Molluscs

blue-rayed limpet/brenigen las (Patella pellucida) © North Wales Wildlife Trust
blue-rayed limpet/brenigen las (Patella pellucida) © North Wales Wildlife Trust

Here you will find ID features of a few more common shelled molluscs; less common, but easily identified shelled species and we introduce some sea slug species which can be identified fairly readily, with care. They are grouped together here as they are all molluscs and seeing them together highlights the variety you can see within the molluscs grouping.

The species in focus are:

dog whelk/gwichiad y cwn (Nucella lapillus)

common whelk/cragen foch fwyaf (Buccinum undatum) 

netted dog whelk sp. (e.g. Tritia reticulata)

blue-rayed limpet/brenigen las (Patella pellucida)

celtic sea slug (Onchidella celtica)

sea lemon/mwsg môr (Doris pseudoargus)

sea hare/môr-wlithen glustiog (Aplysia punctata)

 

common piddock/pidog (Pholas dactylus)

You will see what they look like in different situations and learn a bit about their ecology.

You will then be tested.

There are not many difficult comparisons with this set of mollusc species and some have few similar species at all (ie. blue-rayed limpet). There may be difficulty in seeing the species in the first place apart from rock or algae (e.g. sea hares, celtic sea slug). We tentatively introduce some similarities which may occur and how to tell the difference.

dog whelk/gwichiad y cwn (Nucella lapillus) ©NWWT
dog whelk/gwichiad y cwn (Nucella lapillus) ©NWWT
common whelk/cragen foch fwyaf (Buccinum undatum) ©NWWT
common whelk/cragen foch fwyaf (Buccinum undatum) ©NWWT

The dog whelk can possibly be confused with a small common whelk. The differences are:

1. the common whelk is wider in ratio to its length than the dog whelk
2. the dog whelk can be quite worn-looking and the ridges faded in some specimens
3. the dog whelk, even when exhibiting ridges, has less pronounced markings than the common whelk.

common piddock/pidog (Pholas dactylus) © NWWT
common piddock/pidog (Pholas dactylus) © NWWT
American piddock (Petricola pholadiformis) © NWWT
American piddock (Petricola pholadiformis) © NWWT

The common piddock has a challenger; another piddock (the American/false piddock). When it situ within a tunnel, ID between the two will be difficult. When shells are seen out of situ the differences are:
1. the common piddock hinge is so fragile it makes it less likely to be seen with the two shells still attached on shore. The hinge is also oddly shaped.
2. the common piddock has ridges running from the hinge end down towards the outer opening edge of equal size, whereas the American piddock has ridges which look flattened towards the pointed end of the shell compared to the wider end.
3. there is another piddock but which is much broader and can’t be confused with either of the above.

netted dog whelk (e.g. Tritia reticulata) © NWWT
netted dog whelk (e.g. Tritia reticulata) © NWWT
netted dog whelk (e.g. Tritia reticulata) © NWWT
netted dog whelk (e.g. Tritia reticulata) © NWWT

The netted dog whelk, although can’t be mistaken from most other molluscs, it has several contenders within a close species grouping. The most common is Tritia reticulata, but there are other features to keep an eye out for to decipher whether it’s one of the other species:
1. look to describe the size, as there are smaller species (Tritia pygmaea; Tritia incrassata)
2. take a close look at the opening to note thickening and teeth along outer edge to ID Tritia incrassata.

Some whelk species

dusky doris (Onchidoris bilamellata) ©NWWT
dusky doris (Onchidoris bilamellata) ©NWWT
sea lemon/mwsg môr (Doris pseudoargus) ©NWWT
sea lemon/mwsg môr (Doris pseudoargus) ©NWWT

Although the sea lemon is commonly large and so less likely to be confused with other sea slugs, we have tripped up by thinking a similar-textured slug was a smaller version of the sea lemon. This is the rough-mantled or dusky doris (Onchidoris bilamellata). It’s been found frequently on our North coast shores, so well worth knowing about in readiness for finding it.
1. smaller than sea lemon, this grows to 4cm
2. often found grouped
3. usually mottled browns, but can also be just white or pale.

Some slug species

Two more molluscs

Alternative factoids

You might be keen to know a bit more about the various molluscs you’ve been studying here. This section will not appear in the quiz as questions.

The celtic sea slug is one of the Climate Change Indicators you’ll be learning more about whilst taking part in Shoresearch Surveys. This species is thought to be moving northwards as our seas are becoming generally warmer. 

The dog whelk is one of several species which drills perfectly circular holes in other shells (such as mussels and limpets) to feed on them. These holes can be found in shells left on shore.

Dog whelks are a well-studied intertidal species and are the main species which shows the effects of certain chemicals in the marine environment. Females exhibit what’s called imposex, which means they grow male sexual features, which then means they cannot reproduce.

Blue-rayed limpets are known to migrate as they age from youngsters on pink encrusting algae to wrack and kelp stipes and fronds to finally the kelp holdfast, where they live out their days. 

Piddocks are capable of making tunnels in rock, peat by rotating their shell which has “teeth” on the outer surface, slowly carving a hole in which they live.

Blue-rayed limpets blue lines are created by what’s called structural colour, colour created by fine-scale textures reflecting light in different ways.

The next section is the quiz. You will be tested on what you’ve learnt so far. Don’t worry too much, as once you’ve been out on the shore, the ID features you’ve learnt about will make more sense and after a couple of visits you should feel more confident about your ID.
Good luck!