Shell gravel from the River Loddon

In places, along the stretches of the River Loddon examined during the field meeting on 11 July, the river bed was seen to consist of patches of fine gravel intermixed with dead shells. A small sample of this shelly gravel was taken close to the river bank, about 100 m upstream from Black Bridge on Sandford Lane at SU 77984 72898 (with thanks to Tom Walker for the grid reference and loan of his shell scoop), and examined for its shell content.

As with a shell assemblage taken from an archaeological deposit, such as the bottom of a prehistoric drainage ditch, the species represented in the sample as dead shells might be expected to represent other habitats than the gravel itself. This accumulation of species occurs through the processes of taphonomy. This term is used to describe all the different factors that act on dead shells to produce the actual collection of shells that we see. For example, dead molluscs could have fallen off adjacent aquatic weeds, empty shells could have been swept in by the current from elsewhere in the river system, shells could have been redistributed by flooding, and land snails could have fallen in from the river bank, all over an indeterminate period of time. And this, in fact, appears to have been the case with the sample in hand.

This small sample from the river bed contained the remains of at least 681 individual molluscs (the number of valves being divided by two to produce a Minimum Number of Individuals (MNI) of bivalves – the actual number, of course, since the valves do not necessarily make matching pairs, is likely to be much higher). The overall MNI of 681 shells comprised 30 species (Table 1).

Most of the gastropods were represented by dead shells and most of these again were opaque and chalky with age: some may have been in the assemblage for a relatively long time and might even represent species that do not live in the vicinity today. For example, Acroloxus lacustris was not recorded live anywhere on this occasion or on the previous visit to the Loddon almost exactly one year before. A few small Theodoxus fluviatilis and some adult Bithynia tentaculata were collected live, indicating that the gravel could form part of their natural habitat; the other gastropods were probably washed in from elsewhere, most obviously the four land snails. It is noteworthy that the majority of specimens were dead juveniles, their light shells being easily transported through the water. Species with the highest proportion of live-collected shells were the small bivalves. Out of an MNI of 192 Sphaerium corneum, 72 (37.5%) were represented by joined valves. A similar proportion of Pisidium spp. and three of the four Musculium lacustre were also complete shells which were either live or recently dead on collection. These bivalves appear to have successfully colonised the river gravel.

Numerically, Sphaerium corneum and Pisidium spp. comprised 54.62% of the total number of individuals. The next most frequent species was Bithynia tentaculata (10.87%), followed by Bithynia leachii (6.61%), Potamopyrgus antipodarum (6.31%) and Theodoxus fluviatilis and Ancylus fluviatilis (both at 4.11%). This small sample clearly demonstrates that river gravel can be extremely prolific in terms of shell material. These samples can be easily collected and used to assess species richness and diversity – with the provisos that the assemblages will be biased by their taphonomy and that the dead shells of yesterday may not necessarily represent the living shells of today.

Table 1: Species recorded from the River Loddon gravel.

*Owing to their large number and the presence of juveniles, no attempt was made to identify all the pisidia to species level. However, six species were identified, of which the most frequent appeared to be Pisidium subtruncatum and Pisidium casertanum, followed by Pisidium henslowanum, Pisidium milium, Pisidium amnicum and Pisidium nitidum.