The Mediterranean proliferation of Caulerpa taxifolia provides an opportunity to examine the extent to which an introduced species can modify biodiversity. During 1992, benthic invertebrates were sampled in Roquebrune-Cap Martin area, at depths of 6 and 10 m, from a Caulerpa station and a reference station. Species richness is slightly lower at 6 m within Caulerpa while the number of specimens is much reduced at both depths. Similarity indices compare well between reference and Caulerpa stations.

Caulerpa taxifolia, distributed in tropical seas, was recorded for the first time in the Mediterranean in 1984, as a single 1 m2 patch. This surface increased to 3 ha by 1990, 30 ha in 1991, 100 to 430 ha in 1992, and 1000 to 2000 ha at the end of 1993 (Meinesz & Hesse, 1991; Boudouresque & Meinesz, 1994).

Maximum colonization is between 2 and 6 m depth, but the alga has been observed down to 99 m. Density can reach 350 m of fronds and 14,000 leaves per m2. Caulerpa taxifolia colonizes any type of substrate and invasion results in a drastic impoverishment of the algal components of the communities (Boudouresque et al., 1992; Verlaque & Fritayre, 1994). According to these authors, maximal poverty of communities coincides with the maximal development of C. taxifolia (summer-autumn) and epiflora is negligible, except during spring.

These changes may induce functional and structural modifications of the fauna that lived in the communities before infestation. Invasion, particularly of photophilic algae and Posidonia beds (Ros et al., 1985), can modify quality and intensity of physical, chemical and hydrodynamical factors, number and quality of shelters, quantity and quality of food, and exchange of individuals between communities and populations.

An important aspect of marine biodiversity studies is identification of species and subspecies complexes. Here we present a number of examples from marine algae in which allozymes, DNA sequences and RAPDs are used to identify cryptic species and explore phylogenetic-population level relationships over several biogeographic spatial and temporal scales. In addition to evidence for recent dispersal over large distances, effects of present and past climates are reflected in the phylogenetic history of many biogeographic complexes. In some cases temperature adaptations can also be detected. Phylogeographic studies on coastal seaweeds and oceanic phytoplankton which utilize molecular genetic data contain a wealth of previously unrecognized information relevant to biodiversity.

The spawning period of the nudibranch mollusc Onchidoris bilamellata (L.), although generally extending from December to May, has occasionally been reported to occur also in summer and early autumn. Preliminary data are presented here on the larval development of summer and early autumn spawnings and compared to previous studies of the larval development of ‘normal’ (winter-spring) spawnings. The possible causes and consequences of the rare summer and early autumn spawnings are discussed.

The nudibranch mollusc Onchidoris bilamellata is distributed around the British Isles, the Atlantic coast of France, Iceland, Greenland, and the coasts of North America and Japan (Thompson & Brown, 1976). It exhibits an annual life cycle with a single spawning season, generally extending from December to May in which the adults spawn repeatedly. At the end of May/June the post-spawning mortality of the adults is usually complete (Todd, 1979). Summer and early autumn spawnings have been recorded in intertidal populations only occasionally (Todd, 1979 and references therein; present study). Larval development has been studied previously on larvae hatching from eggs laid during the ‘normal’ spawning season. No information is available on the larval growth and development of summer and early autumn spawnings. In the present study two key points were addressed: the occurrence of individuals and spawn masses in the field throughout the year; and the identification of any distinctive traits of the July and September offspring.

The field observations were carried out on the south-eastern coast of Scotland (56°18′N 2°39′W) from July 1993 to August 1994. The surface sea-water temperatures were recorded from January 1993 to August 1994 in St Andrews (56°20′N 2°47′W), seven miles from the location under study (Figure 1).

An in vitro bioassay has been developed which shows that oogenesis in Harmothoe imbricata is controlled by a gonadotrophic hormone which promotes egg protein synthesis in developing oocytes. A similar endocrine control mechanism is found in Eulalia viridis and analysis of in vivo assays indicate that the hormone acts as a transducer between the environment and the gamete. The timing of gametogenesis is controlled by environmental cues in both species and may be affected by predicted global climate change. With the development of these sensitive hormone assays it is possible to test the impact of climate change on both species, providing a possible early warning system for global warming and potential bio-indicators of climate change. Harmothoe imbricata is likely to be the more sensitive indicator species.

Clumps of macroalgae on sheltered shores exposed at low tide provide a mosaic of micro-habitats for halacarid mites. The predominant pattern is one of zonation, with the level of high water neap tides limiting the distribution of many species of Halacaridae. Within emersed clumps of macroalgae four micro-habitats are recognized: the dry surface layer of the clump; damp fronds within the clump retaining surface moisture; lower stipes and holdfasts; the surface of the substratum between the holdfasts. Halacaridae are spatially segregated, different species being associated with different micro-habitats within clumps. This has implications for the way in which intertidal phytal meiofaunal communities are sampled. In general damp fronds from within seaweed clumps are collected. Species characteristically associated with stipes, holdfasts and the substratum beneath the clumps are probably under-represented in the majority of collections, and may, therefore, be considered rare.

Marine species new to science continue to be discovered around Britain and Ireland. The number of marine species described each year was plotted against time for Pisces, Echinodermata, Anthozoa, Bivalvia, Decapoda, Gastropoda, Bryozoa, Tunicata, Medusozoa, Amphipoda, Porifera, Nudibranchia, Polychaeta, Copepoda, Oligochaeta, and Nematoda. Trends suggest that the latter four taxa in particular, in which individuals generally have a small body size, still have many species remaining to be described. More conspicuous taxa are better known, but new species continue to be described. Whilst the World Wars and advent of new scientific techniques do not appear to have had significant impacts on the general trends in discovery of new species, individual scientists have made major contributions.

The parasitic copepod Sphaeronella? callisomae, usually a single female, was occasionally found in the brood pouch of the amphipod Orchomene nanus. It is possible that this parasite consumes its host's eggs. Infected female hosts were significantly smaller than uninfected ones, so the copepod may retard host growth (or select young female hosts). Other associates (ciliates) were found in the body cavity and epizooitically on the host's gill lamellae.

Extensive collections of Orchomene nanus (Krøyer) (Crustacea: Amphipoda) have been made year-round (April 1993 to April 1994) using funnel-traps baited with crushed shore crabs (Carcinus maenas (L.)) deployed overnight (soak time 17 h) in the shallow sublittoral (5–6 m depth) at Keppel Bight, Millport (54°45·75′N 4°54·48′W: for further details, see Moore & Wong, 1995, in press a,b).

The presence of nicothoid copepods (tentatively identified by Mr M. O'Reilly, Clyde River Purification Board, as Sphaeronella? callisomae Scott; an inadequately described species, see Gotto, 1993) in the brood pouch of this amphipod were recorded routinely. At least two species of ciliates were also encountered. A sessile ciliate protozoan (?Lagenophrys sp.) occurred commonly as an epizoite of O. nanus branchiae (Figure 1A–D); and unidentified parasitic ciliates, possibly of three species, occurred in the body cavity (Figure 1E, F).

The incidence of parasitization of female O. nanus with S. ?callisomae was very low (0·81%; 8/990). There was only ever a single mature female copepod per host, although instances occurred of multiple infestations with nauplii or copepodites. Two female O. nanus were recorded with a female nicothoid coexisting with the amphipod's own eggs inside the brood pouch.