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Robert Cameron (University of Sheffield and Natural History Museum, London)

Biogeographers use logarithmic species area curves to analyse patterns of species diversity both on continents and amongst islands. Island studies are well known, and have made major contributions to theory, but there are virtually no continental studies for molluscs.

Within Great Britain, the number of species of land mollusc increases very slowly with increasing area: based on a starting point in Suffolk, by c.25% for each 10-fold (1000%) increase in area over a range from a few 100 square meters to the island as a whole. This is what is expected in a "province", a biogeographically uniform region.

When the analysis is extended, however, curves get much steeper: if the world were "greater Suffolk", it would have only c. 250 species of land mollusc. Even within Europe, curves derived from Britain underestimate diversity, especially for countries near the Mediterranean. Increased diversity in that area is not, of course, a new discovery, but the use of species/area curves enables one to quantify differences. Local faunas in Britain are amongst the richest in Europe, and the high diversities found elsewhere are a product of very small-scale biogeographical differentiation: Europe is not a "province" in this sense.

Species/area curves have also been used to look beyond Europe, and at a variety of islands. Because they compensate for the varying sizes of various study-areas, they are a very powerful analytic tool.


The dispersal of land snails in north-west Europe during the Late-glacial and Holocene

Rich Meyrick (University of Cambridge)

Land snails are common in calcareous sediments of Late-glacial and Holocene age (the last 10-13,000 years) and are ideally suited for providing detailed palaeoenvironmental data. One of the best types of sediment for preserving land snail assemblages is a calcareous spring precipitate known as tufa. Where tufa accumulation was rapid, the temporal resolution of each sample can be on a centennial scale or less.

A recently completed study has provided detailed Late-glacial and Holocene molluscan records from several regions of Europe that have until now been largely unexamined, including the Rheinland (western Germany and Luxembourg) and Sweden. These studies collectively represent a 1200 km south-north transect of faunal successions across north-west Europe. By comparison with existing data, broad scale similarities in the development of molluscan faunas during the Late-glacial and Holocene across north-west and central Europe have been demonstrated.

The timing of key biostratigraphical events have been used to evaluate the dispersal of terrestrial molluscs from glacial refugia. It has been established that the molluscan assemblages of the British Isles and France developed in parallel with those from the Rheinland. Closed forest faunas, similar in overall composition, appeared approximately synchronously in these two regions, at approximately 7500 radiocarbon years before present (yr BP). Significantly, however, these assemblages were characterised by the occurrence of different species, Spermodea lamellata in Britain and France and Acicula polita in the Rheinland. The establishment od such faunas, which had become progressively more impoverished with increasing latitude, appears to have been delayed until at least 5000 yr BP in Skåne, southernmost Sweden, and until historical times in Östergötland, south-central Sweden.


Evolution and distribution of the terrestrial molluscs of the Açores

António M Frias Martins (Universidade dos Açores)

The Açores are an isolated oceanic archipelago, about 1400 km from the mainland of both Europe (Portugal) and America (Newfoundland). The nine islands are spread along a 600 km SE-NW axis, and form three distinct groups (Fig. 1). The islands have different geological ages, the oldest being Santa Maria (8 my). São Miguel was formed by accretion; the easternmost portion arose 4 mya, other portions followed sequentially, and the youngest, 40 thousand years ago, united what were once two islands. The history of the remaining islands lies within 1-2 my. The geographical location, the clustering, the age and the stepwise formation of the islands constitute a privileged situation for evolution to take place in a variety of ways.


Fig 1 The Açores

The terrestrial molluscs possess the highest degree of endemism of all biological taxa in the Açores and, therefore, are natural candidates for evolutionary and colonization studies. Almost half (44%) of the 107 described species are endemic: of the remaining species, 53% have European distribution and only 3% are American (Table 1). The cause of the overwhelming European character of the Açorian terrestrial malacofauna is not clearly understood in view of the present oceanic current system, for the main current influencing the Açores, the Gulf stream, runs eastwards, from the American continent to Europe. Transoceanic colonization of the islands should, then, have happened at the time of a more favourable current system, probably before the closing of the Panama Isthmus. The age of the Açores precludes, however, the feasability of such a mechanism, unless one postulates a presently sunken arch-archipelago, as was proposed to explain the evolution of Hawaiian drosophilids.

A more probable explanation is that the colonisation of the islands by terrestrial molluscs was mostly dependent on phoresis by birds. Such a method of transportation could account for the present taxonomic assemblage of the Acorian terrestrial malacofauna, for all the resident birds and the great majority of avian visitors are Eauropean.

Three main transoceanic moments of colonization can be identified in the Açores:

  • (a) the one that gave rise to the endemisms that reached genus/subgenus level - the highest endemic category on the islands (the enids, Drouetia and other zonitids, Plutonia and other vitrinids, Helixena and other Leptaxinae, as well as undescribed Geomitrinae);
  • (b) that which produced endemic species and the autochthonous fauna - widely spread and somewhat different from the European or Macaronesian populations (Craspedopoma, Hydrocaena, Carychium, Spermodea, Acanthinula, Euconulus, the Nearctic Punctum,to name but a few);
  • (c) that related to introduction by humans - generally localised (Theba, Otala, Caracollina, Rumina) or sometimes widely distributed (Helix, Deroceras, Testacella, Vallonia, among many other examples) but indistinguishable from the continental mainland populations.

The distribution within the archipelago follows a pattern related to the age of and vulcanic activity on the islands. Three main centres can be identified:

  • (a) Santa Maria, volcanically inactive for about 3.5 my. Its rich endemic malacofauna is relatively distantly related to that of the remaining islands as are the congeneric species among them - apparently, evolutionary radiation has progressed furthest in certain groups (hygromiids, zonitids) and more recent events of the kind are not detectable.
  • (b) Flores and Corvo, the former volcanically inactive for about 700 000 years. The endemics are readily distinguished from those of the other islands, at the species level: the radiation of Drouetia has produced four anatomically closely related species, readily distinguishable from consubgenerics of other islands.
  • (c) São Miguel and the whole central group, volcanically active. Variable species, such as Phenacolimax brumalis, and species complexes, such as the Oxychilus miguelinus, Oxychilus atlanticus, Macaronapaeus pruninus, Macaronapaeus delibutus complexes, spread over several islands and are indicative of relatively recent radiation of these taxa. São Miguel, with a recorded history of major cyclic eruptions, shows a complex pattern of variability, in e.g. Oxychilus atlanticus (Fig. 2), tentatively interpreted as the result of recurrent isolates by those eruptions, which do not remain separate long enough to proceed to reproductive isolation.

The terrestrial malacofauna of the Açores is at present being studied by a team of Açorian, Belgian, English and Spanish malacologists.

This presentation was partially supported by the grant PRAXIS XXI/2/2.1/BIA/169/94.

Figure 2: Oxychilus (Drouetia) atlanticus (Morelet & Drouet, 1857), endemic to São Miguel, Açores.

Table 1. Systematic distribution and geographical affinities of the Açorean pulmonate and terrestrial gastropods.

End = endemic, Hol. = Holarctic, Mac. = Macaronesian, Nea. = Nearctic, Pal. = Palearctic, * = taxa in publication.

Families

Genera

Species

total

End

Mac

Pal

Nea.

Hol.

Hydroconidae

1

1

-

1

-

-

-

Cyclophoridae

1

1

1

-

-

-

-

Ellobiidae

6

7

1

-

6

-

-

Physidae

1

1

-

-

1

-

-

Lymnaeidae

1

2

-

-

1

-

1

Planorbidae

1

1

-

-

-

1

-

Cochlicopidae

1

2

-

-

-

-

2

Vertiginidae

2

3

1

-

1

-

1

Pupillidae

2

6

5

-

1

-

-

Valloniidae

3

5

1

-

1

-

3

Enidae

1

7

7

-

-

-

-

Endodontidae

4

4

1

-

2

1

-

Arionidae

1

3

-

-

3

-

-

Vitrinidae

3*

7

7

-

-

-

-

Zonitidae

7

21

13

-

7

1

-

Milacidae

1

1

-

-

1

-

-

Limacidae

3

7

-

-

7

-

-

Euconulidae

1

1

-

-

-

-

1

Ferussaciidae

1

1

-

-

1

-

-

Subulinidae

1

1

-

-

1

-

-

Clausiliidae

1

2

1

-

1

-

-

Testacellidae

1

1

-

-

1

-

-

Helicidae

3

3

-

-

3

-

-

Helicodontidae

2

3

-

-

3

-

-

Hygromiidae

8

16

9

2

5

-

-

TOTAL

57

107

47

3

46

3

8

%(species)

44%

3%

43%

3%

7%



Hotspots of Molluscan Biodiversity: use of presence/absence data for prioritisation of conservation management in North Africa

Mary Seddon, BioSyB, National Museum of Wales, Cathays Park, Cardiff CF1 3NP. email Mary...@nmgw.ac.uk

Much of the molluscan biogeographical work in North Africa is based on species lists derived from the period 1900 to 1938. The area of coverage in Morocco and Algeria largely reflected the patterns of colonial activity, with some areas remaining poorly recorded due to the difficulties in accessing remote mountains.

New priorities for conservation management usually focus on areas with high biological diversity which has accumulated over long periods of geological time. This method of selecting areas covers widespread species with ranges spanning continental areas but also recognises that prioritisation should reflect distinctive areas with high endemism.

In North Africa, priority areas have been assessed based on information on endemic bird and mammal species (e.g. Algerian Nuthatch, Cuvier's Gazelle, Barbary Macaque), plants (succulents, cedars), and protected areas (e.g. National Parks, Forest Reserves). The sites that meet UN protected area status are Haut Atlas (Morocco; Toukbal, Orientale National Park), Al Jabal al Akdar (Libya; Kouf National Park), Babor Mountains (Algeria; Petit Kabylie). These are all centres of plant diversity for Magrebian Flora RCE (regional centres of endemism) which is part of the Mediterranean/Sahara transition.

Between 1984 and 1988 we carried out detailed mapping work which resulted in the collection of live and dead shells from 765 sites over Morocco, Algeria and Tunisia. There was little standardisation in the method of data collection, as the aim was to gather material for taxonomic revisions. However, the collecting was all undertaken between July and November, with each site visited for about 1 - 2 man hours, and collecting of common species was restricted to 30 individuals. As no material was collected during winter when conditions are wetter and several species breed, the data-set is biased. It is also biased towards rare species, so actual numbers of individuals cannot be used. Thus only the presence data has value: absence data is suspect, and must be used with care.

The North African molluscan fauna is essentially Palaearctic. However, the distinctive habitats support different molluscan faunas comprising widespread Mediterranean species, Iberian peninsula species, western and eastern Mediterranean species. The endemism in the region is usually restricted to montane regions (e.g. Moyen Atlas, Haut Atlas, Djurdjura). The highest levels of endemism and species' richness relate partly to suitable geological conditions (limestone areas) and partly to endemic vegetation zones (cedar forest). Other areas include semi-desert environments, steppe grasslands, marshes, springs and calcareous flushes.

In Morocco, the main hot spot for land-snails is the Moyen Atlas which is selected for both species richness and endemism. This area was not previously highlighted on the basis of other flora and fauna. The Haut Atlas and Agadir to Tin-Tin coast have lower levels of species richness, but the endemic species require protection. The Rif mountains would also be selected on the basis of the links to Iberian molluscan fauna.

In Algeria the main hot spots for land-snails are co-incident with the regions highlighted for their plant diversity: Babor (high molluscan species richness) and the Djurdjura (for the endemism, as well as rare European species living at the edge of their range).

In Tunisia the Zaghouan - Kef region has many Roman archaeological sites which are protected monuments; this area is important for high molluscan species richness and endemism. The other two regions with reasonable levels of endemism are Cap Bon and Matmata. None of these areas are selected as regional centres of plant diversity, although there are Ramsar sites protecting bird species adjacent to Djebel Zaghouan.

Thus it is apparent that this data-set based on molluscs, despite the problems, can be used for assessing conservation needs.

 

Area effects and geographic variation in the land snail Cepaea nemoralis.

Angus Davison ( Institute of Genetics, QMC, University of Nottingham (angu...@nott.ac.uk )

I have used two kinds of molecular variation, mitochondrial and microsatellite DNA, to investigate the relative contributions that selection, gene flow and history play in bringing about differences between neigbouring populations of snails (Davison 1999, Molec. Ecol., in press; Davison & Clarke, in prep.). In particular I have concentrated on the causes of area effects in populations of Cepaea nemoralis.

An area effect refers to unexplained geographical patterns in the frequencies of shell colours and banding (Cain and Currey 1963, Phil Trans Roy. Soc. B 246: 1). In lowland areas, there is a clear correlation between the frequencies of the various snail shell types, the type of habitat in which they live, and the activities of predators, notably the song-thrush (Cain and Sheppard 1954, Genetics 39: 89). However, on chalk downlands such as the Marlborough Downs in Wiltshire, particular colour and banding alleles predominate over large areas, apparently regardless of the habitat in which the snails live. Groups of populations are separated from each other by steep gradients of gene frequencies, over just a few hundred metres, and without any obvious change in habitat. When I mapped Cepaea allele frequencies on the Marlborough Downs, I found that a large proportion of the microsatellite differentiation was associated with the area effects. Analyses of this sort must be interpreted with care because of correlations between geographic and genetic distance, but the results strongly suggest that historical factors have been the most important influence in the generation of geographical patterns of visible as well as microsatellite markers.


Evolution of self-fertilization in Carinarion

Kurt Jordaens1, Sofie Geenen2, Patrick Van Riel1 and Thierry Backeljau1,2

(1University of Antwerp (RUCA), Department of Biology, Evolutionary Biology Group, Groenenborgerlaan 171, B-2020 Antwerp Belgium. Email jord...@ruca.ua.ac.be

2 Royal Belgian Institute of Natural Sciences, Vautierstraat 29, B-1000 Brussels ñ Belgium.)

Allozyme analyses of the hermaphroditic slugs Arion (Carinarion) fasciatus, A. (C.) circumscriptus and A. (C.) silvaticus have suggested that the three species in North America and Northwest Europe predominantly reproduce uniparentally, most probably by selfing. We used allozyme electrophoresis to investigate the population genetic structure of these species throughout a larger part of their supposedly native European distribution. Our results show that the previously ëspeciesí specific markers disappear if populations from S-SE Europe are investigated. Moreover, A. fasciatus and A. sylvaticus appear ëparaphyleticí. We therefore question the species status of the Carinarion segregates. This study is the first consistent demonstration of heterozygotes in Carinarion and it suggests a geographic component in the prevalence of outcrossing with selfing in N-NE Europe and a mixed breeding system (i.e. selfing and outcrossing) in E-SE Europe. In contrast to previous models, high selfing rates do not necessarily result in low gene diversities. We tentatively argue that, after the last ice ages, selfing Carinarion strains invaded N and NW Europe from ice-free refugia in SE Europe where outcrossing is more common.


Freshwater mussel distributions and the importance of river management

David Aldridge (Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ)

It is important to understand the factors that affect mussel biogeography and distribution for two major reasons. First mussel populations are declining worldwide (Bogan, 1994) and we can only develop informed conservation strategies if we have a good understanding of their ecological requirements. Secondly, mussels can be viewed as natural water filters and as such perform a key role in the functioning of many riverine ecosystems.

A large number of biotic and abiotic factors have been proposed as major forces in structuring mussel distributions. While factors such as salinity and calcium hardness can delimit 'potentially suitable' habitats on a biogeographical scale, there is little consensus on the factors which can explain smaller scale distributions of mussels such as patchy distributions within one catchment or even cross-channel distributions. Indeed, such difficulties in explaining small-scale mussel distributions led Strayer et al. (1994) to conclude that they "... seriously doubt whether it is worthwhile to focus on ... traditional habitat descriptors (water depth, current speed, sediment granulometry, etc.) ... in future studies of unionid ecology".

Two factors which are rarely considered as structuring forces in freshwater mussel distributions are (i) the ecology of the host fish of the mussels' parasitic glochidia larvae; (ii) the impact of river management regimes:

(i) Glochidia have an obligate reliance on host fish, and many mussels have a very narrow host range (e.g. Zale and Neves, 1982). Therefore, even if a river appears to be a suitable habitat for a particular mussel species, it will be absent if its hosts are absent. Furthermore, the host fish represents the major dispersal mechanism of unionid mussels and so the distribution of a mussel population may be defined by the distribution and habitat preferences of their major fish hosts rather than by any preferences of the mussel itself.

(ii) A study of the impact of dredging and weed cutting in a Fenland river (Aldridge, submitted) showed river management to be important in explaining small-scale mussel distributions. As well as removing up to 20% of some species, river management was shown to drag a large proportion of the mussels across the river bed. Marked mussels were found not to redistribute themselves across the channel and so the resultant concentration of mussels close to the bankside from where the JCB had operated persisted in the population (Figure 1). It is possible that management regimes are responsible for many other apparently inexplicable distributions of molluscs.

References

Bogan A E (1993) Freshwater bivalve extinctions (Mollusca: Unionoida): a search for causes. American Zoologist 33, 599-609.

Strayer D L, Hunter D C, Smith L C & Borg C K (1994) Distribution, abundance, and roles of freshwater clams (Bivalvia: Unionidae) in the freshwater tidal Hudson River. Freshwater Biology 31, 239-248.

Zale A V & Neves R J (1982) Fish hosts of four species of lampsiline mussels (Mollusca: Unionidae) in Big Mocassin Creek, Virginia. Canadian Journal of Zoology 60, 2535-2542.


Autecological studies of Vertigo angustior (Jeffreys) and Vertigo geyeri (Lindholm) at Whiteford Burrows and Waun Eurad

Eva Catharine Sharland (National Museum of Wales)

Vertigo angustior and Vertigo geyeri are both classified in the British Red Data Books as RBD1, Endangered. Both species are also considered vulnerable throughout Europe and appear on Annex II of the EC Habitats and Species Directive. These are threats of further decline in existing populations duer to habitat loss, for example by the drainage of wetlands. Monthly samples are being taken from two sites in Wales in order to gain valuable information about population dynamics. Fieldwork is concentrating on Whiteford Burrows NNR, Glamorgan (V. angustior) and Waun Eurad SSSI, Anglesey (V. geyeri). This monthly sampling will yield information about the longevity of individuals and the age structure of the population being studied. This monitoring is scheduled to continue until the end of 1999.

Studies are also trying to determine the life cycles of both these species, particularly regarding reproductive biology about which little is known. By using micro-dissection techniques, information is being obtained about fecundity and about the level of hemiphally, which may relate to selfing. Further work to be carried out during 1999 and 2000 will include comparisons with other populations of both species, and more extended sampling in the two chosen study-sites to determine the relationship between vegetation and population density in the two species.

The aim of this project is to provide recommendations for a practical monitoring strategy. This will be used to enable conservation agencies to fulfil their monitoring requirements under the UK Biodiversity Plan and the EC Habitats and Species Directive.

Vertigo angustior; Vertigo geyeri.


The terrestrial molluscs of North East Yorkshire

Tony Wardhaugh (13 Captain Cook's Crescent, Middlesbrough TS7 8NN)

Twenty woodlands in north-east Yorkshire (vice-county 62) have been surveyed for their terrestrial molluscan fauna. The woodlands include twelve considered to be ancient semi-natural (i.e. in existence since at least 1600 AD) and eight secondary deciduous (much more recent in origin and absent from first edition Ordnance Survey maps). In addition, for each wood, a record was made of the presence or absence of six species of flowering plant thought to indicate ancient status (Anemone nemorosa, Oxalis acetosella, Lysimachia nemorum, Galium odoratum, Luzula sylvatica and Carex pendula). For details of survey methods see J. Conchol, Lond. 36, 19-30.

For molluscs, ancient woods were found to be significantly more species rich (mean 34.58 +2.14, range 21-45) compared with secondary woods (mean 26.38 + 1.27, range 20-32. t = 2.897, p<0.01). The same pattern was true for flowering plants, with woods containing five or six of the plant species always containing 32 or more mollusc species. No causal relationship is implied, but this correlation may be useful in predicting which as yet unsurveyed woods might be the most species rich for molluscs where details of flora are already known.

Using data from this survey and elsewhere in VC 62, molluscan species found only in ancient woodland include Spermodea lamellata (15 woods), Limax cinereoniger (9), Columella aspera (8) and Azeca goodalli (5). In addition, the following are strongly associated with such sites: Vertigo substriata, Columella edentula, Leiostyla anglica, Cochlodina laminata, Ashfordia granulata and Perforatella subrufescens. Some of these species show less habitat restriction elsewhere in Britain, for example Cardigan (VC 46) (Chater 1997, J. Conchol. Special Publication No. 2, 165-170).

 


 

 

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