Geographic distribution of the bathypelagic genus paraeuchaeta (Copepoda,Calanoida)

The geographic distribution of the bathypelagic calanoid genus Paraeuchaeta was investigated by examining midwater trawl and plankton net samples collected mostly from depths exceeding 1000 m throughout the world's oceans. Of the 81 species referred to Paraeuchaeta, the geographic ranges of about 50 species could be defined with reasonable certainty. Contrary to early authors, the number of species having a worldwide distribution was surprisingly small (12 species or 15% of the 81 species of the genus) as compared to common species endemic to various geographic regions (34 species or 42% of the total of 81). Almost twice as many species were found in the Indo-Pacific Ocean as in either the Atlantic or the Southern Ocean. Faunistically, the northern Atlantic, mid-Atlantic, northern Pacific, East Pacific, Indo-West Pacific, and Southern Ocean were distinct in terms of endemic species. A number of species were found to be endemic to highly productive areas, where they were usually very abundant. Rare species, on the other hand, were generally found to be widely distributed, although some were too rare for their range to be determined with certainty. To explain these findings, the following hypothesis is proposed: Bathypelagic calanoids endemic to and abundant in eutrophic areas are those adapted to eutrophic conditions of their habitats and therefore cannot expand their ranges into contiguous oligotrophic waters even if the other environmental conditions are favorable. Other species, on the other hand, generally have extensive geographic ranges because of their survival ability in widely expanding oligotrophic conditions and the absence of physicochemical barriers at bathypelagic depths of the world's oceans.     

Worldwide patterns of mitochondrial DNA differentiation in the harbor seal (Phoca vitulina)

The harbor seal (Phoca vitulina) has one of the broadest geographic distributions of any pinniped, stretching from the east Baltic, west across the Atlantic and Pacific Oceans to southern Japan. Although individuals may travel several hundred kilometers on annual feeding migrations, harbor seals are generally believed to be philopatric, returning to the same areas each year to breed. Consequently, seals from different areas are likely to be genetically differentiated, with levels of genetic divergence increasing with distance. Differentiation may also be caused by long-standing topographic barriers such as the polar sea ice. We analyzed samples of 227 harbor seals from 24 localities and defined 34 genotypes based on 435 bp of control region sequence. Phylogenetic analysis and analysis of molecular variance showed that populations in the Atlantic and Pacific Oceans and east and west coast populations of these oceans are significantly differentiated. Within these four regions, populations that are geographically farthest apart generally are the most differentiated and often do not share genotypes or differ in genotype frequency. The average corrected sequence divergence between populations in the Atlantic and Pacific Oceans is 3.28% +/- 0.38% and those among populations within each of these oceans are 0.75% +/- 0.69% and 1.19% +/- 0.65%, respectively. Our results suggest that harbor seals are regionally philopatric, on the scale of several hundred kilometers. However, genetic discontinuities may exist, even between neighboring populations such as those on the Scottish and east English coasts or the east and west Baltic. The mitochondrial data are consistent with an ancient isolation of populations in both oceans, due to the development of polar sea ice. In the Atlantic and Pacific, populations appear to have been colonized from west to east with the European populations showing the most recent common ancestry. We suggest the recent ancestry of European seal populations may reflect recolonization from Ice Age refugia after the last glaciation.      

A biological consequence of reducing Arctic ice cover: arrival of the Pacific diatom Neodenticula seminae in the North Atlantic for the first time in 800 000 years

The Continuous Plankton Recorder survey has monitored plankton in the Northwest Atlantic at monthly intervals since 1962, with an interegnum between 1978 and 1990. In May 1999, large numbers of the Pacific diatom Neodenticula seminae were found in Continuous Plankton Recorder (CPR) samples in the Labrador Sea as the first record in the North Atlantic for more than 800 000 years. The event coincided with modifications in Arctic hydrography and circulation, increased flows of Pacific water into the Northwest Atlantic and in the previous year the exceptional occurrence of extensive ice‐free water to the North of Canada. These observations indicate that N. seminae was carried in a pulse of Pacific water in 1998/early 1999 via the Canadian Arctic Archipelago and/or Fram Strait. The species occurred previously in the North Atlantic during the Pleistocene from∼1.2 to∼0.8 Ma as recorded in deep sea sediment cores. The reappearance of N. seminae in the North Atlantic is an indicator of the scale and speed of changes that are taking place in the Arctic and North Atlantic oceans as a consequence of regional climate warming. Because of the unusual nature of the event it appears that a threshold has been passed, marking a change in the circulation between the North Pacific and North Atlantic Oceans via the Arctic. Trans‐Arctic migrations from the Pacific into the Atlantic are likely to occur increasingly over the next 100 years as Arctic ice continues to melt affecting Atlantic biodiversity and the biological pump with consequent feedbacks to the carbon cycle.     

Evolution of the Northern Rockweed,Fucus distichus,in a Regime of Glacial Cycling: Implications for Benthic Algal Phylogenetics

Northern hemisphere rockweeds (Fucus) are thought to have evolved in the North Pacific and then spread to the North Atlantic following the opening of the Bering Strait. They have dispersed and widely speciated in the North Atlantic and its tributary seas. Fucus distichus is likely near the ancestral member of this genus, and studies have shown that there are several species/subspecies in this complex (i.e. F. evanescens and F. gardneri). We used phylogenetic and haplotype analyses to test the phylogenetic relationships and biogeography of F. distichus. Our data and subsequent analyses demonstrate that, unlike previous studies that lacked samples from an extensive geographical area of the Arctic and Subarctic, there is a distinct Arctic haplotype that is the source of subspecies in both the North Pacific and North Atlantic. Fucus distichus occupies a low tide zone habitat, and in Arctic/Subarctic regions it is adapted to the severe stress of sea ice coverage and disturbance during many months per year. We hypothesize that the very large geographic area of Arctic and Subarctic rocky shores available to this species during interglacials, supported by large Arctic/Subarctic fringe areas as well as unglaciated refugia during glacial cycles, provided a robust population and gene pool (described by the Thermogeographic Model). This gene pool dilutes that of the more fragmented and area-limited Temperate/Boreal area populations when they are brought together during glacial cycles. We suggest that similar subspecies complexes for a variety of Arctic/Subarctic shore biota should be examined further in this context, rather than arbitrarily being split up into numerous species.     

Limited genetic variation and structure in softshell clams (Mya arenaria) across their native and introduced range

To offset declines in commercial landings of the softshell clam, Mya arenaria, resource managers are engaged in extensive stocking of seed clams throughout its range in the northwest Atlantic. Because a mixture of native and introduced stocks can disrupt locally adapted genotypes, we investigated genetic structure in M. arenaria populations across its current distribution to test for patterns of regional differentiation. We sequenced mitochondrial cytochrome oxidase I for a total of 212 individuals from 12 sites in the northwest Atlantic (NW Atlantic), as well as two introduced sites, the northeast Pacific (NE Pacific), and the North Sea Europe (NS Europe). Populations exhibited extremely low genetic variation, with one haplotype dominating (65–100%) at all sites sampled. Despite being introduced in the last 150–400 years, both NE Pacific and NS Europe populations had higher diversity measures than those in the NW Atlantic and both contained private haplotypes at frequencies of 10–27% consistent with their geographic isolation. While significant genetic structure (F _ST = 0.159, P < 0.001) was observed between NW Atlantic and NS Europe, there was no evidence for genetic structure across the pronounced environmental clines of the NW Atlantic. Reduced genetic diversity in mtDNA combined with previous studies reporting reduced genetic diversity in nuclear markers strongly suggests a recent population expansion in the NW Atlantic, a pattern that may result from the retreat of ice sheets during Pleistocene glacial periods. Lack of genetic diversity and regional genetic differentiation suggests that present management strategies for the commercially important softshell clam are unlikely to have a significant impact on the regional distribution of genetic variation, although the possibility of disrupting locally adapted stocks cannot be excluded.     

Species-level phylogeography and evolutionary history of the hyperdiverse marine gastropod genus Conus

Phylogenetic and paleontological analyses are combined to reveal patterns of species origination and divergence and to define the significance of potential and actual barriers to dispersal in Conus, a species-rich genus of predatory gastropods distributed throughout the world's tropical oceans. Species-level phylogenetic hypotheses are based on nucleotide sequences from the nuclear calmodulin and mitochondrial 16S rRNA genes of 138 Conus species from the Indo-Pacific, eastern Pacific, and Atlantic Ocean regions. Results indicate that extant species descend from two major lineages that diverged at least 33 mya. Their geographic distributions suggest that one clade originated in the Indo-Pacific and the other in the eastern Pacific + western Atlantic. Impediments to dispersal between the western Atlantic and Indian Oceans and the central and eastern Pacific Ocean may have promoted this early separation of Indo-Pacific and eastern Pacific + western Atlantic lineages of Conus. However, because both clades contain both Indo-Pacific and eastern Pacific + western Atlantic species, migrations must have occurred between these regions; at least four migration events took place between regions at different times. In at least three cases, incursions between regions appear to have crossed the East Pacific Barrier. The paleontological record illustrates that distinct sets of Conus species inhabited the Indo-Pacific, eastern Pacific + western Atlantic, and eastern Atlantic + former Tethys Realm in the Tertiary, as is the case today. The ranges of <1% of fossil species (N=841) spanned more than one of these regions throughout the evolutionary history of this group.     

Spatial variation and low diversity in the major histocompatibility complex in walrus (Odobenus rosmarus)

Increased global temperature and associated changes to Arctic habitats will likely result in the northward advance of species, including an influx of pathogens novel to the Arctic. How species respond to these immunological challenges will depend in part on the adaptive potential of their immune response system. We compared levels of genetic diversity at a gene associated with adaptive immune response [Class II major histocompatibility complex (MHC), DQB exon 2] between populations of walrus (Odobenus rosmarus), a sea ice-dependent Arctic species. Walrus was represented by only five MHC DQB alleles, with frequency differences observed between Pacific and Atlantic populations. MHC DQB alleles appear to be under balancing selection, and most (80 %; n = 4/5) of the alleles were observed in walruses from both oceans, suggesting broad scale differences in the frequency of exposure and diversity of pathogens may be influencing levels of heterozygosity at DQB in walruses. Limited genetic diversity at MHC, however, suggests that walrus may have a reduced capacity to respond to novel immunological challenges associated with shifts in ecological communities and environmental stressors predicted for changing climates. This is particularly pertinent for walrus, since reductions in summer sea ice may facilitate both northward expansion of marine species and associated pathogens from more temperate regions, and exchange of marine mammals and associated pathogens through the recently opened Northwest Passage between the Atlantic and Pacific Oceans in the Canadian high Arctic.     

Global warming and arctic terns: Estimating climate change impacts on the world's longest migration

Climate change is one of the top three global threats to seabirds, particularly species that visit polar regions. Arctic terns migrate between both polar regions annually and rely on productive marine areas to forage, on sea ice for rest and foraging, and prevailing winds during flight. Here, we report 21st-century trends in environmental variables affecting arctic terns at key locations along their Atlantic/Indian Ocean migratory flyway during the non-breeding seasons, identified through tracking data. End-of-century climate change projections were derived from Earth System Models and multi-model means calculated in two Shared Socioeconomic Pathways: ‘middle-of-the-road’ and ‘fossil-fuelled development’ scenarios. Declines in North Atlantic primary production emerge as a major impact to arctic terns likely to affect their foraging during the 21st century under a ‘fossil-fuelled development’ scenario. Minimal changes are, however, projected at three other key regions visited by arctic terns (Benguela Upwelling, Subantarctic Indian Ocean and the Southern Ocean). Southern Ocean sea ice extent is likely to decline, but the magnitude of change and potential impacts on tern survival are uncertain. Small changes (<1 m s⁻¹) in winds are projected in both scenarios, but with minimal likely impacts on migration routes and duration. However, Southern Ocean westerlies are likely to strengthen and contract closer to the continent, which may require arctic terns to shift routes or flight strategies. Overall, we find minor effects of climate change on the migration of arctic terns, with the exception of poorer foraging in the North Atlantic. However, given that arctic terns travel over huge spatial scales and live for decades, they integrate minor changes in conditions along their migration routes such that the sum effect may be greater than the parts. Meeting carbon emission targets is vital to slow these end-of-century climatic changes and minimise extinction risk for a suite of polar species.     

Gene flow on ice: the role of sea ice and whaling in shaping Holarctic genetic diversity and population differentiation in bowhead whales (Balaena mysticetus)

Sea ice is believed to be a major factor shaping gene flow for polar marine organisms, but it remains unclear to what extent it represents a true barrier to dispersal for arctic cetaceans. Bowhead whales are highly adapted to polar sea ice and were targeted by commercial whalers throughout Arctic and subarctic seas for at least four centuries, resulting in severe reductions in most areas. Both changing ice conditions and reductions due to whaling may have affected geographic distribution and genetic diversity throughout their range, but little is known about range‐wide genetic structure or whether it differed in the past. This study represents the first examination of genetic diversity and differentiation across all five putative stocks, including Baffin Bay‐Davis Strait, Hudson Bay‐Foxe Basin, Bering‐Beaufort‐Chukchi, Okhotsk, and Spitsbergen. We also utilized ancient specimens from Prince Regent Inlet (PRI) in the Canadian Arctic and compared them with modern stocks. Results from analysis of molecular variance and demographic simulations are consistent with recent and high gene flow between Atlantic and Pacific stocks in the recent past. Significant genetic differences between ancient and modern populations suggest PRI harbored unique maternal lineages in the past that have been recently lost, possibly due to loss of habitat during the Little Ice Age and/or whaling. Unexpectedly, samples from this location show a closer genetic relationship with modern Pacific stocks than Atlantic, supporting high gene flow between the central Canadian Arctic and Beaufort Sea over the past millennium despite extremely heavy ice cover over much of this period.     

Climate impacts on transocean dispersal and habitat in gray whales from the Pleistocene to 2100

Arctic animals face dramatic habitat alteration due to ongoing climate change. Understanding how such species have responded to past glacial cycles can help us forecast their response to today's changing climate. Gray whales are among those marine species likely to be strongly affected by Arctic climate change, but a thorough analysis of past climate impacts on this species has been complicated by lack of information about an extinct population in the Atlantic. While little is known about the history of Atlantic gray whales or their relationship to the extant Pacific population, the extirpation of the Atlantic population during historical times has been attributed to whaling. We used a combination of ancient and modern DNA, radiocarbon dating and predictive habitat modelling to better understand the distribution of gray whales during the Pleistocene and Holocene. Our results reveal that dispersal between the Pacific and Atlantic was climate dependent and occurred both during the Pleistocene prior to the last glacial period and the early Holocene immediately following the opening of the Bering Strait. Genetic diversity in the Atlantic declined over an extended interval that predates the period of intensive commercial whaling, indicating this decline may have been precipitated by Holocene climate or other ecological causes. These first genetic data for Atlantic gray whales, particularly when combined with predictive habitat models for the year 2100, suggest that two recent sightings of gray whales in the Atlantic may represent the beginning of the expansion of this species' habitat beyond its currently realized range.     

Trans‐Pacific and trans‐Arctic pathways of the intertidal macroalga Fucus distichus L. reveal multiple glacial refugia and colonizations from the North Pacific to the North Atlantic

Aim We examined the phylogeography of the cold‐temperate macroalgal species Fucus distichus L., a key foundation species in rocky intertidal shores and the only Fucus species to occur naturally in both the North Pacific and the North Atlantic. Location North Pacific and North Atlantic oceans (42° to 77° N). Methods We genotyped individuals from 23 populations for a mitochondrial DNA (mtDNA) intergenic spacer (IGS) (n = 608) and the cytochrome c oxidase subunit I (COI) region (n = 276), as well as for six nuclear microsatellite loci (n = 592). Phylogeographic structure and connectivity were assessed using population genetic and phylogenetic network analyses. Results IGS mtDNA haplotype diversity was highest in the North Pacific, and divergence between Pacific haplotypes was much older than that of the single cluster of Atlantic haplotypes. Two ancestral Pacific IGS/COI clusters led to a widespread Atlantic cluster. High mtDNA and microsatellite diversities were observed in Prince William Sound, Alaska, 11 years after severe disturbance by the 1989 Exxon Valdez oil spill. Main conclusions At least two colonizations occurred from the older North Pacific populations to the North Atlantic between the opening of the Bering Strait and the onset of the Last Glacial Maximum. One colonization event was from the Japanese Archipelago/eastern Aleutians, and a second was from the Alaskan mainland around the Gulf of Alaska. Japanese populations probably arose from a single recolonization event from the eastern Aleutian Islands before the North Pacific–North Atlantic colonization. In the North Atlantic, the Last Glacial Maximum forced the species into at least two known glacial refugia: the Nova Scotia/Newfoundland (Canada) region and Andøya (northern Norway). The presence of two private haplotypes in the central Atlantic suggests the possibility of colonization from other refugia that are now too warm to support F. distichus. With the continuing decline in Arctic ice cover as a result of global climate change, renewed contact between North Pacific and North Atlantic populations of Fucus species is expected.     

Differences in species diversity of Monogenea between the Pacific and Atlantic Oceans

Data from five extensive surveys each in the Pacific and Atlantic Oceans show that relative species diversity (number of parasite species per host species) of gill Monogenea of coastal marine fishes is greater in the northern and southwestern Pacific than in the northeastern and central- and southwestern Atlantic. Relative species diversity is markedly lower in the cold northeastern Atlantic than in the warmer parts of the Atlantic examined, and in the northern Pacific than in the warm southwestern Pacific. The difference between the northern Pacific and Atlantic is entirely or almost entirely due to a much greater number of species of Gyrodactylidae in the northern Pacific. A species-area relationship cannot explain the difference, because the area of the northern Pacific is not larger than that of the northern Atlantic and because Gyrodactylidae are cold-water forms which cannot have immigrated from warmer seas. The difference is tentatively explained by an evolutionary time hypothesis: more species of Gyrodactylidae have accumulated in the much older Pacific than in the Atlantic Ocean. Alternatively, an ecological time hypothesis may explain the difference: ice sheets during the last glaciation covered much more of the continental shelf in the northern Atlantic than in the northern Pacific, possibly extinguishing more Monogenea in the former than in the latter Ocean.     

Phylogenetic relations of the dinoflagellate Gymnodinium baicalense from Lake Baikal

Freshwater dinoflagellates still remain poorly studied by modern biological methods. This lack of knowledge prevents us from understanding the evolution and colonization patterns of these ecologically important protists. Gymnodinium baicalense is the most abundant, and possibly endemic, planktonic dinoflagellate from the ancient Lake Baikal. This dinoflagellate species blooms in the spring under the ice. This study analyzed the origin of this Baikalian dinoflagellate using three markers (two ribosomal and one mitochondrial DNA). It was found that this species is a true member of the order Gymnodiniales and has close relatives in the glacial melt waters of the Arctic Ocean. It seems that G. baicalense has diversified relatively recently from the arctic marine gymnodinioids. These results shed light on dinoflagellate biogeography and their colonizations in Lake Baikala biodiversity hotspot.     

Phylogenetic relationships among calyptraeid gastropods and their implications for the biogeography of marine speciation

Although calyptraeid gastropods are not well understood taxonomically, in part because their simple plastic shells are the primary taxonomic character, they provide an ideal system to examine questions about evolution in the marine environment. I conducted a phylogenetic analysis of calyptraeid gastropods using DNA sequence data from mitochondrial cytochrome oxidase I (COI) and 16S genes and the nuclear 28S gene. The resultant phylogeny was used to examine the biogeographic patterns of speciation in the Calyptraeidae. Parsimony and Bayesian analyses of the combined data sets for 94 calyptraeid operational taxonomic units and 24 outgroups produced well-resolved phylogenies. Both approaches resulted in identical sister-species relationships, and the few differences in deeper topology did not affect biogeographic inferences. The geographic distribution of the species included here demonstrate numerous dispersal events both between the Pacific and Atlantic oceans and across the equator. When parsimony is used to reconstruct the movement from the Pacific to the Atlantic oceans on the phylogeny, there are 12 transitions between oceans, primarily from the Pacific to the Atlantic. When the latitude is coded as north versus south of the equator, the most-parsimonious reconstruction gives the origin of calyptraeids in the north followed by 15 dispersal events to regions south of the equator and no returns to the north. Many clades of the most closely related species are either sympatric or occur along a single coastline. Closely related species can, however, occur in such divergent regions as Southern California and South Africa. There is little evidence for sister-species pairs or larger clades having been split by the Isthmus of Panama or the Benguela upwelling, but the East Pacific Barrier appears to separate the most basal taxa from the rest of the family.     

Novel microsatellite markers used to determine the population genetic structure of the endangered Roseate Tern, <Emphasis Type="Italic">Sterna dougallii</Emphasis>, in Northwest Atlanticand Western Australia

The Roseate Tern, Sterna dougallii, is an endangered species in the Northwest Atlantic, where it has undergone transient reductions in population size over the past 120 years. This population has been slow to regain former size and range, perhaps in part due to the female-biased sex ratio, which results in female–female pairs, reducing the average productivity of the colony. The larger populations of the Western Pacific and Indian Oceans are not endangered and there is no evidence of a biased sex ratio at breeding in Western Australia. We developed four novel microsatellite markers and adapted one other and these are the first used in the genus Sterna. We also determined the utility of these markers for 17 related species. Here we report the population genetic structure within and between two regions, the Northwest Atlantic and Western Australia. A significant finding is that the Northwestern Atlantic region has much lower allelic diversity than the Western Australia region, promoting the recommendation for increased protection of sites in this region in order to preserve remaining genetic diversity and new potential breeding habitats.     

Ancient phylogenetic separation between Pacific and Atlantic cephalochordates as revealed by mitochondrial genome analysis

The subphylum Cephalochordata (lancelets) is a relatively small taxonomic group in contrast to the subphyla Urochordata and Vertebrata. As an initial step to determine whether lancelets exhibit small genetic divergence in keeping with their conservative body organization or large genetic variation, four Branchiostoma species from the Pacific (B. belcheri and B. malayanum) and Atlantic (B. floridae and B. lanceolatum) Oceans were genetically compared using partial mitochondrial DNA sequences of the cytochrome oxidase c subunit I (COI) and 16S ribosomal RNA (16S rRNA) genes. In both genes, large genetic differences were revealed between the Pacific and Atlantic species, as well as within the former. Two maximum-likelihood trees from the COI and 16S rRNA genes showed that the Pacific and Atlantic lancelets were reciprocally clustered into different clades. Furthermore, both gene trees consistently exhibited deep phylogenetic separation between the two oceans. The estimated divergence time suggested that differentiation may have followed the migration of ancestral lancelets from the Pacific to the Atlantic Oceans via the Tethys Sea.     

Widespread gene flow between oceans in a pelagic seabird species complex

Global‐scale gene flow is an important concern in conservation biology as it has the potential to either increase or decrease genetic diversity in species and populations. Although many studies focus on the gene flow between different populations of a single species, the potential for gene flow and introgression between species is understudied, particularly in seabirds. The only well‐studied example of a mixed‐species, hybridizing population of petrels exists on Round Island, in the Indian Ocean. Previous research assumed that Round Island represents a point of secondary contact between Atlantic (Pterodroma arminjoniana) and Pacific species (Pterodroma neglecta and Pterodroma heraldica). This study uses microsatellite genotyping and tracking data to address the possibility of between‐species hybridization occurring outside the Indian Ocean. Dispersal and gene flow spanning three oceans were demonstrated between the species in this complex. Analysis of migration rates estimated using bayesass revealed unidirectional movement of petrels from the Atlantic and Pacific into the Indian Ocean. Conversely, structure analysis revealed gene flow between species of the Atlantic and Pacific oceans, with potential three‐way hybrids occurring outside the Indian Ocean. Additionally, geolocation tracking of Round Island petrels revealed two individuals travelling to the Atlantic and Pacific. These results suggest that interspecific hybrids in Pterodroma petrels are more common than was previously assumed. This study is the first of its kind to investigate gene flow between populations of closely related Procellariiform species on a global scale, demonstrating the need for consideration of widespread migration and hybridization in the conservation of threatened seabirds.     

Patterns of Coral Recruitment and Post-settlement Mortality on Bermuda's Reefs: Comparisons to Caribbean and Pacific Reefs

The recruitment of juvenile corals and post-settlement mortalityare important processes for coral population dynamics and reefcommunity ecology. I monitored juvenile coral recruitment andsurvival on a severely disturbed reef in Bermuda from 1981 to1989 and on adjacent healthy reefs from 1986 to 1990. Poritesastreoides was the dominant recruiting species at all sites,due to the release of brooded planulae that may settle rapidly.The dominant corals on Bermuda's reef, Diploria spp., were poorrecruiters, perhaps due to the broadcast mode of reproductionof these species. However, Diploria spp. have lower juvenilemortality rates compared to P. astreoides, which may explaintheir abundance on Bermuda's reefs. Brooding corals, primarily agariciids, were the dominant recruitson Atlantic reefs compared to high recruitment rates by spawningacroporids in the Pacific, which may be the result of differentenvironmental conditions and/or evolutionary trends in the twooceans. The latter group also suffered high post-settlementmortality compared to brooding coralsin both the Atlantic andthe Pacific. Massive corals in both oceans had generally lowrecruitment rates, related to their spawning mode of reproduction,and low rates of post-settlement mortality. The dominant roleof long-lived massive corals on the Atlantic and Pacific reefscan be understood in terms of their life-history strategy incomparison to the relatively short-lived Pacific acroporidsand Atlantic agariciids that rely on different strategies tomaintain their populations.     

GROWTH RESPONSES TO SALINITY VARIATION IN FOUR ARCTIC ICE DIATOMS1,2

During spring, extensive blooms of microalgae grow on the underside of arctic sea ice. The brownish, algal layer penetrates ca. 2 cm into the bottom surface of the ice and the algae are potentially exposed to very high salinities. Four diatom species, Melosira juergensii Ag., Porosira glacialis (Grun.) Jørg., Navicula transitans var. derasa (Grun.) Cleve, and Coscinodiscus lacustris Grun., isolated from, sea ice samples taken from the Beaufort and Chukchi seas near Barrow, Alaska, were grown at 11 salinities ranging from 5 to 70‰ at 5 C under constant illumination. All of the species grew at 5‰ except N. transitans whose lower growth limit was 15‰. Growth was high over a broad range of salinities, but none of the species grew at salinities above 60‰. These diatom species appear to be well suited to tolerate the salinities in the brine pockets near the bottom of annual arctic sea ice where they are found. High brine-cell salinity, however, may limit the upward, penetration of ice algae into the bottom of sea ice.