Een populatie van het wit muizenoortje Auriculinella bidentata (Montagu, 1808) en één vers aangespoeld leeg huisje van het meertandig muizenoortje Myosotella denticulata (Montagu, 1803) in de Baai van Heist
Dumoulin, E.; Langeraert, W. (2017). Een populatie van het wit muizenoortje Auriculinella bidentata (Montagu, 1808) en één vers aangespoeld leeg huisje van het meertandig muizenoortje Myosotella denticulata (Montagu, 1803) in de Baai van Heist. De Strandvlo 37(2): 83-104
On April 3rd 2016 three empty shells of the Pulmonate two-toothed white snail were found washed ashore in a shell grit bank at the eastern border of the nature reserve Baai van Heist (municipality Knokke-Heist). One subadult specimen was clearly an old shell, very probably of fossil Pleistocene or Holocene origin. The other two snails (1 subad. and 1 juvenile) showed small traces of weathering but certainly had a recent appearance. This prompted our presumption that the species could possibly live in situ in the Baai van Heist. On September 19th 2016 we searched for the species along the eastern mole of the outer port of Zeebrugge, situated within the reserve. At two localities in the mediolittoral zone we could find various live specimens of A. bidentata under stones. The features of the habitats in which they have been found were in conformity with those known from the literature. Moreover we could ascertain that two-toothed white snails living nearer to the coastline occurred lower beneath the mean tide level than their congeners from a more seaward finding spot. Most of the snails were dwelling on the substrate under stones. We also noticed that larger shell fragments, mainly from mussels, created small cavities under the stones. These could serve as shelter or air-chamber to provide oxygen for respiration. For the first time A. bidentata is now recorded alive along the Belgian coast. On the same day we found a fresh empty shell of the multi-toothed mouse-eared snail Myosotella denticulata in a shell grit bank at the shore of the Baai van Heist. We presume that this species may also live in this nature reserve under similar environmental conditions as the two-toothed white snail. So far however, it has not been discovered in situ. In Belgium this species is known from the IJzer estuary, but it is new to the eastern part of the coast. From the late 1960s until the 1990s the beach area of the Baai van Heist was repeatedly raised with sand dredged offshore of the harbour of Zeebrugge. In grit banks consisting of shells and debris washed out from these sediments, empty specimens of the mouseeared snail Myosotella myosotis have been found on rare occasions. These were clearly old shells of a fossil nature, presumed of Pleistocene (Eemian) and/or Holocene age. Nowadays an embryonic saltmarsh is developping in the Baai van Heist which will create potential habitats for mouse-eared snails. Up to now the species has not yet been observed living on this site. Regarding the foraging behaviour of A. bidentata it is assumed that she is a selective deposit feeder, mainly browsing on sediment particles (epipsammic browsing). Perhaps the snail also browses the epilithic microflora and -fauna from stones, on which she often dwells. Characteristic for the three Ellobiidae is that specimens as from 2.5 whorls onward the columella in the top of the spire is resorbed. By this a small cavity develops in the snail's shell. We wonder if this cavity has a function in the life of the animals. For instance it may serve as a kind of float for local dispersal or as an air reservoir for respiration during flood at high tide. We further ask ourselves how A. bidentata, considering her non-planktonic life cycle, could have colonized the Baai van Heist. We suggest two possible dispersal mechanisms: rafting and kinds of zoochory. The latter can take place via birds foraging in the snail's habitat, especially turnstones and rock pipits. Also other bird species may be eligible for zoochory, e.g. oystercatcher, curlew, herring gull and possibly some songbirds which occasionally dwell in the intertidal area during migration or in the winter season.
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