untitled
1. Life under a breaker; the high levels of turbidity and erosion in the hydrodynamic seabed. Photo source: Levi Sofen

Imagine you are a small sea creature beneath the water, with only the support of the sand to keep you fixed in place. You dig yourself within the sediment with all your might as each consecutive wave comes crashing overhead threatening to sweep you away. The sediment becomes increasingly unstable as the water erodes the sand. You feel yourself becoming dislodged. What would you do to stop yourself from being washed away?

A. Source: https://www.youtube.com/watch?v=vFb0XbXdhu0 

It is choppy in the subtidal zone

The exposed sandy shore make up 80% of the world’s coastline. The extremely hydrodynamic sandy shore experiences powerful waves, currents and erosion (image 1). The high level of water movement in this region is as a result of the change in bed level height as the water becomes shallower towards the shore (video A). In the subtidal zone, below the low water line, is dominated by erosion, loosening the sediment. The mobile sands found in the subtidal transition zone making the environment unfavourable to many species. During a storm the top 4cm of the seabed can be eroded away, dislodging any organisms who dare settle in these stressful conditions.

Ensis to the extreme

The American jack-knife clam, Ensis directus. Adult size ranges from 4- 20cm with the food of the clam out the bottom of the shell. The siphons of the clam are positioned at the opposing end of the shell. Photo source: Maria Hayden-Hughes
2. The American jack-knife clam, Ensis directus. Adult size ranges from 4- 20cm with the food of the clam out the bottom of the shell. The siphons of the clam are positioned at the opposing end of the shell. Photo source: Maria Hayden-Hughes

However, there is a species that concurs all the extremes, Ensis directus (image 2), the American razor clam. The narrow subtidal band along the shore from ~7m to 10m water depth is where E. directus is most abundant. Even under the hydrodynamic erosive conditions the American razor clam still manages to settle and establish within the mobile sand. Originally from North America, E. directus was first recorded in the eastern North Sea in the 1970’s, invading the English Channel in 1999. This invasive species has now successfully established itself becoming a highly abundant bivalve along coasts in the North Sea. The worry with most invasive species is the effect that the intruder will have on native species. There has been no indication that E. directus is competing with native species. The environmental conditions found in the current-dominated subtidal zone are too harsh for other species to successfully establish. Therefore, it seems that E. directus is occupying an otherwise vacant niche in the coastal ecosystem.

Put best foot forward

To survive in such an extreme environment the species has developed a physical trait to cope with the conditions. Research has found that E. directus is a very rapid burrower using the foot under a twoanchor system to immediately burrow deep into the sediment when disturbed (video B). The powerful foot of E. directus is able to hold the clam in place to avoid becoming dislodged by the strong water movement. As well as using the foot to burrow, the muscular, fleshy organ can be used to propel the razor clam through the water column to avoid predation or unfavourable conditions. By being highly mobile and an efficient fast burrower, the American razor clam is adapted to successfully inhabit this environment. High disturbance levels mean other species cannot cope under the conditions, therefore E. directus does not have other species to compete with for resources, explaining the rapid establishment of the species along our coasts.

B. Source: https://www.youtube.com/watch?v=xYiYyfeHFDA

Here to save the day

Ensis directus protruding from the sediment siphon feeding. This behavioural trait will alter the flow dynamics surrounding the clam, however it is unknown to what extent. Photo source: http://www.onderwaterwereld.org/library/nl-zout/bivalva/ensis%20directus
3. Ensis directus protruding from the sediment siphon feeding. This behavioural trait will alter the flow dynamics surrounding the clam, however it is unknown to what extent. Photo source: Silvia Waajen

The presence of E. directus in this extremely dynamic area could be highly influential on the stability and erodibility of the sediment, improving the durability of the coastline. Benthos can have a large impact on the coasts. The activities of organisms can change the environment altering the state of resources, controlling abiotic and biotic factors. These organisms are known as allogenic engineers. The behaviour of allogenic engineers, can alter the sediment dynamics and act as a bio-destabiliser. Bioturbation by benthic organisms such as Arenicola marina, the lugworm, rework the sediment, making organic matter available to other organisms increasing food availability. The lugworm inhabits a burrow in the sediment which is flooded by seawater oxygenating the surrounding sediment. Bio-irrigation alters the sediment composition, reducing the anoxic layer improving the environment for communities to develop. The actions of these organisms act as bio-destabilises.Other organisms known as autogenic engineers can act as bio-stabilisers. The presence and structure of the organism can alter the processes being carried out within the environment. The physical body structure of autogenic engineers can possibly provide protection from predation, competition, form nursery and feeding grounds by providing a complex structural network such as seagrass beds. The tube-building worm Lanice cochilega, protrude from the sediment in dense aggregations. The patches can reduce water velocities, currents and turbidity, changing the environmental conditions that can stabilise the sediment.

The American razor clam has been observed to protrude above the sediment surface on the tidal flats in the Netherlands, as few other species do (image 3). Similar to L. cochilega, the protruding structure could alter flow dynamics aiding sediment deposition and stabilisation. However, the burrowing behaviour of E. directus may cause erosion and sediment destabilisation. Overall, the presence of the American jack-knife clam could alter the geology, conceivably reducing coastal erosion as a bio-stabiliser, or intensifying the process as a bio-destabiliser.

There is 41% of the world’s population live and depend on the coast. However, the 70% of sandy shore is suffering erosion at an alarming rate (image 4). It is critical to understand the potential of E. directus to help build up and protect the coastline from erosion especially with the increasing threats of storm events, sea level rise. The presence of E. directus could reduce sediment movement in the subtidal environment acting as a bio-stabiliser and helping to reduce erosion of the sandy shore. Not all invasive species may mean bad news. Maybe the bad guy could end up being the hero that saves our coasts?

pacifica
4. Coastal erosion along a sandy shore eroding a cliff and undercutting people’s homes making them vulnerable to collapse. Photo source: Getty.
print

(Visited 309 times, 1 visits today)