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.     

Genetic variation in the neritic squid Loligo forbesi (Myopsida: Loliginidae) in the northeast Atlantic Ocean

Horizontal starch gel electrophoresis was employed to investigate levels of genetic differentiation between 13 samples of the neritic squid species Loligo forbesi Steenstrup obtained from throughout the majority of its known geographical range. Six enzyme loci identified in a preliminary study as being polymorphic were screened for variation between samples. No significant differences in allele distribution were detected between any of the samples obtained from the Faroe Bank in the north to Lisbon in the south, suggesting that squid throughout this range in the vicinity of the continental shelf are able to maintain panmixia, and effectively belong to a single population sharing a common gene pool. No clinal variation in allele distribution was detected throughout this range, a result which complements the findings of a detailed morphological companion study of the same individuals. Comparison of this homogenous European continental shelf population with squid from the Azores revealed highly significant (P<0.01) differences in allele distribution at five of the six polymorphic enzyme loci studied. A genetic identity value (I) equivalent to 0.93 over 33 loci was obtained. Analysis of F-statistics suggested migration rates between sites to be as low as one individual per five generations, a rate deemed insufficient under most models to prevent divergence by random genetic drift. The large distance and oceanic depths separating the Azores from continental Europe seem to present an effective barrier to gene flow to L. forbesi, a squid belonging to a family considered to be confined in distribution to relatively shallow, near coastal waters. The two populations of squid in the Azores and along the European continental shelf currently both ascribed to L. forbesi should therefore probably best be regarded as relative subspecies.     

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.     

Genetic evidence for cryptic speciation in the marine bryozoan Alcyonidium gelatinosum

The common subtidal marine bryozoan Alcyonidium gelatinosum (L.) is one of the most confusing species of a taxonomically very difficult genus. It shows massive variation for almost all morphological characters. A number of sympatric colonies collected off the coast of Lundy (Bristol Channel) could be clearly divided on morphological grounds into two quite distinct types. Using starch gel electrophoresis the products of a range of genetic enzyme loci were compared between the two morphotypes and also between these and other Bristol Channel populations of A. gelatinosum. Minimal differences were observed between one Lundy morphotype and the other allopatric populations, which it was consequently concluded were conspecific. The second morphotype showed considerable genetic differentiation [genetic identity (Nei, 1972) =0.417] from the other Lundy population, with no common alleles at some loci. The probability of obtaining the observed results by chance in a single population is significantly low (P<10^–14). It is therefore concluded that the two morphotypes of A. gelatinosum from Lundy are not conspecific.     

Multivariate analysis of the mud crab Scylla serrata (Brachyura: Portunidae) from four locations in Southeast Asia

The large, edible mud crab Scylla serrata (Forskål) exhibits different phenotypes which are recognised by the fishermen of Southeast Asia and are given local names such as “white”, “green”, “red” or “black” crabs on the basis of their colouration. A preliminary study using multivariate techniques was undertaken in order to examine the degree of dissimilarity between the different morphs of commercially fished stocks of S. serrata. Twenty-two morphometric and 20 meristic characters were measured on male crabs from four locations in three countries in Southeast Asia, including Surat Thani in Thailand where “black” and “white” morphs exist sympatrically. Canonical variate analysis (CVA) on the morphometric data discriminated three groups with no chain-linking. Surat Thani “white” crabs and those from south Vietnam were phenotypically similar and formed one distinct group; a second group contained crabs from Ranong (Thailand) and Sarawak which were phenotypically “black”; the third group contained “black” crabs from Surat Thani. CVA on meristic data confirmed two groupings, but implied that the Surat Thani “black” morph may only be a variant of the Surat Thani/Vietnam “white” form, rather than a third species. These findings are discussed in relation to the ecology, fishery management and aquaculture potenial of S. serrata.