The Lawn Shrimp cometh

I quite often receive invertebrates in the post, but they are usually leaf beetles (Chrysomelidae) sent to me for identification/verification in my capacity as organiser of the UK's Chrysomelid Recording Scheme - like here for example. However, a couple of days ago I came home to find something quite different awaiting me on the doormat - a crustacean looking like a small shrimp, or to be more precise an amphipod of the family Talitridae (a group usually associated with seashore habitats rather than inland terrestrial ones).

In this case, it had already been identified as Arcitalitrus dorrieni, the 'landhopper', 'woodhopper' or 'lawn shrimp' by the finder/sender, Dennis Trunecka of the Southampton Natural History Society. This is an interesting find as it is Australian in origin (New South Wales & Southern Queensland), with the first UK record being from the Scilly Isles in 1924. Since then, it has been found in a number of sites across southern England, and also in Ireland, the Channel Isles, west Wales and western Scotland (coastal when north of southern England). However, it is not entirely clear how widely it has established itself in the last couple of decades, although individuals can move tens of metres per day as well as being moved over longer distances by the plant trade etc. (Cowling et al. 2004).

Arcitalitrus dorrieni found in woodland leaf litter in Hampshire. The seven segments of the peraeon and the three segments of the pleon are indicated. Length (head to rear of body in this curved position) approx. 6.5mm.

It is most often found under stones and dead wood or among damp material (detritus, debris, leaf litter) in gardens, damp scrub and woodland. It most likely arrived (and to some extent spread) in the UK through the transport of plants/soil to and between plant nurseries and garden centres. It is uncertain whether this non-native species has a significant ecological impact in the UK, although it is possible that it competes with (and maybe replaces) native detritivores in woodlands. In some locations, it can be found in high densities - up to approximately 2,500 per square metre in Dicksonia antartica litter on the Scilly Isles (Richardson 1980). It certainly can be a significant detritivore, consuming 24.7% of annual litter fall in a coniferous woodland in Ireland (O'Hanlon & Bolger 1999) - more than any of the native macrofaunal species.

Identification is fairly straightforward, especially given the small number of possible confusion species. Orchestia cavimana  is an introduced semi-terrestrial Mediterranean amphipod (Konopacka et al. 2009) but much paler in colour - A. dorrieni is variably dark, and orange when dead as here, though pale if preserved). However, there is another introduced terrestrial amphipod, A. sylvaticus, although this is much rarer in the UK. Using the key in Peart & Lowry (2006), the two species can be separated by looking at the epimera (the three segments of the pleon, singular 'epimeron'). In A. dorrieni, the 2nd epimeron is longer than the 3rd while in A. sylvaticus they are more-or-less equal. In the top photo, this is unclear as the rear edge of eipermon 3 is obscured by one of the legs, but with some legs (re)moved, it is clear that this is A. dorrieni. There are other features which might be required to separate further species but these do not (yet) occur in the UK, although it is possible they could be imported with plants.

A. dorrieni - from the green lines, it is clear that epimeron 2 is longer than epimeron 3.

The head bears numerous appendages including two pairs of antennae (typical of crustaceans) and a complex array of mouthparts - I won't go into the details here but there are plenty of resources online and in print providing introductions to crustacean anatomy. The lateral compression is clear (flattened side-to-side) and is generally a good way of separating amphipods from isopods (e.g. woodlice which are flattened top-to-bottom i.e. dorso-ventrally). The antennae are inserted in front of the eye which is black and not especially well developed, being covered by a transparent plate. This is likely to be an adaptation to its life within/under leaf-litter and under damp material where vision is less likely to be useful than senses such as touch - note the long antennae and various bristles.

A. dorrieni showing its lateral compression.

Head of A. dorrieni (side view)

Mouth and mouthparts of A. dorrieni (ventral view)

So, an interesting find and thanks to Dennis for passing it on to me for closer scrutiny. As ever, finds such as this are useful in determining the distribution (and in this case, spread) of species, so it is worth keeping an eye out - especially in case a third Arcitalitrus finds its way here.

References

Cowling, J.E., Spicer, J.I., Weeks, J.M. & Gaston, K.J. (2004). Current status of an amphipod invader, Arcitalitrus dorrieni (Hunt, 1925) in Britain. Journal of Natural History 38: 1665-1675. 
Konopacka, A., Grabowski, M., Bącela-Spychalska, K. & Rewicz, T. (2009). Orchestia cavimana Heller, 1865 (Amphipoda: Talitridae) enters freshwater inland habitats in the Vistula River, Poland. Aquatic Invasions 4(4): 689-691.
O'Hanlon, R.P. & Bolger, T. (1999). The importance of Arcitalitrus dorrieni (Hunt) (Crustacea: Amphipoda: Talitridae) in coniferous litter breakdown. Applied Soil Ecology 11: 29-33.
Peart, R. & Lowry, J.K. (2006). The amphipod genus Arcitalitrus (Crustacea: Amphipoda: Talitridae) of New South Wales forests, with descriptions of six new species. Records of the Australian Museum 58: 97-118.
Richardson, A.M.M. (1980). Notes on the occurrence of Talitrus dorrieni Hunt (Crustacea: Amphipoda: Talitridae) in south-west England. Journal of Natural History 14: 751-757.