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3. Bryozoa and Marine Molluscs

Figure 3: Colony of Conopeum seurati (Canu 1928) encrusting an Ecrobia ventrosa (Montagu 1803) shell

Bryozoa often occur as epibiont organisms on the shells of marine molluscs, and are often recorded in assemblages derived from food waste. In these cases, the ecological tolerances and geographical range of the bryozoa might give some additional information about the origin of the shells. Published records of bryozoa on marine shells include Buckquoy, Orkney (Evans and Spencer 1977, 217); Ardnave, Islay (Evans 1983, 356); Chanctonbury Ring, Sussex (Somerville 2001, 109); Shapwick, Somerset (Light 2007, 928); Whitefriars Street, Norwich (Ayers and Murphy 1983, 36); and Gilberd School, Colchester (Murphy 1992). In most of these cases the bryozoa were not identified beyond phylum level, however. The mere presence of bryozoa may give useful information: in a survey of modern shells from Poole in Dorset, Winder (1997, 198) found that encrusting colonies of bryozoa were more frequent on oysters dredged from the harbour than those from the bay. Winder does not offer an explanation for this; however, it may be that there was a wider availability of suitable surfaces for the colonies to encrust in the harbour than on the soft substrate of the bay.

The colonies may reveal information about a shell's likely condition at the time of collection, as the presence of bryozoa on the inside of a bivalve shell shows that it could not have been collected as an intact animal and so is unlikely to be food waste (Thomas 1978, 155; Thomas 1981, 50; Murphy 1992). This was also the case with the 82,000 year-old perforated Nassarius gibbosulus (Linnaeus 1758) (a gastropod) shells from Grotte des Pigeons, Taforalt, Morocco (Bouzouggar et al. 2007, 9967). Claassen (1998, 149) has suggested that bryozoans on shells may be indicators of seasonality, stating that large quantities of Conopeum sp. on bivalve shells are a winter phenomenon. This seems too general a statement, however, as once a bryozoan colony forms on a shell, it is a permanent fixture.

Bryozoans that are often found on shells from European waters are Bugula flabellata (Thompson, in Gray 1848), Bugula plumosa (Pallas 1766), Celleporina hassalli (Johnston 1847), Conopeum reticulum Gray 1848, Conopeum seurati, Crisiidae, Electra spp., Flustrellidra hispida (Fabricius 1780), Membranipora membranacea (Linnaeus 1767), and Walkeria (=Valkeria) uva (Linnaeus 1758) (Ryland 1962, table 1). Bryozoan colonies may affect the settlement, growth and mortality of shellfish such as oysters (Valero 2006, 4). In addition to encrusting forms, a number of bryozoans leave borings in shells (Boekschoten 1966, 359-67). In the lower Pleistocene crag at Levington (Suffolk), shells of Neptunia contraria (Röding 1798), Glycymeris glycymeris (Linnaeus 1758) and Astarte spp. showed the characteristic borings of Immergentia (=Terebripora) orbignyana (Fischer 1866) (Boekschoten 1966, 360). These are more-or-less cylindrical holes in the shell up to 0.1mm in diameter (called shell pores by Boekschoten) with a slit-like caudal extension, through which the zooid makes contact with the environment. These are connected by stolons within the shell 0.45–0.9mm apart (Boekschoten 1966, 359). As with encrusting forms, the ecological tolerances of boring bryozoans may reveal clues about the source of the shell, and its condition at time of collection.

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