Chris Ashelby_Marine Laboratory Scientist_Technical Specialist

Non-native species are introduced in a variety of different ways, often accidentally. But no matter how they are introduced, finding them and identifying them is another matter.

Early detection of newly introduced species offers the best chance of controlling or eradicating the species before it can become established. However, it is a very rare occurrence that an invasive and non-native species (INNS) will be detected as soon as it is introduced.

More often species need to reach sufficient abundance to be encountered before they are reported; this may be years after their initial introduction, by which point they are likely to have become established and widespread. For example, the first time the shrimp Palaemon macrodactylus was found in Europe was in samples collected in the Orwell Estuary, Suffolk in December 2001 (Ashelby et al., 2004).

At that point it was already abundant in the estuary and looking through archived zooplankton samples, we found larvae, showing that it was already present in 1999. At the same time, researchers in Southern Spain also found the same species in archived samples from 1999.

Palaemon_macrodactylusLater, when examining archived samples from a monitoring programme in the Thames, APEM found a single individual from November 1992 (Worsfold & Ashelby, 2008).

It had therefore been in Britain and elsewhere in Europe for at least nine years before anyone had found it!

Likewise, photographs on a website showed that the Japanese brush-clawed crab, Hemigrapsus takanoi was present in Britain for at least two years before the first specimens were reported (Ashelby et al., 2017).

This illustrates how even large, relatively distinctive species can escape attention. Many NNS introduced to Britain are much more difficult to detect and require microscopical examination to determine.

There are some established monitoring programmes for detecting INNS in certain areas and associated with specific activities, but these usually focus on a small number of easily identified and already known INNS, rather than being specifically designed to detect new arrivals.

More frequently, detecting new INNS is a chance occurrence and most of the species that have been recently reported from Britain have been detected in surveys designed to address other questions.

The window shell Theora lubrica was found in an impact assessment study for a new river crossing, and the Asian Date Mussel, Arcuatula senhousia was first found in samples collected as part of Water Framework Directive Monitoring (Worsfold et al., 2020). Other NNS have also been found by amateur naturalists.

Sampling in certain habitats increases the chances of finding INNS. For example, estuaries, particularly those near ports are more likely to have INNS than deep-water, offshore habitats. In fact, in several recent surveys conducted by APEM in estuaries, INNS have outnumbered native species.

Detecting INNS

The first step to detecting INNS is an in-depth knowledge of the species composition and morphology of the existing flora and fauna of the area (both native and previously introduced). The differences between species can be very subtle; recognising when something doesn’t fit is key and may indicate that you have found a new INNS. Even once an INNS is encountered, often putting a name to the species isn’t a simple task.

Reviewing relevant literature helps keep our scientists abreast of species that are spreading elsewhere in the world and may help put a name to a newly encountered species.

Similarly, ‘horizon-scanning’ exercises can provide a useful starting point of species likely to be introduced. But often even before these lists are completed, some of the ‘horizon’ species are already introduced into the new environment but undetected, as occurred with Hemigrapsus takanoi which was included in the horizon-scanning exercise conducted by Roy et al. (2014) but in reality, was present in Britain since at least 2011 (Ashelby et al., 2017).

Besides, literature reviews and horizon-scanning cover only a minute part of the potential species that could be introduced. Species could come from anywhere in the world, but often successful introductions come from similar habitats and similar latitudes.

This can help narrow the search but when the biota of potential source regions is itself poorly understood it presents challenges.

An example of this is a small aquatic snail that was first found in Europe in the Thames in 2003 by APEM’s Technical Specialist Tim Worsfold.

Tim very quickly recognised the family and even had a suspicion about the genus that the snail belonged to but putting a species name to it proved problematic for more than 15 years.

Using a combination of morphological analysis and DNA it was eventually identified as the South American species Heleobia charruana (van Haaren et al., in press). Another snail that has been found in the same samples in the Thames is still without a name!

APEM has a strong track record of recognising and reporting new INNS from our routine surveys and sample analyses. We have provided the first British or European records of several INNS in both marine and freshwater environments. Our standard data outputs highlight records of INNS, along with other notable species.

We have designed surveys specifically to detect INNS, helping regulators achieve their obligations of reporting on the establishment and spread of INNS in Britain. To find out more about our INNS or laboratory services please contact Dr Chris Ashelby to discuss your requirements.

Relevant publications

The publications listed below are cited in the text of this blog. Authors whose names are highlighted in bold work for APEM.

Ashelby CW, Worsfold TM & Fransen, CHJM (2004) First records of the oriental prawn Palaemon macrodactylus (Decapoda: Caridea), an alien species in European waters, with a revised key to British Palaemonidae. Journal of the Marine Biological Association of the United Kingdom, 84(5): 1041-1050.

Roy HE, Peyton J, Aldridge DC, Bantock T, Blackburn TM, Britton R, Clark P, Cook E, Dehnen-Schmutz K, Dines T, Dobson M, Edwards F, Harrower C, Harvey MC, Minchin D, Noble DG, Parrott D, Pocock MJO, Preston CD, Roy S, Salisbury A, Schönrogge K, Sewell J, Shaw RH, Stebbing P, Stewart AJA, Walker KJ (2014) Horizon scanning for invasive alien species with the potential to threaten biodiversity in Great Britain. Global Change Biology, 20: 3859–3871,

Van Haaren T, Worsfold TM, Stelbrink, B, Collado GA, Gonçalves ICB, Serra WS, Scarabino F, Gittenberger A, & Gittenberger E (in press) Heleobia charruana (Gastropoda, Truncatelloidea, Cochliopidae), a South American brackish water snail in northwest European estuaries. Basteria.

Worsfold TM & Ashelby CW (2008) Additional UK records of the non-native prawn Palaemon macrodactylus (Crustacea: Decapoda). Marine Biodiversity Records, 1(e48): 1-3. [originally published as: Worsfold TM & Ashelby CW JMBA2 Biodiversity Records, 5547: 1-3. 2006]

Worsfold TM, Pennisi N & Ashelby CW (2020) Theora lubrica Gould, 1861 (Bivalvia: Semelidae), new to the UK, with notes on associated non-native species, and an earlier date of introduction for Arcuatula senhousia (Bivalvia: Mytilidae) to the UK. Journal of Conchology 43(6): 665-674.

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