Nuclear ITS region of the alga Heterosigma akashiwo (Chromophyta: Raphidophyceae) is identical in isolates from Atlantic and Pacific basins

The internal transcribed spacer (ITS) region from 19 isolates of the algal genus Heterosigma (Chromophyta: Raphidophyceae) was amplified by polymerase chain-reaction (PCR) and sequenced. Isolates were obtained from both the Atlantic and Pacific basins, including Europe, eastern North America, western North America, Japan and New Zealand. This study presents evidence that all Heterosigma isolates in this study are representatives of one species (H. akashiwo). All 19 isolates, except one (LB 2005) had identical ITS sequence (98.31% similar by pairwise comparison); Isolate LB 2005 may represent a separate subspecies. Such high degree of ITS sequence identity implies that the organism has spread between oceanic regions in geologically recent times, possibly by human means. In addition to those from Heterosigma spp., the ITS regions from other marine Raphidophyceae (Chattonella antiqua, C. marina, C. subsalsa, Fibrocapsa japonica, and Olisthodiscus luteus) were amplified and sequenced using PCR. Total ITS lengths differed among the Raphidophyceae (C. antiqua, 577 base pairs (bp); C. marina, 577 bp;. C. subsalsa, 579 bp; F. japonica, 830 bp; H. akashiwo, 561 and 563 bp; O. luteus, 829 bp), but 5.8S rDNA sequences were similar in size (13 to 142 bp). The high ITS sequence identity between C. antiqua and C. marina (>99.9% by pairwise comparison) suggests the need for a taxonomic review of these species encompassing all morphological, genetic, physiological and biochemical information. Additionally, a number of cultures of Raphidophyceae were positively identified. In general, ITS comparisons among the Raphidophyceae may be most useful at the level of species determination rather than at the population level. Received: 12 July 1999 / Accepted: 16 March 2000     

Geographical range and taxonomic divergence in North Atlantic Calanus (C. helgolandicus,C. finmarchicus and C. glacialis)

The known distribution of Calanus helgolandicus in the North Atlantic Drift is difficult to explain in the absence of a reproductively active population inhabiting continental waters off eastern North America. New evidece indicates that this population, overlooked in the past, does in fact exist. The species has been found in a study of zooplankton samples from a number of MARMAP (Marine Resource Monitoring, Assessment, and Prediction Program) cruises surveying ichthyoplankton between Cape Hatteras and the New York Bight. Sexual activity in these stocks of C. helgolandicus was indicated by the ripeness of ovaries, the frequency of males, the presence of sperm in the females' seminal receptacles and the appearance of a female bearing a Calanus spermatophore. The new records provide a likely origin for the presence of the species in the vicinity of the Labrador Grand Banks as well as further east in the North Atlantic Drift. C. finmarchicus, similarly sexually active, was found to be sympatric with C. helgolandicus in the MARMAP collections which were taken from a region contiguous with the southernmost, known distribution, of C. glacialis. The distribution of integumental organs (i.e., pore signature patterns) was examined in the three species to determine whether they would be taxonomically useful. Strikingly different patterns were found on the female urosome. Pore signature differences between the polar species C. glacialis and the temperate C. helgolandicus proved to be as pronounced as those between the boreal C. finmarchicus and its two neighboring species. The successively overlapping ranges and the distinctive differences in pore signature patterns suggest that divergence from the generic pattern of integumental organ distribution has been a product of selection against hybridizing among the three species. If this is in fact the case, the reproductive range of C. helgolandicus has overlapped with those of C. finmarchicus and C. glacialis for appreciable periods in the history of the three species.     

Allozyme and mitochondrial DNA variation in orange roughy,Hoplostethus atlanticus (Teleostei: Trachichthyidae): little differentiation between Australian and North Atlantic populations

Allozyme and mitochondrial DNA (mtDNA) genetic variation was compared in orange roughy (Hoplostethus atlanticus Collett) collected from waters off southern Australia and from waters about 22 000 km away in the North Atlantic west of Scotland. Samples were screened for 11 polymorphic allozyme loci and with 9 restriction enzymes. Significant heterogeneity between the two areas was detected for three allozyme loci (ADA ^*, CK ^* and GPI-1 ^*), and the overall G _ST (gene-diversity statistic) value of 1% was small but significant. Significant mtDNA haplotype heterogeneity was observed after ²- of haplotype frequencies but not after a G _ST analysis. Nucleotide sequence-diversity analysis showed very low net divergence (0.0023%) between the two samples. The Australian orange roughy had a lower allozyme heterozygosity and a lower mitochondrial DNA nucleon diversity than the North Atlantic sample. The very limited, although significant, allozyme and mitochondrial DNA heterogeneity between these areas suggests that there is some gene flow between these two populations. The species appears to be widespread, with its presence reported from the southern Pacific, southern Indian, and northern and southern Atlantic Oceans, and it is likely that gene flow between the antipodes is mediated by stepping-stone exchange between adjacent populations rather than by direct migration.     

Continuous plankton records: The occurrence of apostome ciliates (Protozoa) on Euphausiacea in the North Atlantic Ocean and North Sea

The phoronts (resting cysts) of apostome ciliates have been recorded on Euphausia hemigibba, E. krohni, Meganyctiphanes norvegica, Nematoscelis megalops, Nyctiphanes couchi, Thysanoessa gregaria, T. inermis, T. longicaudata and T. raschi taken in the Continuous Plankton Recorder Survey of the North Atlantic Ocean and the North Sea. Variations in the percentage of host euphausiids infested by apostomes have been studied in relation to species, geography, body size and season. The percentage infestation was found to be related mainly to the body size of the hosts. The apostomes are believed to be species that develop either in the exuviae discarded exoskeletons or corpses of the euphausiids, or in the digestive systems of predators of the euphausiids.     

Nitrogen-fixation by cyanobacteria associated withCodium fragile (Chlorophyta): Environmental effects and transfer of fixed nitrogen

N₂-fixation associated with the green macroalgaCodium fragile subsp.tomentosoides (van Goor) Silva from Long Island, New York, USA, was attributable to several species of endophytic cyanobacteria. Rates of N₂-fixation ranged from 0.03 to 3.2µg N g^–1 dry wt h^–1 in freshly collected plants from several sites. Growth of the cyanobacteria appeared to be light-limited, due to the transmission of only 5 to 10% of incident light through the pigmented surface-layer of the macroalga. Daily irradiance was the most important factor determining both abundance of cyanobacterial cells and rate of N₂-fixation. The rate was also affected by instantaneous irradiance, and increased twofold from dark to ambient surface irradiance. Rates were reduced at low temperature (8°C) but showed no temperature effect between 12° and 26°C. External concentrations of dissolved inorganic nitrogen (DIN) up to 20µM did not influence N₂-fixation rate, but long-term exposure to 60µmol l^–1 d^–1 of NH ₄ ⁺ caused a reduction in the rate. InC. fragile grown under high daily irradiance and low external DIN concentration, ~50% of the assimilated-N was attributable to N₂-fixation. However, chlorophyllb extracted from plants grown with¹⁵N₂ showed an atom % excess¹⁵N of less than 0.1, suggesting that only a small proportion of the bacterially fixed-N was transferred to the seaweed. The association betweenC. fragile and its endophytic cyanobacteria appears to be based primarily on microhabitat suitability, rather than mutual metabolic dependence. It is doubtful that N₂-fixation by cyanobacteria is important to the ecological success of this seaweed species.     

Genetic divergence between east and west Atlantic populations of<Emphasis Type="Italic"> Actinia</Emphasis> spp. sea anemones (Cnidaria: Actiniidae)

The sea anemone Actinia equina was considered a highly variable species with a wide geographical distribution, but molecular systematic studies have shown that this wide distribution may be the result of the lumping of cryptic species. In this work enzyme electrophoresis was used to analyse the genetic variability of A. equina from the Atlantic coasts of Europe and Africa, as well as the relationships between those populations and other species of the genus. Samples of A. equina from the United Kingdom and France were compared with supposedly conspecific populations from South Africa and a recently described species from Madeira, Actinia nigropunctata. The South African and Madeiran populations were genetically very divergent from each other (genetic identity, I=0.15), as well as from the A. equina population from the United Kingdom and from the other species studied (I between 0.23 and 0.78), indicating that the African population of A. equina belongs to a different species. High genetic similarities were found between west Atlantic A. bermudensis and east Atlantic A. sali, in spite of the great geographical distance between them, and between the European A. equina and A. prasina. The results confirm the presence of the new species A. nigropunctata from Madeira and suggest a new species within the A. equina species group, from South Africa.Communicated by O. Kinne, Oldendorf/Luhe     

Reproductive isolation with little genetic divergence between Urticina (=Tealia) felina and U. eques (Anthozoa: Actiniaria)

There has long been doubt as to whether there are one or two British species of the sea anemone genus Urticina. In the present study, populations of both putative species, which occur sympatrically in the Isle of Man, have been compared by electrophoresis of isozymes and nematocyst analysis. The analysis of fourteen isozyme loci exhibited a high genetic identity between the two taxa (I=0.907), but four loci showed significant differences in gene frequencies; thus indicating reproductive isolation. The populations displayed highly significant differences in the sizes of the various nematocyst types. The individual analysis of data for each type of nematocyst revealed that the basitrics from the actinopharynx and from the tentacles contribute most to the observed difference. Consequently, U. felina (L.) and U. eques (Gosse) are assumed to be valid species, as suggested by Manuel (1981). The mean heterozygosities for each species (0.410 and 0.436, respectively) are the highest reported for cnidarians.     

Genetic differentiation in a pelagic crustacean (Meganyctiphanes norvegica: Euphausiacea) from the North East Atlantic and the Mediterranean Sea

Meganyctiphanes norvegica (M. Sars)is a pelagic crustacean that plays a key role in marine food webs of North Atlantic Ocean and marginal seas. We studied eight population samples collected in the European Atlantic and Mediterranean Sea. By means of single strand conformation polymorphism analysis (SSCP) and direct sequencing, we investigated a segment of 158 base pairs of the mitochondrial gene coding for the subunit 1 of NADH dehydrogenase. We found 12 sequence variants among the 385 individuals studied. Analysis of molecular variance (AMOVA) showed that 14.75% of the total genetic variability was explained by differences between populations, thus indicating absence of panmixia for these populations. Pairwise comparisons revealed three distinct genetic pools: the first one represented by Cadiz Bay, the second one by the Ligurian Sea, and the third one included all the NE Atlantic samples. We also investigated one population from the Alboran Sea (within the Mediterranean basin, east of the Strait of Gibraltar). This population was found to be genetically intermediate between the NE Atlantic samples and the Ligurian sample, suggesting that the restriction to the gene flow is not associated with the Strait of Gibraltar, but possibly with the Oran–Almeria oceanographic front. The present work indicates that M. norvegica, although endowed with a high dispersal capacity because of its pelagic habit, can develop separate breeding units inside the same oceanic basin (the Atlantic). Furthermore, the Ligurian sample should be considered as a distinct evolutionary entity, separated from the Atlantic population. Received: 2 May 1999 / Accepted: 26 November 1999     

Molecular phylogeny of mud crabs (Brachyura: Panopeidae) from the northwestern Atlantic and the role of morphological stasis and convergence

 Mud crabs of the family Panopeidae are common organisms in coastal soft-bottom, vegetated, rubble, and oyster-bed communities along the temperate and tropical coastlines of the American continent. Similar morphology among many species renders their distinction and classification difficult. Here, we present phylogenies of western Atlantic Panopeidae based on DNA sequences of the mitochondrial large subunit rRNA (16S; 529 basepairs) and cytochrome oxidase I (COI; 640 basepairs) genes. Results suggest that the speciose genera Panopeus and Eurypanopeus are not monophyletic and that their taxonomy does not accurately reflect evolutionary partitions. In two cases (P. herbstii complex and E. depressus and allies), the molecular findings strongly support sister-species relationships that differ from previous morphology-based assumptions. We suggest that convergence or morphological stasis are responsible for the phenotypic similarities between divergent evolutionary lineages. Received: 23 July 1999 / Accepted: 5 April 2000     

Phylogeography of two Atlantic squirrelfishes (Family Holocentridae): exploring links between pelagic larval duration and population connectivity

Genetic surveys of reef fishes have revealed high population connectivity within ocean basins, consistent with the assumption that pelagic larvae disperse long distances by oceanic currents. However, several recent studies have demonstrated that larval retention and self-recruitment may be higher than previously expected. To assess connectivity in tropical reef fishes, we contribute range-wide mtDNA surveys of two Atlantic squirrelfishes (family Holocentridae). The blackbar soldierfish, Myripristis jacobus, has a pelagic juvenile phase of about 58 days, compared to about 71 days (~22% longer) in the longjaw squirrelfish, Holocentrus ascensionis. If the pelagic duration is guiding dispersal ability, M. jacobus should have greater population genetic structure than H. ascensionis. In comparisons of mtDNA cytochrome b sequences from 69 M. jacobus (744 bp) and 101 H. ascensionis (769 bp), both species exhibited a large number of closely related haplotypes (h=0.781 and 0.974, π=0.003 and 0.006, respectively), indicating late Pleistocene coalescence of mtDNA lineages. Contrary to the prediction based on pelagic duration, M. jacobus has much less population structure (φ_ST=0.008, P=0.228) than H. ascensionis (φ_ST=0.091, P<0.001). Significant population partitions in H. ascensionis were observed between eastern, central and western Atlantic, and between Brazil and the Caribbean in the western Atlantic. These results, in combination with the findings from 13 codistributed species, indicate that pelagic larval duration is a poor predictor of population genetic structure in Atlantic reef fishes. A key to understanding this disparity may be the evolutionary depth among corresponding taxonomic groups of “reef fishes”, which extends back to the mid-Cretaceous and encompasses enormous diversity in ecology and life history. We should not expect a simple relationship between pelagic larval duration and genetic connectivity, among lineages that diverged 50–100 million years ago.     

Complex patterns of population structure and recruitment of <Emphasis Type="Italic">Plectropomus leopardus</Emphasis> (Pisces: Epinephelidae) in the Indo-West Pacific: implications for fisheries management

Here the population genetic structure of an ecologically and economically important coral reef fish, the coral trout Plectropomus leopardus, is investigated in the context of contemporary and historical events. Coral trout were sampled from four regions (six locations) and partial mtDNA D-loop sequences identified six populations (Fst = 0.89209, P < 0.0001): Scott Reef and the Abrolhos Islands in west Australia; the Great Barrier Reef (GBR), represented by northern and southern GBR samples; New Caledonia and Taiwan, with Taiwan containing two genetic lineages. Furthermore, this study identified source and sink populations within and among regions. Specifically, the northern population in west Australia (Scott Reef) was identified, as the source for replenishment of the Abrolhos population, whilst New Caledonia was a source for recruitment to the GBR. Based on these insights from a single mtDNA marker, this study will facilitate the development of rational management plans for the conservation of P. leopardus populations and therefore mitigate the risk of population declines from anthropogenic influences.     

Benthic amphipods (Crustacea: Malacostraca) in Icelandic waters: diversity in relation to faunal patterns from shallow to intermediate deep Arctic and North Atlantic Oceans

Patterns in diversity, species replacement and species composition of gammaridean amphipods (Crustacea: Malacostraca) were studied in two areas on different sides of the Greenland–Iceland–Faeroe Ridge. One was sited south of the Ridge in the North Atlantic Ocean off southern Iceland and the other north of the Ridge in the Arctic Ocean (subarctic and arctic waters of the Iceland Sea) off northern Iceland. Samples were taken with a Rothlisberg–Pearcy epibenthic sled within the depth range of about 50 to 1200 m at a total of 28 stations. In all 21 150 individuals were found belonging to 156 species. Of these 102 were found in the northern area with 64 of the species restricted to this area. In all 94 species were found in the southern area with 52 of the species restricted to this area. This suggests a similar regional diversity, but a different faunal composition. The amphipods showed similar species replacement patterns with depth on both sides of the Ridge, while the diversity patterns differed between the areas. In the North Atlantic the number of species increased with depth, while in the Arctic there seemed to be no pattern at all. It is suggested that diversity patterns of amphipods on each side of the Ridge are shaped by different factors, and these may not be the same as those determining diversity patterns of other groups in the area. Salinity (as indicative of water masses) was found to be the most important environmental variable in explaining the species composition when both areas were considered, suggesting a number of contributing factors (adaptation to water masses, dispersal, etc.). Water temperature was the most important variable in explaining the variance in the species composition in the northern area, while depth was the most important variable in explaining the variance in the species composition of the southern area. Received: 9 March 1997 / Accepted: 10 December 1997     

Latitudinal variation in aspects of the biology of Cyclothone braueri and C. microdon (Pisces: Gonostomatidae) in the eastern North Atlantic Ocean

Samples from a series of RMT-8 tows along 20°W in spring and autumn of various years were used to study the variability in aspects of the breeding biology of two species of Cyclothone: C. braueri and C. microdon in the eastern North Atlantic Ocean. Variables compared within each species included length-frequency distribution, maximum size, size at maturity, sex ratio, egg size and fecundity. Populations of both species at lower latitudes were generally smaller, reached maturity at a smaller size, and probably spawned earlier in the year and over a longer period than populations at higher latitudes. In both species, fecundity was highest at 40°N in the spring. The results suggested that C. microdon may not spawn at the northern extent of its range.     

Population structure of short-beaked common dolphins (<Emphasis Type="Italic">Delphinus delphis</Emphasis>) in the North Atlantic Ocean as revealed by mitochondrial and nuclear genetic markers

The understanding of population structure and gene flow of marine pelagic species is paramount to monitoring, management and conservation studies. Such studies are often hampered by the potentially high dispersal behavior of the species, the lack of obvious geographical barriers in the marine environment and the scarce sample availability. Short-beaked common dolphins (Delphinus delphis) are widespread in coastal and open-ocean habitats of the North Atlantic Ocean, nevertheless population structure and migratory patterns are poorly understood. Furthermore, concern has been raised about the status of the species because large numbers of dolphins have been taken incidentally in several fisheries throughout the North Atlantic in the past decades. In the present study, a large number of individual samples were obtained from seasonal and spatial aggregations of common dolphins from western (wNA) and eastern North Atlantic (eNA) regions, mostly using opportunistic sampling (i.e. from incidental entanglement in fishing gear or beach-cast carcasses). Genetic variability was investigated using nuclear (14 microsatellite loci) and mitochondrial (360 bp of the control region) genetic markers. Levels of genetic diversity were relatively high in all sampled areas and no evidence of recent reduction of effective population size (i.e. bottleneck) was detected at the nuclear loci. Significant population structure was detected between the two main regions (wNA and eNA) where it appeared to be more pronounced at mitochondrial (F _ST = 0.018, P < 0.001) than nuclear markers (F _ST = 0.005, P < 0.05), indicating the presence of at least two genetically distinct populations of common dolphins in the North Atlantic Ocean. In contrast, no significant genetic structure was detected between temporal aggregations of dolphins from within the same region, suggesting possible seasonal movement patterns at a regional scale. The observed levels of genetic differentiation between classes of markers are discussed here as a possible consequence of migratory patterns or recent population subdivision. An erratum to this article can be found at     

Calorific,ash, carbon and nitrogen content in relation to length and dry weight of Parathemisto gaudichaudi (Amphipoda: Hyperiidea) in the North East Atlantic Ocean

The calorific, ash, carbon and nitrogen content, length and dry weight were determined for the hyperiid Parathemisto gaudichaudi (Guerin). Regression equations for all these variables were determined so that they can be estimated by calculation from measurements of length of the hyperiid. Mean values for total nitrogen and carbon were 7.79±0.85% and 36.80±4.18% of the dry weight, respectively. The carbon to calorific equivalent for P. gaudichaudi was 10.37 kcal g^-1 carbon (9.13 kcal g^-1 when corrected for nitrogen). The calorific value for ash-free adult P. gaudichaudi was 5.138 kcal g^-1±1.309 (4.510 kcal g^-1 when corrected for nitrogen). This large variation in the calorific content (coefficient of variation of 25.84%) can be accounted for largely by variation in the ash content (coefficient of variation of 21.84%). The calorific value determined for P. gaudichaudi is similar to that measured for other carnivorous crustaceans and adds support to the hypothesis that animals with high calorific content have a low fecundity and an energy-rich store which can be used as a buffer during unfavourable periods in their life.     

Analysis of ribosomal ITS1 sequences indicates a deep divergence between Rhodactis (Cnidaria: Anthozoa: Corallimorpharia) species from the Caribbean and the Indo-Pacific/Red Sea

This paper explores the implications of ribosomal ITS1-sequence analysis for evolutionary relationships between four species of corallimorphs which are morphologically very similar but have distinct distributions. The analyses highlight the unreliability of morphological criteria in establishing systematic relationships amongst tropical corallimorphs. Both parsimony and distance methods of phylogenetic analysis strongly support a deep divergence between the Caribbean species Rhodactics sanctithoma and the three morphologically-related species R. howesii and R. indosinensis from the Indo-Pacific and R. rhodostoma from the Red Sea. The analyses also strongly support a closer relationship between the Red Sea species, R. rhodostoma, and one of the Indo-Pacific species, R. howesii, than between these two and the second Indo-Pacific species, R. indosinensis. The first of these results is presumably a consequence of the closure of the Central American Seaway, the event which led to the separate development of Atlantic and Indo-Pacific coral faunas. However, the lack of detailed distribution data represents a major barrier to reconstructing the evolutionary history of the genus.     

Molecular relationships and species divergence among <Emphasis Type="Italic">Phragmatopoma</Emphasis> spp. (Polychaeta: Sabellaridae) in the Americas

Phragmatopoma spp. are marine, reef-building polychaetes that inhabit the intertidal and shallow subtidal zones of both coasts of the Americas. Phragmatopoma californica is found in the Pacific Ocean along the California coast south to Mexico, while Phragmatopoma caudata inhabits the Caribbean islands and Atlantic Ocean from the Florida coast south to Brazil. Although apparently geographically isolated, P. californica and P. caudata have been found to interbreed (Pawlik 1988a) and thus their specific taxonomic relationship has been unclear. In this study, two genes, cytochrome c oxidase subunit I (COI) and the first internal transcribed spacer region (ITS-1), were sequenced to assess the specific status of P. californica and P. caudata as well as Phragmatopoma virgini. Comparison of sequences revealed that samples of P. californica shared no COI haplotypes or ITS-1 sequences with P. caudata. Phylogenetic analyses, including maximum parsimony and Bayesian methods, clustered each species in separate, well-supported clades with genetic distances between them being greater than between either contested species or the uncontested, valid species, P. virgini. Thus, the molecular data support that P. californica and P. caudata are separate species. However, the sample of individuals of P. virgini included one genetically divergent individual, whose morphology was found to match that of a species formerly recognized as P. moerchi but since synonymized with P. virgini. Divergences among lineages were dated using the COI sequences, after adjustment for non-clock-like behavior. Consequently, we suggest that P. virgini and P. caudata are sister taxons and that P. californica diverged from the P. virgini/P. caudata clade ∼5.7 mya with P. virgini diverging from P. caudata ∼3 mya.     

Genetic divergence between colour morphs in populations of the common intertidal sea anemones Actinia equina and A. prasina (Anthozoa: Actiniaria) in the Isle of Man

A distinctive morph ofActinia equina (L.) is found at low frequency among populations on intertidal hard substrata on some shores on the Isle of Man. This morph is red with rows of green oval spots or elongate markings running longitudinally down the column. Starch gel electrophoresis of 21 allozyme loci was used to compare samples of this morph from two localities with other sympatricActinia spp. Collections were made in early summer 1986. For one sampling site the comparison was with redA. equina and greenA. prasina Gosse, whilst at the other site with redA. equina only. At both sites significant genetic differences in allele frequencies at several loci were found between the new morph and sympatricA. equina andA. prasina. The results also confirm the reproductive isolation ofA. prasina. As with much previous work on sea anemones, levels of mean heterozygosity per locus (H) were found to be high (H=0.157 to 0.342). A surprising feature of the results, although bearing out earlier unpublished data, is that high levels of genetic differentiation are found between populations of what appear to be the same morphs collected from shores only a few km apart. Among theActinia spp. studied, differentiation between populations of the same morph on different shores was of the same order as between sympatric, reproductively isolated populations.     

Dispersal and use of sponges and ascidians as camouflage by Cryptodromia hilgendorfi (Brachyura: Dromiacea)

From May 1977 to February 1979, the use of sponges and ascidians by Cryptodromia hilgendorfi was studied in Moreton Bay, Queensland, Australia. The aim was to investigate patterns of seasonal use, cap making behaviour, cap turnover, the effect of intraspecific interactions on cap life and the effect of movement of crabs between hosts on background matching. C. hilgendorfi uses 12 (of 16 available) species of sponge and 3 species of ascidians to construct caps, which are carried by the crabs using their last two pairs of legs. Cap area increases non-linearly with crab size, and caps are normally two to three times as large as the crabs. Cap making behaviour is described. It occurs during intermoult periods, with females making most of their caps at night. Caps decrease in size with time, but conceal the crabs which commonly occupy exposed sites on sponges. Cap life is independent of crab size, differs between different cap species and is influenced by the presence of other crabs who can dislodge caps through aggressive behaviour. Caps are made from the sponge Suberites carnosus more often than from other available sponges. S. carnosus caps also decay less rapidly than caps made from other sponges. Use of sponge and ascidian species varies seasonally, with Halichondria sp. and S. carnosus being used in all months. C. hilgendorfi exhibits a preference for certain sponges. The majority of crabs carried caps which matched their host sponge or ascidian, but mis-matches varied seasonally with a winter peak following the breeding season. Young C. hilgendorfi settle only on S. carnosus sponges and disperse from this host to other species in the environment. Males and females differ in their rate of discovery of new hosts. Males, despite their greater mobility, find new hosts slower than females. It is hypothesized that males occupy “home ranges” which females do not. Crabs frequently move between sponges, mostly at night. Sponges and ascidian species grow in intimate association with each other, and sponge crabs act a selective asexual propagation mechanism. Depending upon the nature of the interactions between sponge and ascidian species (co-operative or competitive) and whether competitive hierarchies or networks are involved, the sponge crabs may have either stabilizing or destabilizing effects on the sponge community.