P. J. Mill

Partial sequence of the mitochondrial genome of Littorina saxatilis: relevance to gastropod phylogenetics.

Abstract. A 8022 base pair fragment from the mitochondrial DNA of the prosobranch gastropod Littorina saxatilis has been sequenced and shown to contain the complete genes for 12 transfer RNAs and five protein genes (CoII, ATPase 6, ATPase 8, ND1, ND6), two partial protein genes (CoI and cyt b), and two ribosomal RNAs (small and large subunits). The order of these constituent genes differs from those of other molluscan mitochondrial gene arrangements. Only a single rearrangement involving a block of protein coding genes and three tRNA translocations are necessary to produce identical gene orders between L. saxatilis and K. tunicata. However, only one gene boundary is shared between the L. saxatilis gene order and that of the pulmonate gastropod Cepaea nemoralis. This extends the observation that there is little conservation of mitochrondrial gene order amongst the Mollusca and suggests that radical mitochondrial DNA gene rearrangement has occurred on the branch leading to the pulmonates.

Mitochondrial DNA CoI haplotype variation in sibling species of rough periwinkles

Three sibling species of rough periwinkles are currently recognized: Littorina arcana, L. compressa and L. saxatilis. Certain forms of L. saxatilis are also argued by some to deserve species status, such as the barnacle-dwelling ‘L. neglecta’ and the lagoonal ‘L. tenebrosa’. Relationships between these taxa, and between and within representative populations, are investigated using sequence analysis and restriction fragment length polymorphism of a mitochondrial DNA fragment spanning the cytochrome oxidase I and cytochrome oxidase II gene boundary. These data show that there is some sharing of haplotypes between species, with L. arcana haplotypes paraphyletic with respect to L. saxatilis haplotypes, and L. compressa haplotypes paraphyletic to both L. arcana and L. saxatilis haplotypes. Such sharing of mtDNA haplotypes could be a consequence of either persistent hybridization or episodes of hybridization, or incomplete lineage sorting of ancestral polymorphisms. On the balance of evidence it is suggested that the latter, rather than hybridization events, is the more likely causal agent of the observed distribution. Intraspecific variation is extensive and it is suggested that the patterns of intraspecific polymorphism are explainable by a combination of historical factors (the impact of the Pleistocene ice-age) and contemporary restrictions to gene flow. It is argued that Littorina haplotypes evolved in at least two separate glacial refugia and became scattered by the subsequent range expansion around most of the coastline. Recent factors such as restricted gene flow and bottlenecks would then be capable of affecting the haplotype distribution, resulting in the pattern observed.

Lifetime egg production and egg mortality in the damselflyPyrrhosoma nymphula (Sulzer) (Zygoptera: Coenagrionidae)

Artificial oviposition sites were used to estimate egg deposition rates in the field. Females laid an average of 10.76 eggs/minute with a mean duration of 22.81 minutes, giving an average clutch size of 245 eggs. Since one mating corresponded to one clutch of eggs, lifetime mating success was used as a measure of the number of clutches produced. Mean lifetime clutch production was 5.91 clutches per female, equating to 1447 eggs per female per lifetime. Eggs were hatched in the laboratory at temperatures comparable with those in the field. Hatching was highly synchronised and the overall hatching success was 75.1%. Causes of egg mortality in the laboratory were limited to infertility and unhatchability. Since no other sources of egg mortality could be found at the study site, this value was a good reflection of hatching success in the field. Lifetime egg production and hatching success were used to estimate the number of viable offspring produced per female, giving a higher order estimate of reproductive success than has previously been published for a zygopteran.

Geographical variation in heat coma temperatures in Littorina species (Mollusca: Gastropoda)

Upper critical thermal limits were measured as heat coma temperatures in Littorina species (Mollusca: Gastropoda) from sites around the British Isles. Thermal tolerance was found to be stable within a population but extensive variation occurred between populations and species. The majority of species examined displayed heat coma values of around 30°C and did not show a positive correlation with shore height. The highest capacity for thermal tolerance was found along the coast of South Wales, while the lowest was found on the south-west coast of Ireland and on the east coast of Scotland.

Littorina neglecta Bean: ecotype or species?

The taxonomic status of the littoral prosobranch Littorina neglecta has been investigated using animals from the northeast coast of England. Rough periwinkles were collected from barnacle-covered rock platforms near the northern (Ness Point) and southern (Old Peak) ends of Robin Hoods Bay, and from small and large boulders at the southern end. Shape was analysed in shells from all sites, and the activity levels and heat stability of aspartate aminotransferase (Aat) and alanine aminotransferase (Alat) were determined in samples from Peak Steel and a nearby boulder habitat. Of the shells from Ness Point, 98% could be separated visually, using colour, sculpturing and banding, into three categories (the oviparous L. arcana, and the ovoviviparous L. saxatilis and L. neglecta). The categories were confirmed using discriminant analysis on shell measurements and cross-validation indicates at least 70% accuracy. On Peak Steel L. saxatilis breeds at a small size and its shells are visually quite distinct from thos...

Adaptive and non-adaptive variation in two species of rough periwinkle (Littorina) on British shores

Rough periwinkles are notoriously variable in shell characters. There are many reports of substantial local variation on single shores which not only make identification difficult but also may be difficult to understand in terms of likely selective pressures. We show that despite local variation in southern Britain there is evidence of a broader scale of change which is likely to be explicable in adaptive terms. At the same time, along an extreme environmental gradient on a single shore in south Wales we show that there are changes in morphology which are related to avoidance of water loss.

A geographically-based study of shell shape in small rough periwinkles

A study using principal component analysis and discriminant analysis was carried out on shell shape variation in 3093 specimens of rough periwinkles, 2500 of which were below 5.5 mm in columella length, from around the North Atlantic. Using a combination of colour plus sculpture, and life history trait, the snails were classified by inspection and examination into Littorina nigrolineata, L. arcana, L. saxatilis and L. neglecta. Principal component analyses indicated that similar aspects of variation were important in the different taxa, but these were sometimes of differing levels of importance between L. saxatilis and L. neglecta. Crossvalidation in a discriminant analysis showed classification of shells larger than 5.5 mm to have at least an 88% accuracy. That of shells below 5.5 mm showed an accuracy of 49% in L. arcana, increasing to 54% in L. saxatilis and 63% in L. neglecta, with 76% accuracy for small L. nigrolineata. This last was a special case as only one site was sampled, therefore comparative data are not available. This geographically-based study reveals that L. neglecta is more homogeneous over its range than recently reported by other workers and shows greater differences from L. saxatilis than the latter does from either L. nigrolineata or L. arcana. Size effects do not account for these differences because L. neglecta is morphometrically distinct from both large and small L. saxatilis. Furthermore, small, mid-shore L. saxatilis classify with large high-shore L. saxatilis in discriminant analysis, not with L. neglecta. These results provide evidence that the taxon L. neglecta is more distinct than has sometimes been suggested.

Heat stability and activity levels of aspartate aminotransferase and alanine aminotransferase in British Littorinidae

Aspartate aminotransferase and alanine aminotransferase activity levels were determined in crude homogenates of Littorina arcana, Littorina littorea, Littorina compressa, Littorina neglecta, Littorina obtusata, Littorina fabalis, Melarhaphe neritoides and three forms of Littorina saxatilis; high shore thin-shelled H, midshore thick-shelled M and barnacle dwelling B. In all the groups investigated, aspartate aminotransferase activity was higher than alanine aminotransferase, and high shore species appeared to have higher activity levels of aspartate aminotransferase relative to alanine aminotransferase than did low shore species. Aspartate aminotransferase was more heat stable than alanine aminotransferase at 45°C, and it was more heat stable in high shore than in low shore groups. Heat stability also varied within a species; the enzymes of L. saxatilis H were more heat stable than those of L. saxatilis M from the same site. The enzymes of a small-barnacle dwelling form of L. saxatilis showed greater heat stability than sympatric samples of L. neglecta. The possible factors involved in these differences are discussed.

Nucellicolidae: A New Family of Endoparasitic Copepods (Poecilostomatoida) from the Dog Whelk Nucella lapillus (Gastropoda)

ABSTRACT The external morphology of the adults of both sexes and of the late male copepodid of Nucellicola holmanae, new genus, new species, parasitic in a gastropod mollusc, Nucella lapillus, is described. A new family within the order Poecilostomatoida is erected to accommodate this monotypic genus. The adult parasite is found within the viscera of the host and is presently recorded only on the North Sea coast of England.

Distribution, abundance and shell morphology of Littorina saxatilis (Olivi) and Littorina arcana Hannaford Ellis at Robin Hood's Bay, North Yorkshire

The distributions of the rough periwinkles L. saxatilis and L. arcana are considered in the northern half of Robin Hoods Bay, North Yorkshire, England. The relative proportions of these two species at different sites have been determined, using the criterion of the female reproductive system for their identification. The commonest shell colour morphs are grey (both species) and orange-banded (L. saxatilis). Hence information has been obtained not only on the proportion of banded animals in the total population, but on the proportion of banded L. saxatilis.The northern end of the bay comprises a long, almost continuous, boulder field within which the proportion of L. arcana increases with greater exposure. Most of the boulder field comprises largely sandstone boulders but shale predominates at the most exposed site and, at the most sheltered site, which is separated from the rest of the boulder field by a short stretch of shingle, the boulders are exclusively shale. Banded morphs predominate in those regions where most of the boulders are sandstone but grey morphs predominate in the shale areas. A possible explanation for the distribution of these morphs is given in terms of gene flow. One site near the centre of the bay is unusual in that it contains L. arcana exclusively. However it comprises an artificial vertical substratum which is probably quite exposed.