Seawalls in urban harbours are doing more than just protecting coastlines from waves and preventing erosion – they are hosting some weird looking invasive and non-native species (INNS) who are now major components of our coastal waters. Seawalls provide perfect substrate for early-colonising and fast-growing INNS (which is not a good thing!).

Do you know that feeling you get when you approach an intertidal seawall during a spring low tide – the feeling that’s telling you there are going to be loads of INNS living on the surface of that wall? I love that feeling. I know – as a coastal ecologist specialising in INNS – that I really shouldn’t love that feeling. In fact, my job is to stop the spread of these species. So why do I enjoy the rush I get when walking up to a heavily biofouled seawall?

It’s because before I even get close to the seawall, running through my head are all the INNS that I predict I will see. I base this predicted suite of species on multiple factors: seawall composition, tidal height of seawall, presence of overhangs or other structures on the wall, nearby vessel traffic, to name a few.  It’s like a game – how many species did I correctly predict would be on this wall?

I use this game to improve my ability to rapidly assess a structure for species that don’t belong there. In this way, I can effectively assess multiple structures within a harbour in one low tide. Once I locate their ideal habitat, I often find that I have more invasive species in front of me than I know what to do with.

To briefly set the scene for you…

Most coastal areas of the UK have extraordinarily large tidal ranges – usually around 5m, but up to 15m in the Severn Estuary. This means that at two times during every day, large sections of the coast are exposed at low tide, revealing thousands of organisms, such as sponges, tunicates and molluscs, that live attached to hard substrate such as natural rocky shore but also man-made infrastructure. This exposed section at low tide is called the “intertidal zone”.

Many of the INNS found in these exposed intertidal areas are “encrusting”, meaning that they fasten themselves to hard substrate, or sometimes other organisms, where they spend their entire life. These encrusting organisms need direct access to water so that they can filter feed tiny microorganisms out of the water column at high tide. So, if one species overgrows another, the species on top will survive and the species below will perish.

Many encrusting marine INNS overgrow and smother native species to the point where they cannot filter food out of the water. Invasive species are even known to grow on top of other INNS (Figure 1), so they are pretty easy to spot if you know what you’re looking for. It is this characteristically aggressive smothering behaviour that contributes to invasive species thriving in habitats that are not their own.

Figure 1. Both photographs showing an invasive species overgrowing another invasive species. Top photograph: Didemnum vexillum (carpet sea squirt) overgrowing Watersipora subatra (red ripple bryozoan). Bottom photograph: W. subatra overgrowing Botrylloides diegensis (San Diego sea squirt).

 

Many of these species look so odd and out of place, that the term “alien” might be more appropriate than “invasive” (Figure 2). This may be because many encrusting marine invasive species come from far away places such as the Indo-Pacific and the north western Pacific.

Figure 2. Many marine invasive species look “alien” and out of place here in UK waters. Top photograph: Styela clava (leathery sea squirt) attached below a large wharf. Bottom photograph: Didemnum vexillum (carpet sea squirt) growing from below a pier.

 

INNS are so opportunistic that they tend to find hard substrate before any of their native equivalents. For example, I have often documented that hard man-made structures, such as piping and small jetties, placed in natural areas were often riddled with invasive species – probably due to their ability to establish more rapidly than natives (Figure 3).

Figure 3. A small man-made pipe located within natural sandy habitat. The pipe is not visible because it is entirely inhabited by Watersipora subatra (red ripple bryozoan), which has Botrylloides diegensis (San Diego sea squirt) growing on it.

 

Coastal man-made structures are thus contributing to the ability of invasive species to gain a foothold and out-compete our native UK species. This is starting to result in a decrease in native biodiversity in coastal systems, especially those that are within heavily urbanised harbours.

“An ounce of prevention is worth a pound of cure.” This saying could not be truer for coastal and marine structures that have experienced invasion by fast-growing, encrusting invasive species. Thus, early detection of an invader is key in controlling its spread and minimising its impact on marine infrastructure such as seawalls, floating pontoons and vessel docking areas.

Fortunately, APEM has a team of skilled marine taxonomists that specialise in identifying marine non-native and invasive species, with a specialty in identifying species within recreational marinas and larger urbanised harbours. We use rapid assessment surveys and deploy settlement tiles to monitor marina infrastructure for early signs of invasive species. Moreover, we remain up-to-date on harmful species predicted to enter the UK so that we are ready to detect them as soon as they arrive (i.e., ‘horizon scanning’).

APEM has extensive experience in identifying marine INNS in harbours and marinas. In fact, our scientists were the first to detect Theora lubrica, a bivalve from the Northwest Pacific, right here in UK waters in the Lowestoft area. Armed with the knowledge of its presence, rapid response efforts targeting T. lubrica within the area can now be implemented.

Learn more about how APEM can help you with marine invasive species identification and early detection here.