J. P. Thorpe

The use of allozyme electrophoresis in invertebrate systematics

The role of enzyme electrophoresis is discussed as it applies to taxonomy and systematics. particularly of invertebrates. Details are given of methods for distinguishing and identifying cryptic or sibling species and the different approaches to sympatric and allopatric populations are reviewed. The calculation and uses of genetic distance measures are outlined. as are the empirical relationship of such measures to different levels of taxonomic separation. Defficulties. drawbacks and limitations of the technique are explained together with the advantages. Evidence for molecular clocks is outlined briefly and their role in systematic studies is discussed, as are methods of analysing genetic divergence data for systematic purposes. References to studies covering a wide range of invertebrate taxa are tabulated.

The genetic structure of intertidal populations of two species of nudibranch molluscs with planktotrophic and pelagic lecithotrophic larval stages: are pelagic larvae “for” dispersal?

Abstract Planktonic larvae of benthic marine invertebrates may confer larval dispersal and colonization potential on the reproductive adult. Analyses of genetic heterogeneity of local populations provide an indirect means of assessing gene flow and the scale of realized dispersal of larval migrants. Here we compared populations of two species of nudibranchs with long term planktotrophic (Goniodoris nodosa) and shorter term pelagic lecithotrophic (Adalaria proxima) veliger stages. Both species have annual life cycles, population turnover is absolute and free-swimming larvae hatch from benthic spawn masses. G. nodosa larvae are pelagic for up to ∼3 months, whereas those of A. proxima can metamorphose within 1–2 d of release although, at least in the laboratory, they may delay metamorphosis for several weeks. The broad ranges of pelagic larval transport potential were estimated at 102–103 km for G. nodosa and 101–102 km for A. proxima: significant differentiation of populations of G. nodosa was predicted to occur on a scale of 103–104 km, whereas for A. proxima differentiation was expected on a scale of perhaps only 102–103 km. For both species, analyses were undertaken of allozyme variation among sites throughout the 1600 km British Isles range of A. proxima. F-statistic analyses showed no evidence of population differentiation for G. nodosa (11 populations). By contrast, comparable data for 19 populations of A. proxima showed very high levels of differentiation, but no obvious pattern of variation or significant isolation by distance. More detailed sampling of nine populations of A. proxima over a 26.18 km range in W Scotland – in a location characterised by highly dispersive tidal currents – revealed (1) similar levels of differentiation to the 1600 km range, (2) clear distance-related clinal patterns of variation, and (3) a significant inverse relationship between genetic differentiation and geographic distance. For Adalaria proxima there was evidence of significant differentiation of populations on a scale of ≤101 km. This was counter to our expectations of larval transport potential for A. proxima and implies that these larvae are behaviourally adapted to avoid dispersal in the water column and thereby recruit locally. Whilst the planktotrophic species, G. nodosa, conformed to our expectations, the clear implication of the data for A. proxima is that possession of a pelagic larval stage does not necessarily facilitate dispersal and colonisation potential for this sedentary invertebrate. Extrapolations and predictions of dispersal potential, and gene flow in natural populations, based on laboratory observations of the larval phase duration may be markedly error-prone.

Genetic and cytological divergence between colour morphs of the Mediterranean sponge Oscarella lobularis Schmidt (Porifera, Demospongiae, Oscarellidae)

The demosponge Oscarella lobularis (Schmidt, 1862) has been reported from various localities throughout the world and is abundant in many areas. It is considered to be morphologically very variable, and morphs of different colour or texture can be found living sympatrically. Samples of a soft violet morph and of cartilaginous blue, yellow and green morphs were collected from the Mediterranean Sea, near Marseilles. The genetic divergence of the four morphs was estimated by enzyme electrophoresis, and cytological differences were studied using light and transmission electron microscopy. The cartilaginous morphs were genetically practically identical, but the soft violet morph was diagnostically different from the sympatric samples of the blue and green morphs and from the allopatric yellow morph at most of the genetic loci studied (genetic identity, I ≈ 0·25). The high genetic divergence was associated with consistent differences in morphology and cytology between the violet morph and the other colour morph...

Genetic evidence for cryptic speciation in allopatric populations of two cosmopolitan species of the calcareous sponge genusClathrina

Many sponge species are considered to be cosmopolitan. However, the systematics of marine sponges are very difficult because of the paucity of taxonomically useful characters, and hence the apparently cosmopolitan nature of many species may be simply a consequence of this. In this paper, geographically distant populations of two pairs of cosmopolitan calcareous sponges of the genusClathrina were compared genetically.C. clathrus andC. cerebrum were collected by SCUBA diving between January and March 1989 from two localities: the Mediterranean Sea at La Vesse, near Marseille, Frances, at 9 to 12 m depth, and from the South West Atlantic at Arraial do Cabo, about 200 km east of Rio de Janeiro, at 2 to 10 m depth. Very high levels of gene divergence were found between the allegedly conspecific populations. The levels of genetic identity,I, observed are so low (I=0.128 and 0.287) that the populations clearly cannot be considered conspecific. New species names ofC. aurea sp. nov. andC. brasiliensis sp. nov. are therefore assigned to the southwest Atlantic counterparts ofC. clathrus andC. cerebrum, respectively. It is concluded that, at least for the species studied, and probably for many other species in taxonomically difficult groups, the actual distributions of single species may be far more geographically restricted than is generally assumed.

Effects of reduced salinity on growth and body composition in the European bass Dicentrarchus labrax (L.)

Abstract The effects of various salinities (0.5–33‰) on growth, food intake, conversion efficiency and body and blood composition of bass were investigated over a period of 1 year. Young bass were found to survive in salinities between 33 and 5‰. In freshwater the fish died after a few days. Over the period of 12 months, growth rate was maximal at 30‰ salinity and lower at 25, 33, 20, 10 and 5‰ salinity in order of decreasing growth rate. Maximum ration (Rmax) was found to be minimal at about 20–25‰ salinity. Optimum (Ro) and maintenance (Rm) rations were minimal at 30‰. Relative food intake decreased with increasing age or weight of the fish, and increased with increasing salinity. Length—weight relationships varied with salinity. The a and b values from the length—weight equations were found to be related to the proximate biochemical constitution of the fish, particularly the ash content. Salinity did not affect the proximate or amino acid compositions of the white muscle of the fish. Food conversion efficiency and protein conversion efficiency were maximal at 25‰ and 30‰ salinity, respectively. Haematocrit values were found to be affected by salinity only during the last months of the experiment. The levels of ionic constituents of the plasma examined (Na+, K+, Ca2+, Mg2+) remained constant throughout the experiment and also did not vary over the range of salinities (5–33‰) used.

Genetic differentiation between morphotypes of the marine sponge Suberites ficus (Demospongiae: Hadromerida)

Sponges of three morphotypes of Suberites ficus (Johnston, 1842) were collected during February and March 1985 off the south-west of the Isle of Man, and were compared by using spicule size distributions and genetic allele frequencies of isozyme loci. The populations did not show any significant differences of spicule size or type, but could be easily differentiated into three separate species based on isozyme patterns. Samples of pale orange S. ficus growing on gastropod shells inhabited by hermit crabs (Pagurus spp.) were reproductively isolated from the redorange and the pale yellow colour morphs encrusting the bivalve Chlamys opercularis. These latter two colour morphs were genetically similar, but significant differences were observed at two of the 19 gene loci assayed. All the sponges studied were sympatric, and therefore the genetic differences, indicating reproductive isolation, are strong evidence for separate gene pools and, hence, that they are different species. The genetic identity between the two colour morphs of S. ficus on C. opercularis shells was 0.977, whilst between each of these and S. ficus on hermit crabs it was about 0.65. In all three species genetic variability was high, with mean expected and observed heterozygosity values per locus ranging from 0.17 to 0.36.

Biological control of fouling in suspended scallop cultivation

Abstract Fouling of scallop shells and cultivation nets by living organisms is costly to remove and can reduce scallop growth. Here we investigate biological control of fouling in suspended scallop ( Pecten maximus ) cultivation. In preliminary trials in the Irish Sea, off the Isle of Man, sea urchins ( Echinus esculentus and Psammechinus miliaris ) and hermit crabs ( Pagurus spp.) removed fouling from nets more efficiently than did a range of other invertebrates. Sea urchins and hermit crabs were subsequently deployed from August 2000 to January 2001 at various densities (hermit crabs and E. esculentus at 1, 2, or 3 per net and P. miliaris at 3, 5 or 7 per net) in pearl nets containing scallops. After 6 months, the survival of biological control organisms and their effect on scallop growth and mortality, and fouling of nets and scallop shells, were assessed. Only P. miliaris was associated with increased scallop shell growth, but no biological control organism reduced scallop growth or survival. All three biological control organisms significantly reduced the weight of fouling on nets (often by as much as 50%) and fouling of scallop shells; results were largely independent of biological control organism density. Sea urchins were most effective, removing hydroids and solitary tunicates efficiently; they could thus be commercially exploited alongside scallops in a form of polyculture. These results suggest that biological control could be an efficient and environmentally sound method of addressing the problem of fouling in scallop cultivation.

Experiments on the artificial regulation of the amino acid and fatty acid contents of food organisms to meet the assessed nutritional requirements of larval, post-larval and juvenile Dover sole [Solea solea (L.)]

Abstract The probable amino acid and fatty requirements of larval Dover sole [ Solea solea (L.)] were assessed by analysis of the amino acid and fatty acid content of the yolk of Dover sole eggs. The amino acid and fatty acid contents of the brine shrimp, Artemia salina , and the the rotifer, Brachionus plicatilis , were analysed to assess their suitability for rearing sole larvae. The fatty acid composition of the egg yolk was found to be dominated by large quantities of fatty acids of the 22:6 ω 3 series, but these were present only in trace amounts in Artemia and Brachionus . Similarly, the amino acid proline, which is present in the yolk in significant quantities, is almost completely absent from both the food organisms. Experiments were carried out, using fatty acid enriched media to rear yeast species under a variety of conditions, to establish whether the yeasts might absorb some of the molecules required by the sole larvae so that these molecules could then be transferred to the food species by feeding them on the enriched yeasts. It was found that whilst the total lipid content of the yeasts could not be significantly increased, their fatty acid composition was influenced by the culture media. It was also found that the fatty acid composition of the yeasts was reflected in the fatty acid composition of the Artemia cultured on them, although the larger molecules were absorbed less readily. Trials showed that growth rates of Dover sole larvae were significantly increased by the addition of ω 3 or 18:3 ω 3 fatty acids to the yeasts used to feed Artemia . Brachionus alone proved to be an inadequate diet for the sole larvae. In terms of amino acid composition, decapsulated Artemia cysts appeared to be a promising food, but unfortunately these were not readily digested by the sole larvae.

Population structure of the Dover sole, Solea solea L., in a background of high gene flow

Abstract To investigate the genetic population structure of the Dover sole, Solea solea L., allozyme electrophoresis was performed on 303 fish collected from seven locations ranging from Cumbria, Great Britain, to Greece. A total of 22 enzyme systems were analysed, coded by 33 loci. Of these, 27 loci were polymorphic using the P 99 criterion. A phenogram using Prevostis Distance generated by the Wagner method exhibited a geographic pattern in the clustering of populations. Estimates of N m (effective number of migrants per generation between populations) were sufficiently high to imply near-panmixia between the North Sea, Bay of Biscay and the Irish Coast populations, indicating a probable movement of migrants through the English Channel. However, despite this high level of gene flow, striking patterns of geographic differentiation were observed at a few loci. Allele frequencies at loci ACOH, EST-I-1, PEP-I-2 exhibited genetic patchiness on both local and range-wide (within the northern and southern European basins) scales. This pattern of genetic patchiness could be the result of localised selection, genetic drift or single-generation sampling effects. Estimates of mean heterozygosity ( H ) were inversely related to latitude. Evolutionary processes such as genetic drift and founder effect, and/or selection, may have produced the observed difference in the number of alleles between the basins. Moreover, the absence of isolation by distance provides support for a model of geographic isolation. Such a pattern of genetic patchiness, revealing a slight reduction of genetic variability in the northern European basin, may suggest a population bottleneck, or local reduction in population size. Various physical parameters, especially water temperature during the reproductive period, vary within the range of the species, and may produce or maintain this genetic differentiation. These results indicate the role of both ecological and evolutionary structuring mechanisms in determining the genetic population structure of S . solea .

Population structure and genetic variation in two tropical sea anemones (Cnidaria, Actinidae) with different reproductive strategies

Genetic variation and population structure of two tropical sea anemones, Bunodosoma caissarum Correa and Actinia bermudensis McMurrich were related to their different dispersal capabilities and reproductive modes. B. caissarum reproduces sexually and has a long-lived planktotrophic larva; A. bermudensis can reproduce both sexually and asexually, supposedly with short-range dispersal. Both species were sampled along 1150 km of Brazilian coastline between 1990 and 1991 and analyzed by horizontal starch gel electrophoresis for 16 enzyme loci in B. caissarum and 19 in A. bermudensis. B. caissarum had higher levels of heterozygosity (H=0.35) and lower levels of population structuring (FST=0.042) than A. bermudensis (H=0.17; FST=0.262). At one locality large genetic differences between two sympatric colour morphs of A. bermudensis provide evidence of possible cryptic speciation. Overall levels of genetic variation and heterozygosity in the two species are compatible with their known modes of reproduction.