Nature

Tokyo bayside beaches are a trove of molluscs and crustaceans

A palette of grey and mud-green mollusc and crustacean shells cover the sandy shores of Tokyo Bay, together forming an intricate ecosystem with other less visible organisms and non-living components of their habitat.

Tokyo Bay was once a major hotspot for Tokyo’s fishing industry. The water area was significantly reduced in the 1960s when a major land reclamation project to develop Ota Market commenced. A large number of birds started visiting the ponds that formed as a result of rainwater collecting on pockets of the construction site. As Tokyo modernised over the decades, Tokyo Bay changed from being a fishing port to an industrial zone. Despite the economic rationale to develop every last bit of valuable land surrounding the bay, locals and bird enthusiasts fought hard to conserve the area that later became known to be Tokyo Bird Park.

The far east side of Tokyo Bird Park‘s East Wing is a small estuarine beach. Freshwater from Arakawa, Edo, Obitsu and Yoro River empty into Tokyo Bay and mix with the influx of seawater from Pacific Ocean, creating an unique habitat that is ideal for certain types of intertidal molluscs and crustaceans that thrive in brackish water.

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This photograph is of reed grass (of the genus Calamagrostis) on the beach in the East Wing of Tokyo Bird Park

Barnacles are a common site on estuarine shores. Two key types of barnacles include the goose barnacle and the acorn barnacle and they are often mistakenly thought to be closely related to oysters and mussels as they are also sessile during their adult phase and are often seen inhabiting the same substrate. However, they are actually a type of crustacean and are more closely related to crabs.

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This photograph is of barnacles and mussels that are inhabiting eroding tree stumps on the beach in Tokyo Bird Park.

Another key similarity between barnacles, oysters and mussels is that they are all filter feeders, albeit the exact feeding mechanism utilised differs slightly. While oysters and mussels draw water by flapping organelles (Cilia) attached to their gills, barnacles release their modified legs (Cirri) into the water and subsequently draw plankton into their shell for consumption.

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This photograph is of acorn barnacles on an eroding tree stump during low tide. Barnacles are well evolved to prevent water loss during low tide when they are exposed to sunlight  – their calcite shells are impermeable and the two plates close over the aperture when not immersed in water.

Although competition for food between barnacles and mussels is fairly indirect due to their filter feeding tendencies and sessile nature, they compete directly for space as both organisms require a substrate to settle on during their adult phase. Barnacles grow faster than mussels and are better at colonising substrate higher up the water column. As such, larger barnacles are commonly seen further up tree stumps in the intertidal zone, where they avoid displacement and have more space to grow.

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This photograph is of a cluster of mussels that have settled on an eroding tree stump at the beach in Tokyo Bird Park

Oyster larvae, on the other hand, commonly settle on shells of other oysters and form an oyster reef. Oyster reefs have recently received a lot of attention due to the ecological benefits that they bring. Oysters are recognised as a keystone species as they provide homes to a wide range of organisms and also clean water by filter feeding. They consume nitrogen containing compounds, phosphates and bacteria, thereby reducing eutrophication and pollution. Ecologists and scientists around the world are beginning to create oyster colonies around harbours in attempt to clean the environment and boost biodiversity.

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This photograph is of a bed of oysters on the beach in Tokyo Bird Park.

Who else benefits from an increase in oyster population? Crabs are common predators of intertidal oysters. Coincidentally, healthy and well-fed crabs then go on to reproduce and release thousands of larvae into the water, which form plankton and are later consumed by adult oysters – what a food web!

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This photograph is of an Asian Shore Crab (Hemigrapsus sanguineus) resting on an oyster. It’s so still that there is a small insect resting at the top of its right claw. 

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This photograph is of a hole that had been dug by the Asian Shore Crab. Rather than flinging loose sand out of the hole, the Asian Shore Crab rolls balls of sand to the surface near the entrance. 

 

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