in their surroundings, and isn’t that a good thing?

In their natural habitat their job as filter feeders is absolutely amazing; in other habitats, it can have devastating effects. In fact, zebra mussels are so efficient as filter feeders that they can clean a body of water of particulates in record time, faster than the native filter feeders. But this is not a good thing.

The environments that the mussels invade have a special balance that is maintained by the native populations of animals. If you change one aspect of that balance, then it creates a domino effect.

Let’s say that we have an imaginary river, the River Sága, which is home to large, healthy beds of freshwater bivalves called blue purses (not a real bivalve). In this river there are also a few species of fish that go there to spawn and where the juvenile fish live until they’re big enough to move on. One day, an old fisherman dumps water into the River Sága from his boat and unknowingly releases several thousand eggs of the zebra mussel. A couple of years later, the river is no longer the same. The once-healthy beds of blue purses are now completely covered in smaller bivalves, smothering the native species. The water of the river is the clearest it’s ever been, but downstream there are enormous patches of algae, and there are no fish to be seen. What was once a nice fishing spot for man and animal alike is now barren, save for the zebra mussels and the algae.

Zebra mussels, like any invasive species, are horrible for the environments that they infiltrate because they have no natural predators, and they often outcompete the native species. Because zebra mussels are so good at filtering the water, it makes it easier for predators to find their prey in the water, whether it’s a larger fish or a bird hunting the juveniles that have spawned there. And because zebra mussels reproduce so much, they can easily smother their competitors, becoming the dominate species of the environment and changing it for the worse.

Zebra mussels also have an impact on human property. They have been known to block the drainage pipes of factories. They can incapacitate boats by clogging pipes and engines, or even by covering the sides of the boat and making it too heavy to float properly. It can take an absurd amount of money to remove them, and we have to do it often because they regularly come back and are so hard to eliminate.

I wanted to talk about zebra mussels because they have been noticed in the Chesapeake Bay, which is an important part of my life, and because it helps introduce the topic of invasive species. From what I understand, there is not much you can do once the zebra mussels appear, only that we must strive to prevent their spread elsewhere. But this also means that there is a potential opportunity for you, because maybe you can find a way to remove them from their nonnative habitats.

More information can be found:
https://www.chesapeakebay.net/discover/field-guide/entry/zebra_mussel
https://www.tn.gov/twra/fishing/twra-fish-species/zebra-mussel.html
https://www.invasivespeciesinfo.gov/profile/zebra-mussel
https://www.usgs.gov/faqs/what-are-zebra-mussels-and-why-should-we-care-about-them?qt-news_science_products=0#qt-news_science_products
https://www.nps.gov/articles/zebra-mussels.htm

Shell Beach, Australia

Not every beach is made of sand.

In fact, there are beaches that are made up of volcanic rocks, pebbles, shells, and coral. I’ve been lucky enough to snorkel from a beach that was made up of dead, broken pieces of coral that were piled up from thousands of years of heavy coastal storms. What materials a beach is comprised of can tell you a lot about the area, including how much energy is involved through wind and wave action, what the waves are like, and the history of the beach—but those are topics for another post!

Today we head over to Australia, the continent of many species that could and will kill you. However, the beach that I’ll be talking about is probably one of the safest places to swim, especially for those who aren’t strong swimmers.

Shell Beach, is found within Western Australia’s Shark Bay, making it an embayed beach. This beach is unique for a few reasons.

The first unique feature is that the immediate water has a salinity that is twice that of the ocean! This occurs because the rate of evaporation is greater than the rate that rain falls, so more water is lost due to the heat than is replaced. When the salt water evaporates, the salt stays behind. Add to this the fact that a massive sea grass bed sits at the mouth of the bay, blocking a lot of tidal flow, and this makes for a super salty environment.

But it’s okay, because this leads into the second unique thing about this place. The salty water conditions have created a safe haven for a specific kind of shelled creature, Fragum erugatum, which is a species of cockle. A cockle is a bivalve—its shell is divided into two halves—and it is very similar to oysters and clams.

The f. erugatum cockle can survive the hypersalinity of the waters of Shell Beach, but its natural predators cannot, meaning that this species thrives in this place. In fact, they’ve survived here in L’Haridon Bight for thousands of years with no decline in their population.

How can we tell? When shelled organisms die, their bodies are consumed or they decay, leaving only their shells behind. So when f. erugatum cockles die, their shells remain in the area, and over thousands of years their shells eventually replaced all the sand and other sediments of the beach.

Today, Shell Beach stretches for about 44 miles and is comprised of only cockle shells, and the shells extend about 26−30ft down below the immediate surface. That’s a lot of shells! The shells even make up the sea bed and stretch quite a ways into the bay. On the back part of the beach there are so many shells that they’ve fused together in places to form large hard shapes, which were mined for a while to make decorative blocks until Shark Bay became a protected site.

I think this beach would be a cool place to go because it is so different. Not a lot of creatures can survive the water, so you don’t have to worry so much about potential animal accidents. And the water is easy to float in, much like the Dead Sea in Jordan, so it’s a great place to relax and float in peace. Also, the beach is a pretty snow-white color and was created in such an inspiring way, at least to someone like me!

Sources and cool links:
Ocean: The Definitive Visual Guide made by the American Museum of Natural History
https://www.australiascoralcoast.com/destination/shell-beach
https://www.atlasobscura.com/places/shell-beach
https://parks.dpaw.wa.gov.au/park/shell-beach