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.

Ants round my home, woodlice in theirs...

My entomological side is getting twitchy - after months of near-invertebrateless cold, I can see a variety of multi-appendaged beasts crawling, flying and generally being busy. So, during a trip to dig the allotment, I couldn't help noticing quite a bit of ant activity. They haven't found the house (i.e. the kitchen) yet but are busily working in the garden, around all kinds of cracks and various bits and pieces. As I left through the back gate, there were even some that had helpfully started work at head height (in and around two bolt-holes in a gatepost) allowing me to look at them closely without crouching down: most thoughtful...

Black ants busy around a hole in a gatepost.

Looking more closely, it seemed as if they were clearing out the remains of previous occupants - there looked to be bits of woodlouse and spider coming out. Closer still, building work was going on with 'cement' plates being formed inside the hole - luckily I managed to capture one ant placing a mouthful (mandible-ful?) of building material...

Note on the left - one ant placing building material

They look like one of the common dark garden species, but it's always worth checking so I brought a couple indoors to see if I could key them out (using Skinner & Allen 1996). The single-segment waist with well-developed petiole (a little plate sticking up from the waist), circular spiracle towards the rear of the propodeum (segment just in front of the waist) and a lack of rows of bristles on the underside of the hind tibia brought me to the genus Lasius (not unexpectedly...). From there, various features including the presence of standing hairs on the antennal scape and hind tibia (but not the double rows seen in other groups) took me to the common species L. niger.

Hind tibia with standing hairs

Head, including antennal scape with standing hairs (see below the eye)

OK, so I've been distracted - time to dig the allotment and off I go... digging (finally) takes place and I'm almost finished when I notice (a) some orange-yellow ants and with them (b) some tiny white woodlice. This looks straightforward enough - the ant is Lasius flavus nesting in the grassy path and the woodlouse is the Ant Woodlouse Platyarthrus hoffmannseggi.

L. flavus and P. hoffmannseggi - both around 3-4mm long

P. hoffmannseggi - the yellow colour is just due to lighting/image processing...

P. hoffmannseggi is an interesting beast - it is white (in the top photo you can just see dark gut contents through its dorsal surface), blind and lives closely associated with ants of almost any species (Hopkin 1991), often found in their nests where it may feed on pellets regurgitated by the hosts (Williams & Franks 1988) and/or may be a more general scavenger in the 'rubbish-dump' parts of nests (Pontin 2005). It has also been noted as following lines of ants when nests have moved (Donisthorpe 1927). This is unusual behaviour for a woodlouse (it is the only myrmecophilous - 'ant-associated' - woodlouse in Britain), but not unique - in Malaysia there is a species of Exalloniscus which 'hitchhikes' (maybe 'stows away' is more accurate) on the pupae of ponerine army ants. Although not often seen without digging, it is fairly common in southern England and Wales, becoming scarcer northwards (will climate change extend its range, and if so, how does it disperse?). Beyond this, little is known about P. hoffmannseggi - an area for a keen researcher to work in maybe?

References

Donisthorpe, H. St. J. K. (1927). The Guests of British Ants.Routledge, London. [interesting if you can find it, but can be expensive]
Hopkin, S. (1991). A key to the woodlice of Britain and Ireland. Field Studies 7(4): 599-650. [this has also be published in separate reprint form]
Pontin, J. (2005). Ants of Surrey. Surrey Wildlife Trust, Woking. [another volume in the excellent series of Surrey atlases]
Skinner, G. J. & Allen, G. W. (1996). Ants. Richmond, Slough. [excellent keys to British species]
Williams, T. & Franks, N. R. (1988). Population size and growth rate, sex ratio and behaviour in the ant isopod Platyarthrus hoffmannseggi. Journal of Zoology 215: 703-717.