Knee deep in Malacostraca

This week I have been mostly thinking about Malacostraca, specifically Amphipods - "different feet" - referring to the different types of legs they have, some for eating and some for swimming. Amphipods are highly important invertebrates - primary decomposers which carry out the bulk of shredding of vegetation such as dead leaves, speeding up nutrient cycling by other organisms. The most commonly encountered Amphipod is Gammarus pulex/fossarum agg., which is in fact a species complex of many cryptic species distinguishable only by DNA analysis (Continental-scale patterns of hyper-cryptic diversity within the freshwater model taxon Gammarus fossarum (Crustacea, Amphipoda). (2020) Scientific reports, 10(1), 16536). 

I find Gammarus pulex/fossarum agg. pretty much everywhere I look, and around here that means fairly degraded aquatic environments. Urban rivers are some of the most anthropogenically modified landscapes on Earth, resulting in characteristic low diversity communities ("urban stream syndrome"). Several recent projects have aimed to clean up Leicester's watercourses. Although well-meaning these are mostly cosmetic since nothing has been done to reduce harmful inputs, and there has been no real progress in reversing the damaging concrete canalisation of past decades. A few wild flowers and shrubs have been planted and litter, such as the inevitable shopping trolleys, removed. Surprisingly, litter removal in itself might be counter productive. Invertebrates readily colonise artificial surfaces as long as they are non-toxic. An article from Nottingham and Loughborough looked at several urban streams including Saffron Brook and showed that anthropogenic litter in urban rivers supports distinct and diverse macroinvertebrate communities (Anthropogenic litter is a novel habitat for aquatic macroinvertebrates in urban rivers. (2021) Freshwater Biology, 66(3), 524-534). Compared with the harshness of the bare concrete channel, building waste, broken bottles and shopping trolleys are invertebrate paradise. Once again wildlife and the tidy brigade are in conflict. 

 

Erratum: Demons not Killers

Updated: 30.01.2025

All Dikerogammarus species are non-native invasive species which are potentially harmful to other aquatic life. There are at least two species present in the UK and microscopic examination of the projections on the urosome is required to distinguish between them. In June 2024 I found a specimen of Dikerogammarus in the River Soar in Leicester (described below). At the time I identified this as D. villosus, however, recent correspondence with the Environment Agency has resulted in correction of my I.D. to D. haemobaphes, the "Demon Shrimp".  At one level this makes no difference, but D. haemobaphes was previously known from the Soar so my find is not in fact a new invasive species for VC55. 

Reference: Dobson, M. (2012) Identifying invasive freshwater shrimps and isopods. Freshwater Biological Association. Available at: https://www.nonnativespecies.org/assets/InvasiveFWShrimpsIsopods.pdf

 

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Yesterday I found a single specimen of Dikerogammarus villosus in the River Soar near Leicester. Popularly known as the "Killer Shrimp", this species was first identified in the UK in 2010. It is true that this is a voracious predator, killing invertebrates and small fish. It is also a highly successful colonizer, having swept across Europe from their origin in eastern Europe. This isn't the first time this has happened, other Gammarids have colonized the UK in successive waves, e.g. Gammarus pulex and Crangonyx pseudogracilis. A closely related species, Dikerogammarus haemobaphes, is well established along the Soar, but my recent find is the first VC55 record for D. villosus, which has been demonized even more than its congeners (Killer!). Papers published in the years after its UK arrival predicted dire consequences, including problems with biotic indices for water quality assessment - after the Killer Shrimp has eaten everything else ("Killer shrimps", dangerous experiments and misguided introductions: how freshwater shrimp (Crustacea: Amphipoda) invasions threaten biological water quality monitoring in the British Isles. (2012) Freshwater Reviews, 5(1), 21-35. https://doi.org/10.1608/FRJ-5.1.457). 

D. villosus is certainly tough as old boots - pollution tolerant (which is *very* helpful in British waterways), salt-tolerant and can survive in damp conditions for up to five days so it can potentially be spread by equipment including fishing gear, nets, boats, kayaks and trailers. And yet, fourteen years after its introduction, the distribution of this species still appears to be relatively restricted - it certainly hasn't spread as rapidly as first predicted. Part of the reason for this is certainly under-recording - identifying D. villosus requires close examination - but is relatively straightforward (see: https://www.nonnativespecies.org/assets/Uploads/ID_Dikerogammarus_villosus_Killer_Shrimp_v2.1-1.pdf ). 

Even so, this species has not swept across the country quite as easily as predicted. We need to take sensible biosecurity precautions such as disinfecting nets and equipment - but that doesn't deal with wild birds, etc. Even so, maybe the Killer Shrimp!, like the Killer Harlequin Ladybird! and the Killer Asian Hornet! - may not be quite as bad as first feared. It is another undesired pressure on aquatic ecosystems, but I'm a lot more concerned about pollution than I am about Killer Shrimp!