| Overview of species recorded | Recording Effort | Important Sites | Significant Species | Conservation |
More than 700 species, subspecies and varieties have been noted from our area, including those which are regarded as errors of identification or localisation. It was considered important to include all species mentioned in the literature or found as museum specimens, however doubtful, in order to give an assessment of their value as records. The West Scotland area covered by the present Atlas, contains nearly 100 additional mostly deepwater species, not including subspecies and varieties, which are not in the Species Directory, and it is considered very important to include these.
Just over 430 species are gastropods, including 95 nudibranchs (70% of the wider British fauna) and there are 225 bivalves. Approaching 575 species have been found alive since 1950. This does not necessarily mean that they are permanently resident in the area, as some are clearly adventitious and may not appear again for many years. About 125 (100 living) only occur in deep water on the Hebrides Terrace or the Hebridean slope. In such a large and complex area as this, it is likely that more species new to the area will be reported, largely southern or deep water ones, and the range of others will be extended.
Although some records, both old and new, have been considered more than somewhat unlikely, being far out of their recognised range and habitat, all such records must be treated with an open mind. Twenty or so species are of doubtful records which we have not been able to verify, or of species which have been found adjacent to but not actually within the area, but which are considered likely to be present. About 100 species have only been found as dead shells or as old records, of which 30 come from deep water.
Three groups of species which do not form an intrinsic part of the present molluscan fauna of the area have been included. The first is that of aliens washed to our shores, such as Brachydontes exustus and Ctena decussata. The second is of cold water/glacial species, of which the most obvious example is the fossil Chlamys islandicus. The third comprises warm water Lusitanian or Mediterranean species such as Osilinus lineatus and Striarca lactea which seem to have been in shell sand deposits in the Clyde, and which were recorded by the older workers. Specimens of these are in the collections at the Millport laboratory.
In any estimate of diversity of an area, a major criterion is the amount of recording effort which has taken place (see MAP below). So far as we are concerned, for a rectangle to be regarded as well worked, ideally it should have been subject to a good level of effort on the shore, by divers, and by remote means such as dredging. The methodology of these should include attention to infauna and small species - not just to the large and obvious - and the standard of identification should be very high indeed. This level of perfection has seldom been reached. Therefore at a more practical level, recording effort has been assessed as: poor, usually one visit using one method; moderate, below what could reasonably be expected given the nature of the rectangle, its accessibility, variety of habitats, etc.; good, a level of recording high enough to make an assessment of the true diversity of the location possible.
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Although the offshore, and thus less accessible areas have attracted little attention except one small area on the Hebrides Terrace and Hebridean Slope, most of the landward rectangles have been well covered. The map shows very clearly the honeypot areas, where, once it was established that diversity was naturally high, more and more work was attracted, certainly on occasion leading to disregard of other possibly more interesting locations. A case in point is that of Loch Sween, where a great deal of time and money was spent researching its possibilities as a marine nature reserve. The maximum number of living species in the major rectangle is just under 200. Several of the other sea lochs already boast figures of about 150 to 180 species, mainly as the result of one or two shoe-string surveys, and, since they usually have a greater variety of habitat, should have greater diversity than that of Loch Sween.
The MAP below shows the numbers of living species recorded per rectangle. A more reasoned, although subjective, analysis, is shown here by relating the number of species to the recording effort by omission of data from poorly-recorded areas. What cannot be shown on maps on the scale used here is the variety of habitat within a rectangle - clearly the more habitats the greater diversity. On the other hand, the heads of sea lochs, although well worked, may hold less than 10 species.
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Four areas require particular mention.
It will be seen that St Kilda, a World Heritage Site, does not show up on our maps as anything remarkable. This is because most of our records have come from diving surveys. Should what littoral zone there is be fully worked, the infauna studied, and some dredging done, its real diversity would be seen and conclusions could be drawn as to whether the mollusc diversity be outstanding or whether it might suffer from the combined effects of having few habitats and the oceanic effect of remoteness.
The Clyde area around Millport has been studied for very many years, particularly in the 19th and first half of the 20th century, the records for which do not come into the present discussion. However, on-going work has been quite intensive, producing over 200 species. Most adjacent rectangles, although scored good for recording effort, have only moderate diversity, and further research is required to establish whether this is fact, certainly so on the shores, or artifact.
One may contrast the Clyde with the Firth of Lorn and Loch Linnhe, over most of which effort has been good or exceptional. As a result, diversity appears very high indeed, with over 270 species recorded living in the Oban rectangle. The north coast of Ireland has been worked very well, yet diversity, although high, is not exceptional, except perhaps in Mulroy Bay. This almost certainly because there is not a great deal of variety of habitat.
At this stage of research, it seems likely that recording of the larger and more easily identified species found on the shore and inshore does show real distribution, if overlaid by distribution of recording effort. However, smaller species such as rissoids, pyramidellids and small bivalves are, except on the shore, using the methodology employed by Smith and Nunn, likely to be under-recorded. Offshore recording is far from complete. Records of nudibranchs are significantly patchy, being dependent on where experts on the group have dived. Dredging in the Oban area between 1992 and 1998 was done with care and at a standard of sorting of material obtained not carried out anywhere else. This has provided a wealth of data: species rarely recorded, if at all, elsewhere, are not uncommon. This indicates that similar work elsewhere should produce equally good data, with the caveat that the Firth of Clyde may well prove to be the exception.
Few records have been obtained from the south part of the area, and that part west of the Hebrides to the shelf edge and beyond is almost untouched although we have obtained valuable data collected during the cruises working out of Oban. Nearly all of this comes from a small area of the lower continental slope at 400-1300m west of 09°W and between 56°N and a little north of 57°'N. The suite of species identified probably represents the general composition of the molluscan fauna at these depths along the whole of the shelf edge in our area, as it is known that these species extend both north and south and many are in even deeper water to the west. Owing to lack of data, we have no idea if they come into shallower water or if their distribution is determined by depth or the (unrecorded) nature of the bottom. This is lamentable, as knowledge of the change of fauna from inshore to shelf to slope to abyss is of considerable importance. Species assemblages found in the deeper water are very different from those in shallower water nearer the shore. Many might not be considered part of the West Scotland fauna, but they are included here not only because they can be plotted within the area covered by the map but also because they are part of the logical progression of knowledge from shore to abyss. There are many additional species, some new to the British molluscan fauna, and it will be noted that some were collected as long ago as 1973, but many samples were not sorted or analysed until recently (Smith, in press).
The volume of data obtained to date suggests that the perceived overall picture of the distribution of the Mollusca reasonably reflects actual circumstances. Probably all the shores with particularly high diversity (rich sites) have been identified, likewise those of peculiarly low diversity. The high diversity sites have several features in common; clean water with good flow (rapids) and a variety of habitats close together, including rock, boulders, gravel of various types, possibly sand and mud, and good algal cover with good diversity. Within a small area there may also be variations in salinity such as with brackish lagoons. In fact lagoons of all kinds feature at high diversity sites. Sites of particular mention include Linne Mhuirich, Loch Sween; Clachan Sound, Seil Island; Sound of Arisaig; Ob Mheallaidh, Loch Torridon; Loch Roag, Lewis; Loch Erisort, Lewis; Loch Creran; Loch Carron west of Plockton; Strollamus, Skye; Loch Maddy, North Uist; and The Hassans, Mulroy Bay in Co. Donegal. All of these have been visited by one or both of the authors.
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Linne Mhuirich has been worked on between 1982 and 1993 (Smith, 1982d, 1985a, 1986, 1990, 1996). It is a large lagoon about 4 km long with a wide rapids exit into Loch Sween. Its more important habitats include extensive Zostera marina beds, those at the head being on mud, those nearer the rapids being on pebbles or sand; native oyster beds; and the rapids themselves. These, originally celebrated for their sponge fauna (last seen much diminished), have a considerable although not exceptional variety of Mollusca, again which seems to have diminished both in quality and quantity over the years. Within the rectangle containing Linne Mhuirich, nearly 200 species have been found alive.
Clachan Sound is a shore site par excellence (Smith, 1996, Smith & Nunn, 1985, 1992). It has good years and bad years, depending largely on rainfall (high rainfall = lower diversity), and when last seen, in 1998, was considerably poorer than on previous occasions with most of the larger and rarer bivalves absent. The impression also is that there are occasional large adventitious influxes of larvae from outwith the area, the last major one being a little before 1985. The Sound is a narrow gut about 1km long with the main current flowing in from and out to the south. It is mostly at low water under 1m deep, with considerable areas which dry out a low tide, especially at the sills at the south and north ends. Habitats include slate tips, boulders on gravel gravel and shell gravel, mud, brackish patches, rock and a rock pool which is sometimes excellent but at others full of rotten weed. There is good cover of algae. Important species include the white lyonsii variety of Calliostoma zizyphinum. More than 200 living species have been recorded within the rectangle.
The Sound of Arisaig (Smith, 1981c) has been designated a special site. It is at great risk from the tourist industry (boating, camping and caravans). Occupying about 16km2, the main bay is almost landlocked and sheltered, with tidal islands protecting it. At the mouth there are extensive maerl beds and a fairly weak current. The important habitats are sedimentary - mud, sand, maerl, which reflect the bivalve population. It is considered that lack of work is reflected by only about 140 species being recorded between 1971 and 1985. Revisiting of the area in 2004 suggested that there had been a reduction in the number of large bivalves.
Ob Mheallaidh on the south side of Loch Torridon (Smith, 1978c, 1981c, 1985c) was, when investigated, the prime mainland site north of Skye, hosting 163 species of Mollusca. It is a lagoon about 2km2 in area, at most 10m deep with a short steep sill leading to a complex rock platform shore. Within the lagoon there is a wide range of habitats including rock with crevices, mud, sand shell sand gravel, boulders, much algae especially small algae, and more brackish areas at the head. The shellsand contained a 'tenement' of Mollusca inhabiting the surface and different layers down to at least 0.5m. It is feared that this lagoon could be depauperated due to the presence of aquaculture, although on a recent drive-past, no aquaculture was seen.
The Loch Roag complex on the west side of Lewis is fascinating. It was visited in 1973, 1976, 1977 and 1982 when there was a major diving expedition (Smith, 1977, 1983b; Dipper, 1983). A recent visit, in 2005, on rather poor tides, did not add anything to the existing data. It is a huge area, over 150km2, with depths to about 50m, and with largely rocky shores with small sandy beaches, gravel and boulders. There is a variation in salinity due to input from rivers. The more interesting areas are staircases of small lagoons separated by waterfalls or rapids. Unfortunately, because of deepwater access, there are proposals to develop the loch, possibly by the oil industry. This complex is worthy of much more research, particularly offshore.
It is more difficult to state categorically which offshore areas are of particular significance. The area around Oban, including Lochs Etive and Creran, appears to be particularly rich and to have a fauna not found elsewhere. This is probably partly because it has been investigated so much, although when compared with the Clyde around Cumbrae and Loch Sween, it is still outstanding. Unfortunately in is being extensively damaged by aquaculture, particularly Mytilus edulis which are blanketting other species and Mytilus trossulus which is an unfortunate invader from Canada. Elsewhere, the Loch Carron complex is of particular note, and worthy of much more research.
The Oban area has received much attention and dredging (mostly unpublished) has been extensive. Although the shores are rocky and there are some rocky reefs offshore, most of the sea bed is silt or muddy gravel, reaching depths of over 200m. Around Dunstaffnage conditions have been found to change quite dramatically from year to year due to presence or absence of kelp forest or organic detritus. Many interesting mollusc species have been found, including Amauropsis islandicus, Aclis gulsonae (Killeen & Smith, 1992), Eulima glabra and Melanella frielei. All this work has resulted in the finding of 279 species, and it is likely that this is a particularly rich region, a catch area for larvae coming up the Firth of Lorn, but it is worth speculating that other unknown areas, not yet and probably never to be worked as well, might produce similar riches.
Loch Etive has comparatively few mollusc species. It is a narrow sea loch about 45km long with several sills and a considerable inflow of fresh water from the river Awe and at the upper end some km from the open sea. The shores are thus depauperate. Sublittorally where more saline conditions are present, the sides are rock and the basins are soft mud to 100m in depth. The most interesting species is Thyasira gouldi (Schedule 5 of the Wildlife and Countryside Act, 1981), but near the outer sill, the falls of Lora, where there are boulder beds, the rare nudibranch Aldisa zetlandica has been found (Holt, 1991b).
Loch Creran, much smaller, only about 15km long and up to 30m deep and mostly fully saline, has a wide range of habitats and considerable current areas especially at the mouth around Shian and at the bridge. There are hard and soft substrates, pebble beds, Modiolus modiolus beds and an important Serpula vermicularis bed (Moore, 1996). Hard blue clay (Clyde beds) is also present providing a habitat for Barnea candida and Zirfaea crispata. Over 200 species have been found, including those on the shore.
The Cumbrae area has been worked for many years from the marine station, and is known in great detail, despite this only having 219 living species of Mollusca, including shore species. Certain areas have been overworked (to protect other areas) to provide specimens for teaching and for sale. Much of the sea bed is composed of muddy gravel, but there are more gravelly areas between the islands.
Loch Sween, including Linne Mhuirich, was the targeted subject of research in the 1980s to provide data to support an application for it to become a marine nature reserve, which never came to fruition due to overwhelming local opposition. Apart from earlier works, 12 reports have come out of the recent research, most of the molluscan research being done by Smith (1982d, 1985a, 1986, 1990). Linne Mhuirich is described above. Loch Sween is a sill-less basin with upper arms about 20km long and up to 30m deep, with not much flushing, prone to pollution due to natural run-off compounded by tree felling and human intervention. Much of the shore is rocky or bouldery, with some sand and gravel, the sublittoral is mostly rock, muddy gravel, mud or hard clay. There are nearly 200 species, mainly on the shore.
The outer parts of our area lack data, but samples from several stations on the Hebridean slope reveal that this area has a quite extensive molluscan fauna quite different from that found nearer the coast. One sample at 56°46'N 09°17'W in 1271m contained about 80 species of Mollusc, alive and dead. The nature of the seabed was not given.
Although some species appear to be rare, we feel that a better term would be those of especial significance. It is difficult to judge the status of many. They fall into several categories: the genuinely rare or adventitious; those hard to find; those with particular habitat requirements; northern or southern species. Although the maps suggest that some species have rather restricted ranges, such as those apparently confined to sea lochs, judgement on this is reserved because further research is likely to extend distributions, for instance into the Minches' troughs. Also most sea lochs have not been worked (dredged) sufficiently for conclusions to be drawn.
The most ambitiously rare species are the southern littoral species Acanthochitona fascularis which has been found only in the Firth of Lorn area (some workers dispute the identification of this), and Onchidella celtica, of which only three records have ever been obtained (Smith, 1987a). Nunn doubts both these identifications. Likewise Calliostoma granulatum, found around Skye has only been recorded by McKay (1995). Janthina janthina is an example of a species rarely washed to our shores and included with this could be the great cephalopod Ommastrephes bartrami. Of the nudibranchs, perhaps only Embletonia pulchra and Aeoldiella sanguinea are genuinely rare. The sole record of the former may be an artifact due to Nunn's superior recording technique required for this cryptic meiofaunal species. Otina ovata has only been recorded from Islay (Smith, 1982c).
Any species which has to be dredged and is found rarely comes into the hard to find category; for example Amauropsis islandicus, Eulima glabra and Melanella frielei, some turrids and pyramidellids. Others which live sublittorally on hard substrates are difficult to find; these include Emarginula crassa, Velutina plicatilis, Aclis gulsonae (Killeen & Smith, 1992), Graphis albida and Nucula hanleyi together with Pholadidea loscombiana (Smith, 1983a). Also in this group are most Aplacophora, many small cryptic nudibranchs and small species especially bivalves which may have been sieved away.
There are many species with limited habitat requirements - many nudibranchs with specific prey needs, deep water species, etc. Calliostoma zizyphinum var. lyonsii is restricted to current areas. Here the greatest concentrations of this variety are in and around Clachan Sound.
Few species can be regarded as genuinely northern; possibly Yoldiella philippiana. However, there are many southern species at the northern ends of their ranges, of which most occur only in the north of Ireland, including: Leptochiton scabridus, Osilinus lineatus, Barleeia unifasciata, Rissoa lilacina ss, Haminoea navicula, Okenia elegans, Polycera elegans [Greilada elegans], Thecacera pennigera, Cuthona genovae, Cumanotus beaumonti, Caloria elegans, Venus verrucosa and Gastrochaena dubia.
Eatonina fulgida can be locally very common, and is expected to spread northwards. The occurrence of Calyptraea chinensis (Smith, 1998) seems to be related to oyster culture, although it is not at all common in relation to the numbers and movement of oysters. Modiolus adriaticus reaches as far north as the Firth of Lorn and Barnea candida to Loch Creran, although its limited distribution may be related to lack of habitat. Tapes decussatus, common in some locations, reaches its northern limit in Lewis (Smith, 1983b).
Common species not or very rarely found north of Skye on the mainland include: Leptochiton cancellatus, Diodora graeca, Tricolia pullus, Dikoleps pusilla, Marshallora adversa, Trophonopsis muricatus, Chrysallida fenestrata [Tragula fenestrata], Runcina coronata, Catriona gymnota, Gari depressa, Scrobicularia plana and Paphia aurea [Tapes aureus].
Much research has been undertaken in order to assess the conservation potential and needs of West Scotland. This has been summarised in Marine Conservation Review. Benthic marine ecosystems of Great Britain and the north-east Atlantic (Hiscock (ed.), 1998). The information given in this work for this area is purely factual and conservation aspects are not discussed. Many areas have been designated Special Areas of Conservation. These include:
Firth of Lorn (Lismore)
Loch Creran
Southeast Islay Skerries
Outer Hebrides (Loch Roag Lagoons)
Monarch Isles
Obain Loch Euphoirt
Sound of Barra
Loch Maddy
St Kilda
Inner Hebrides and mainland (Skye (Ascrib, Isay and Dunvegan))
Sound of Arisaig
Loch Sunart
Treshnish Isles
Loch Duich
Loch Alsh
Loch Long
Loch Laxford
Sanday
Note that Loch Sween is not on this list. There are many SSSIs and more are being added. For further information on these, it is suggested that Scottish Natural Heritage be contacted directly. Bigger areas are now being considered. Of these, the Clyde, with possibly the exception of Loch Fyne, is not of the best, but other places such as the Outer Hebrides are very much worthy of attention.
| Overview of species recorded | Recording Effort | Important Sites | Significant Species | Conservation |
Copyright © Shelagh Smith & Julia Nunn