The control of the development of a marine benthic community by predation on recruits

Recruitment is an important process in regulating many marine benthic communities and many studies have examined factors controlling the dispersal and distribution of larval immigrants. However, benthic species also have early post-settlement life-stages that are dramatically different from adult and larval stages. Predation on these stages potentially impacts measured recruitment and the benthic populations and communities that ultimately develop.We examined the consequences of post-settlement predation on 1-day-old to 1-month-old recruits of sessile invertebrates at two field sites in southern New England. One site (Breakwater) was in a protected area with few predators and the other (Pine Island) was <1 km away in an open coast area with three different predator guilds: small and large invertebrates and fish. The Breakwater site had been dominated for >10 years by colonial and solitary ascidians. These species were absent from the Pine Island site which was dominated by bryozoans. Our goal was to examine whether post-settlement predation influenced the development and subsequent structure of the epifaunal community.Here we examine long-term changes in community development resulting from post-settlement predation, and contrast these results to those of earlier experiments examining the reductions in observed recruitment by post-settlement predation. Our first long-term experiment examined natural community development at the two sites and whether transplanted communities changed when exposed to the different levels of predation at these sites. The communities that developed at both sites were consistently different from each other and similar to resident communities at their respective sites. On panels transplanted from the Breakwater to Pine Island, solitary ascidians and the colonial ascidian, Botryllus schlosseri, suffered high mortalities on both caged and uncaged treatments, indicative of predation by small predators that could enter cages. Some solitary ascidians did survive inside cages and the colonial ascidian, Botrylloides violaceus, became dominant on all transplanted treatments. On panels transplanted from Pine Island to the Breakwater, ascidians invaded and dominated all treatments except those that were originally caged at Pine Island.In the second long-term experiment, natural communities were allowed to develop on panels exposed at the Breakwater for 1, 2, 3, and 4 weeks. Each set was transplanted to three treatments at Pine Island: open uncaged pilings, caged pilings to exclude fish and large invertebrates, and racks suspended above the bottom to exclude all predators. When 1-week-old communities were transplanted, after 2-3 weeks only bryozoans were found on the open and caged pilings, while colonial ascidians dominated the suspended rack treatment. When older 2-week-old communities were transplanted, colonial ascidians also became dominant in the caged piling treatment and when 3- and 4-week-old communities were transplanted colonial ascidians dominated all three treatments. Solitary ascidians were never abundant on open pilings exposed to fish and large benthic invertebrate predators.Post-settlement predator-prey interactions involved newly settled and juvenile life-stages of a variety of prey species and many invertebrate and vertebrate predator species. The effects of these interactions on recruitment did result in differences in the development and eventual species composition of the communities, even though predators had little if any effect on the adults of the prey species.     

The colonial ascidian Didemnum sp. A: Current distribution, basic biology and potential threat to marine communities of the northeast and west coasts of North America

Didemnum sp. A is a colonial ascidian with rapidly expanding populations on the east and west coasts of North America. The origin of Didemum sp. A is unknown. Populations were first observed on the northeast coast of the U.S. in the late 1980s and on the west coast during the 1990s. It is currently undergoing a massive population explosion and is now a dominant member of many subtidal communities on both coasts. To determine Didemnum sp. A's current distribution, we conducted surveys from Maine to Virginia on the east coast and from British Columbia to southern California on the west coast of the U.S. between 1998 and 2005. In nearshore locations Didemnum sp. A currently ranges from Eastport, Maine to Shinnecock Bay, New York on the east coast. On the west coast it has been recorded from Humboldt Bay to Port San Luis in California, several sites in Puget Sound, Washington, including a heavily fouled mussel culture facility, and several sites in southwestern British Columbia on and adjacent to oyster and mussel farms. The species also occurs at deeper subtidal sites (up to 81 m) off New England, including Georges, Stellwagen and Tillies Banks. On Georges Bank numerous sites within a 230 km² area are 50-90% covered by Didemnum sp. A; large colonies cement the pebble gravel into nearly solid mats that may smother infaunal organisms. These observations suggest that Didemnum sp. A has the potential to alter marine communities and affect economically important activities such as fishing and aquaculture.     

Benthic foraminifera associated with the invasive ascidian, Didemnum sp. A

The invasive ascidian, Didemnum sp. A, first appeared in New England bays and harbors in the early 1990s, and in the waters around Cape Cod in 1993. While ship traffic was the likely vector introducing the species, its origin and precise date and location of its introduction are presently unknown. Colony surfaces of Didemnum sp. A appear very clean and not favorable substrates for epibiota settlement, but closer inspection revealed the presence of benthic foraminifera. During 2003 and 2004, 52 samples of Didemnum sp. A and other ascidians were collected to determine whether or not the foraminiferal assemblages might also be non-native and thus provide a potential clue to the place of origin of Didemnum sp. A. Sample locations included the New England coast from Connecticut to Maine (with a concentration in the Cape Cod area), northern California, Zeeland, The Netherlands, and Shakespeare Bay, New Zealand. From New England samples, 18 species of benthic foraminifera were identified. The most common species represented were Cornuspira involvens, C. planorbis, Elphidium galvestonense, E. margaritaceum, Glabratellina lauriei, Miliolinella subrotunda, Quinqueloculina bicornis, and Rosalina floridana. Foraminiferal assemblages on Didemnum sp. A from other regions sampled were composed of the same cosmopolitan species found in New England, plus other species which were indigenous to each region. Because no exotic foraminifera species were found it is concluded that Didemnum sp. A likely did not introduce non-native foraminifera originating from their native habitats into the New England region.     

Spatial and temporal effects of pre-seeding plates with invasive ascidians: Growth, recruitment and community composition

Many shallow water subtidal habitats in Massachusetts, USA have recently been invaded by five non-indigenous ascidian species: Ascidiella aspersa, Botrylloides violaceus, Didemnum sp., Diplosoma listerianum and Styela clava. This study examined the effects of seawater temperature, as a proxy for climate change, on B. violaceus and D. listerianum and the impact these ascidians have on native sessile fouling communities. Field experiments were conducted over a four month period at two locations (Lynn and Woods Hole, MA) to examine growth dynamics over regional thermal and geographic ranges. Invasive ascidians occupied as much as 80% of the primary substratum and accounted for the majority of species richness. B. violaceus and D. listerianum growth were similar at both study sites, but initial colony growth of D. listerianum was positively affected by temperature. B. violaceus and D. listerianum exhibited rapid two-week growth rates during the summer months with more rapid growth at the warmer Woods Hole site. Competition for space between B. violaceus and D. listerianum typically resulted in neutral borders between colonies. Overgrowth occurred if the colony of one species was disproportionably larger than the colony of the other species. Recruitment and growth of native species influenced the long-term composition of experimental communities more than the pre-seeding with B. violaceus or D. listerianum colonies. Elevated temperatures, however, increased initial growth of B. violaceus and D. listerianum and may have facilitated the species success to invade the communities during crucial periods of introduction. With projected global climate change, a rise in sea surface temperatures may exacerbate the cumulative impacts of invasions on benthic communities and facilitate the invasion of other non-native ascidian species.     

Distribution and long-term temporal patterns of four invasive colonial ascidians in the Gulf of Maine

Invasive ascidians are a growing concern for ecologists and natural resource managers, yet few studies have documented their short- and long-term temporal patterns of abundance. This study focuses on the invasion of the Gulf of Maine by the colonial ascidians Botryllus schlosseri, Botrylloides violaceus, Diplosoma listerianum and Didemnum sp. A. We examined the time of arrival and potential vectors for these four invasive ascidians using survey data (collected from 1969 onwards) and literature documentation. We also compared the dominance and seasonal patterns of abundance of these species using data from two identical panel studies; one conducted from 1979 to 1980, the other from 2003 to 2005. Didemnum and Botrylloides were most likely first introduced into the Damariscotta River, Maine in the early 1970's through oyster aquaculture while Botryllus and Diplosoma were probably transported by commercial and recreational vessels. The overall abundance of colonial ascidians has increased since 1979 and 1980. Botryllus was the only invasive colonial ascidian present during the 1979 to 1980 study and accounted for an average of 6.16% cover on panels. From 2003 to 2005, the more recently arrived colonial ascidians Botrylloides and Didemnum accounted for 7.38% and 2.08% cover respectively, while Botryllus covered only 1.16%. Our results reveal a shift in seasonal abundance between 1979 to 1980 and 2003 to 2004. In 1979 and 1980, colonial ascidians had the highest percent cover in fall and winter while in 2003 and 2005 they had highest percent cover in summer and fall. Seasonal patterns of space occupation by colonial ascidians were correlated with seasonal changes in seawater temperature.     

Invasion dynamics on a temperate rocky shore: from early invasion to establishment of a marine invader

Populations of invading Asian shore crabs (Hemigrapsus sanguineus) and resident crabs (European green crab Carcinus maenas or species in the family Panopeidae) were monitored for up to 12 years along a south to north orientation on the open coast of Massachusetts and within the Narragansett Bay estuary, Rhode Island. At all sites, densities of resident crabs declined as H. sanguineus increased in abundance. Population dynamics were divided into 3 stages of the invasion (early, mid, late) based on statistically different densities of invading and resident crabs. Early in the invasion on the open coast, the relatively few H. sanguineus present had a wide range of individual sizes. By late in the invasion, relatively small crabs [<10 mm carapace width (CW)] constituted half of the population. Few seasonal differences occurred. Carcinus maenas, dominant at the coastal sites early in the invasion, showed strong recruitment of small individuals in the fall early in the invasion. Overall numbers of C. maenas declined as the H. sanguineus invasion progressed, and very few crabs >10 mm CW were present late in the invasion. In Narragansett Bay, a recruitment peak of H. sanguineus occurred shortly before it surged in population size, and large crabs were abundant late in the invasion. Geographic comparisons of H. sanguineus populations in southern New England showed similar growth trajectories. Monitoring populations of resident and invading species at multiple locations from early invasion through clear establishment of the invader allows a more complete understanding of the population dynamics of marine species invasions.     

Feeding preference of Strongylocentrotus droebachiensis (Echinoidea) for a dominant native ascidian, Aplidium glabrum, relative to the invasive ascidian Botrylloides violaceus

Subtidal benthic communities show distinct patterns of community structure related to substrate angle. Suspension-feeding invertebrate communities often dominate vertical and undercut rock surfaces, while macroalgae dominate horizontal surfaces. One factor that may shape this pattern is sea urchin grazing, which is often more intense on horizontal surfaces. The native Gulf of Maine ascidian Aplidium glabrum, like other native ascidians, is generally restricted to vertical and undercut rock walls, whereas the introduced ascidian Botrylloides violaceous is often abundant on horizontal surfaces. We tested the hypothesis that this pattern could be due to differing predation intensity on these two ascidians by Strongylocentrotus droebachiensis, a dominant omnivore in the Gulf of Maine. Feeding preference of S. droebachiensis on the native A. glabrum vs. B. violaceous was estimated in the laboratory and in field experiments. Laboratory results show that S. droebachiensis prefers to feed on the native ascidian A. glabrum over B. violaceous. In the field, potential differences in the impact of S. droebachiensis on the two species were masked by the much greater growth rate of B. violaceus compared to A. glabrum. These results may help explain observed patterns in ascidian distribution in the Gulf of Maine, and ultimately the overall success of B. violaceus as a major invader throughout New England.     

Recent population expansions of non-native ascidians in The Netherlands

Over the last 50 yrs seven non-native ascidians have settled in The Netherlands, concentrated in the two periods 1974-1977 and 1991-2004 (i.e., Styela clava, Aplidium glabrum, Diplosoma listerianum, Didemnum sp., Botrylloides violaceus, Molgula complanata and Perophora japonica). The year of the introduction of B. violaceus remains a matter of dispute because many of the Botrylloides specimens that are recorded in western Europe, have been identified as the closely resembling species B. leachi. Only Didemnum sp. has become a true invasive species and has become a threat to native ecosystems, especially in the province of Zeeland, by its ability to overgrow virtually all hard substrata present. This includes rocks, stones, sand, algae and almost all sessile marine animals. The sudden population expansion of the didemnid from 1996 onward, coincided with the cold winter of 1995-1996, which caused decreased population sizes of many marine animals. The resulting increase in the availability of suitable substrates for settlement and the strong decrease of grazing sea urchins, may have triggered the population expansion. Studying its population dynamics, the optimal growing temperature for Didemnum sp. appears to be 14-18 °C. Virtually all colonies die when the water temperature gets colder than 5 °C. Colonies growing on live marine animals seem to be more resistant to the cold, than those growing on rocks, stones and plants. Two potential predators of the didemnid have also been recorded in Dutch waters: the gastropods Trivia arctica and Lamellaria sp.     

Effects of marine organisms extracts on microtubule integrity and cell cycle progression in cultured cells

Marine organisms have been reported to be a rich source of biologically active compounds, but when compared with plants, their use is much more restrict in popular medicine. Among marine animals, sponges and ascidians are two of the most prominent sources of new compounds with cytotoxic potential. In this study, we tested 40 extracts of marine sponges and ascidians from southeastern Brazilian coast aiming to evaluate their anti-proliferative effects on cultured cells. The extracts of Amorphinopsis sp., Arenosclera brasilensis, Cystodytes dellechiajei, Cliona aff. celata, Didemnum sp., Hadromerida and Scopalina ruetzleri showed an IC₅₀≤30 μg/mL and produce strong effects on microtubules organization and on the cell cycle progression.     

Phylogenetic diversity of bacteria associated with ascidians in Eel Pond (Woods Hole, Massachusetts, USA)

Recent studies have revealed that many marine invertebrates are closely associated with diverse microorganisms, frequently resulting in the production of compounds of biomedical interest. Thus far, ascidians have not been widely examined for the presence of bacterial associations, although the production of secondary metabolites is well documented. In the present study, we examined the gonad of Molgula manhattensis and the tunic surfaces of Botryllus schlosseri, Didemnum sp., and Botrylloides violaceus for the presence of associated bacteria. These ascidians are common inhabitants of the coastal ocean of Cape Cod, Massachusetts. We used denaturing gradient gel electrophoresis (DGGE) as well as cloning and sequencing of 16S rDNA to analyze the microbial communities. There is a strong evidence that spiroplasma-like bacteria colonize the gonad of the solitary ascidian M. manhattensis. The bacteria might be vertically transmitted and may be involved in the production of secondary metabolites that deter predators of the ascidians. The bacterial communities found on the tunic surfaces of the colonial ascidians were found to be more diverse. However, in all cases the bacterial communities were predominated by alpha-proteobacteria. Alpha-proteobacteria related to the obtained sequences have been identified as symbionts in a variety of hosts, suggesting a specific role for these bacteria. However, based on our data it is difficult to differentiate between persistently and only transiently associated bacteria. Overall, this study demonstrates that ascidian species are associated with diverse bacterial populations. Future studies will aim to elucidate the precise relationships between bacteria and ascidians and to identify bioactive compounds that might be produced as a result of these relationships.     

The Southern Hemisphere ascidian Asterocarpa humilis is unrecognised but widely established in NW France and Great Britain

Non-native ascidians can be a major feature of sessile communities, particularly in artificial habitats, but may be overlooked because of poor understanding of species’ taxonomy and biogeographic status. The styelid unitary ascidian Asterocarpa humilis, up to now only reported in the Southern Hemisphere, has been found on the coast of NW France from St Malo to Quiberon, on the south coast of England from Falmouth to Brighton, and also in north Wales. The first documented occurrence was in 2005 in Brittany, but the species was found to be relatively widespread at a regional scale and common in many places during surveys in 2009, 2010 and 2011. It has possibly been present but overlooked for some time. The identification based on morphology was confirmed by comparison with specimens from New Zealand, within the species’ presumed native range, by molecular barcoding based on mitochondrial (COI) and nuclear (18S) genes.     

Home and Away: Comparisons of Resource Utilization by a Marine Species in Native and Invaded Habitats

The exotic Asian shore crab, Hemigrapsus sanguineus, was recently introduced to the northeastern coast of North America and during the 1990's breeding populations were established throughout southern New England. In 1997–1998, ecological studies of several co-occurring brachyuran crabs were conducted and in native (Tanabe Bay, Japan) and invaded (Long Island Sound, USA) habitats of H. sanguineus. Standardized comparisons of H. sanguineus were made between the 2 habitats using data on crab sizes, utilization of space, and food habits. Results revealed that (1) the resource use of H. sanguineus was quite different from that of other resident species in its invaded habitat, and (2) there were no substantial changes in resource utilization by H. sanguineus after it became established in the invaded habitat (relative to native Tanabe Bay). Differing patterns of resource use by H. sanguineus and other crabs in the invaded habitat, the lack of restriction in resource use by H. sanguineus following its introduction, and the climatological and physical similarities between native and invaded regions likely contributed to the successful invasion of H. sanguineus into rocky intertidal habitats in southern New England.     

Recruitment limitation of native species in invaded coastal dune communities

Recruitment limitation may limit the ability of sites to regenerate after disturbances such as weed invasion and weed management. We investigated seed bank constraints and dispersal limitation in coastal dune communities on the east coast of Australia. The ability of sites to regenerate naturally following weed removal was assessed in coastal dune communities invaded by the invasive alien, bitou bush (Chrysanthemoides monilifera subsp. rotundata). To investigate recruitment limitation, seed banks and vegetation of invaded, native, intensively managed (selective application of herbicide and some re-vegetation) and extensively managed (large-scale, non-selective herbicide application) sites were compared. We investigated the dispersal mechanisms of species in the seed bank and vegetation to determine if communities might be dispersal-limited, i.e. contain significant numbers of species with only short-distance dispersal capabilities. Species richness and composition of soil seed banks differed from the vegetation in foredunes and hinddunes. Invasion depleted seed banks further. About half of the species had short-distance dispersal mechanisms indicating the potential for dispersal limitation. Secondary weed invasion following management was evident although alien species occurred in both seed banks and vegetation. Our results indicated that coastal dune communities suffer recruitment limitation. Native, managed and invaded dune communities appear to be both seed bank and dispersal-limited although management and invasion exacerbates recruitment. Regeneration of coastal dune communities will require active reintroduction of species, particularly those with short-distance dispersal mechanisms.     

Usefulness of tissue nitrogen content and macroalgal community structure as indicators of water eutrophication

We tested the hypothesis that the community structure and biochemical composition of macroalgae reflect the degree of nutrient concentrations in the water column. Benthic community structure and tissue nitrogen (N) content of macroalgae on intertidal rocky shores at three sites were investigated in relation to sewage effluents on Mireuk Island, Tongyeong city, on the southern coast of Korea. Ulva australis clearly dominated at site 1, which was close to a sewage treatment plant, where higher dissolved inorganic N and dissolved inorganic phosphate concentrations were observed. U. australis-dominated communities also appeared at site 2 (intermediate levels of nutrient enrichment). The macroalgal assemblage at site 3 (unimpacted site) was significantly different from those at sites 1 and 2. Five species (U. australis, Sargassum fusiforme, Grateloupia elliptica, Gelidium amansii, and Sargassum horneri) were dominant at site 3, representing 87 % of the total coverage throughout the study period. Species richness (d), evenness (J'), and diversity index (H') were highest at site 3, intermediate at site 2, and lowest at site 1, showing a negative relationship with nutrient levels. These results indicate that macroalgal community structure can be used as a bioindicator in water quality assessment. The tissue N content of green and red algae was responsive to nutrient availability, while the tissue N content of brown algae was relatively unchanged among the sites. This suggests that tissue N content as a bioindicator for detecting the influence of sewage effluent should be considered to reflect the N storage capacity of macroalgae.     

Effect of an invasive crab upon a marine fishery: green crab, Carcinus maenas, predation upon a venerid clam, Katelysia scalarina, in Tasmania (Australia)

Nonindigenous species are increasingly recognized as altering marine and estuarine communities, causing significant changes in abundance and distribution of native species. Such effects are of particular concern to coastal fisheries. We experimentally determined the effect of the nonindigenous European green crab, Carcinus maenas, upon the stepped venerid clam, Katelysia scalarina, the basis for a fledgling clam fishery in Tasmania, Australia. First, we observed a trend of decreased juvenile (<13-mm shell length or SL) abundance of K. scalarina at sites with C. maenas relative to those without this invasive predator. Additionally, relative predation intensity on these juveniles was significantly higher in invaded areas. To better understand the dynamics of predation by this invader, we conducted a number of manipulative experiments. In cage experiments testing per capita predation rates, we found that: (1) of the various sizes of C. maenas, large C. maenas were the most significant predators; (2) the smallest size class of K. scalarina tested (6-12-mm SL) was preferred by C. maenas; (3) C. maenas had much higher predation rates than any native predator tested; and (4) while the native shore crab, Paragrapsus gaimardii, was found to have a constant predation rate over an eightfold range of densities of juvenile K. scalarina (16-128 individuals·m⁻²), C. maenas significantly increased its per capita predation with increasing prey density. Notably, in open field plots at a site where C. maenas was abundant, predation was constant over the range of tested prey densities. We predict, therefore, that the invasion of C. maenas will have significant negative consequences for the Tasmanian K. scalarina fishery.     

Impacts of Argentine ants on invertebrate communities with below-ground consequences

The Argentine ant, Linepithema humile (Mayr), is an invasive species that has been associated with various negative impacts in native communities around the world. These impacts, as for other invasive ants, are principally towards native ant species, and impacts on below-ground processes such as decomposition remain largely unexplored. We investigated the relationship between Argentine ants and invertebrate fauna, litter decomposition and soil microbial activity between paired invaded and uninvaded sites at two locations in Auckland, New Zealand, where there has been no research to date on their impacts. We examined the diversity and composition of invertebrate and microorganisms communities, and differences in soil and litter components. The composition of invertebrates (Order-level, ant and beetle species) was different between invaded and uninvaded sites, with fewer ants, isopods, amphipods, and fungus-feeding beetles at the invaded sites, whereas Collembola were more abundant at the invaded sites. There were significant differences in soil chemistry, including higher carbon and nitrogen microbial biomass at uninvaded sites. Several litter components were significantly different for Macropiper excelsum. The fibre content of litter was higher, and key nutrients (e.g. nitrogen) were lower, at invaded sites, indicating less breakdown of litter at invaded sites. A greater knowledge of the history of invasion at a site would clarify variation in the impacts of Argentine ants, but their persistence in the ground litter layer may have long-term implications for soil and plant health in native ecosystems.     

The marine macrobenthic communities of Wallis and Smiths Lakes,New South Wales

The shallow macrobenthic communities of Wallis and Smiths Lakes on the central N.S. W. coast have been studied by replicate grab samples. Wallis Lake is a coastal marine lagoon constantly open to the sea, whereas Smiths Lake, also a coastal lagoon, is periodically closed to the sea for several months at a time. The benthic communities were described using indices of dominance and diversity. Between site communities were compared using MULTBET and a GOWER ordination. Generally, sites in Wallis Lake had a more diverse fauna than those in Smiths Lake. Weed beds supported more species and individuals than sandy substrates. The benthic fauna of these two lakes is compared with other coastal rivers and lagoons which have been studied in temperate eastern Australia.     

TELOSCHISTES FLAVICANSIN GREAT BRITAIN: DISTRIBUTION AND ECOLOGY

Over the last century, the distribution ofTeloschistes flavicanshas contracted from being widespread in the southern half of England and Wales to being limited to South-West England with outlying populations in Pembrokeshire and North Wales. Twelve core sites have been identified where the species is well established: ten of these are saxicolous/terricolous habitats on windy coastal cliff tops; the other two are lines of sycamore trees near the coast. At 39 further localities, some inland, the species is in very small amounts (often on one tree) and vulnerable to extinction. It is normally a member of theParmelietum revolutaeor theRamalinetum scopularisassociations. The conservation of the species is discussed.     

Ecological observations on the colonial ascidian Didemnum sp. in a New England tide pool habitat

The colonial ascidian Didemnum sp. has colonized northwestern Atlantic coastal habitats from southern Long Island, New York, to Eastport, Maine. It is also present in offshore habitats of the Georges Bank fishing grounds. It threatens to alter fisheries habitats and shellfish aquacultures.Observations in a tide pool at Sandwich, MA from December 2003 to February 2006 show that Didemnum sp. tolerates water temperatures ranging from ≤ 1 to > 24 °C, with daily changes of up to 11 °C. It attaches to pebbles, cobbles, and boulders, and it overgrows other tunicates, seaweeds, sponges, and bivalves. From May to mid July, colonies appear as small patches on the bottoms of rocks. Colonies grow rapidly from July to September, with some growth into December, and they range in color from pink to pale yellow to pale orange. Colony health declines from October through April, presumably in response to changes in water temperatures, and this degenerative process is manifested by color changes, by the appearance of small dark brown spots that represent clumps of fecal pellets in the colony, by scavenging by periwinkles, and by a peeling-away of colonies from the sides of cobbles and boulders. At Sandwich, colonies died that were exposed to air at low tide. The species does not exhibit this seasonal cycle of growth and decline in subtidal habitats (40-65 m) on the Georges Bank fishing grounds where the daily climate is relatively stable and annual water temperatures range from 4 to 15 °C. Experiments in the tide pool with small colony fragments (5 to 9 cm²) show they re-attach and grow rapidly by asexual budding, increasing in size 6- to 11-fold in the first 15 days. Didemnum sp. at Sandwich has no known predators except for common periwinkles (Littorina littorea) that graze on degenerating colonies in the October to April time period and whenever colonies are stressed by desiccation.The tendencies of the ascidian (1) to attach to firm substrates, (2) to rapidly overgrow other species, (3) to tolerate a wide temperature range, (4) to be free from predation, and (5) to spread by colony fragmentation combine to make it a potential threat to benthic marine habitats and aquacultures. Didemnum sp. is known to overgrow mussels, oysters, and sea scallops, and it likely envelops other bivalves too.     

Impacts of the invasive Argentine ant on native ants and other invertebrates in coastal scrub in south‐eastern Australia

Invasive ants threaten native biodiversity and ecosystem function worldwide. Although their principal direct impact is usually the displacement of native ants, they may also affect other invertebrates. The Argentine ant, Linepithema humile (Dolichoderinae), one of the most widespread invasive ant species, has invaded native habitat where it abuts peri‐urban development in coastal Victoria in south‐eastern Australia. Here we infer impacts of the Argentine ant on native ants and other litter and ground‐dwelling invertebrates by comparing their abundance and taxonomic composition in coastal scrub forest either invaded or uninvaded by the Argentine ant. Species composition of native ants at bait stations and extracted from litter differed significantly between Argentine ant‐invaded and uninvaded sites and this was consistent across years. Argentine ants had a strong effect on epigeic ants, which were either displaced or reduced in abundance. The native ant Rhytidoponera victoriae (Ponerinae), numerically dominant at uninvaded sites, was completely absent from sites invaded by the Argentine ant. However, small hypogeic ants, including Solenopsis sp. (Myrmicinae) and Heteroponera imbellis (Heteroponerinae), were little affected. Linepithema humile had no detectable effect upon the abundance and richness of other litter invertebrates. However, invertebrate group composition differed significantly between invaded and uninvaded sites, owing to the varied response of several influential groups (e.g. Collembola and Acarina). Floristics, habitat structure and measured environmental factors did not differ significantly between sites either invaded or uninvaded by Argentine ants, supporting the contention that differences in native ant abundance and species composition are related to invasion. Changes in the native ant community wrought by Argentine ant invasion have important implications for invertebrate communities in southern Australia and may affect key processes, including seed dispersal.