Reduced performance of native infauna following recruitment to a habitat-forming invasive marine alga

Despite well-documented negative impacts of invasive species on native biota, evidence for the facilitation of native organisms, particularly by habitat-forming invasive species, is increasing. However, most of these studies are conducted at the population or community level, and we know little about the individual fitness consequences of recruitment to habitat-forming invasive species and, consequently, whether recruitment to these habitats is adaptive. We determined the consequences of recruitment to the invasive green alga Caulerpa taxifolia on the native soft-sediment bivalve Anadara trapezia and nearby unvegetated sediment. Initially, we documented the growth and survivorship of A. trapezia following a natural recruitment event, to which recruitment to C. taxifolia was very high. After 12 months, few clams remained in either habitat, and those that remained showed little growth. Experimental manipulations of recruits demonstrated that all performance measures (survivorship, growth and condition) were significantly reduced in C. taxifolia sediments compared to unvegetated sediments. Exploration of potential mechanisms responsible for the reduced performance in C. taxifolia sediments showed that water flow and water column dissolved oxygen (DO) were significantly reduced under the canopy of C. taxifolia and that sediment anoxia was significantly higher and sediment sulphides greater in C. taxifolia sediments. However, phytoplankton abundance (an indicator of food supply) was significantly higher in C. taxifolia sediments than in unvegetated ones. Our results demonstrate that recruitment of native species to habitat-forming invasive species can reduce growth, condition and survivorship and that studies conducted at the community level may lead to erroneous conclusions about the impacts of invaders and should include studies on life-history traits, particularly juveniles.     

Replacement of native seagrass with invasive algal detritus: impacts to estuarine sediment communities

Biological invasions modify the quality and supply of detrital subsidies to aquatic and terrestrial ecosystems. Where the invader has very different traits to native species, major changes in associated consumer communities may result, as a consequence of differences in their nutritional value and effects on the sedimentary habitat. We assessed how the replacement of seagrasses with the invasive alga Caulerpa taxifolia in modified Australian estuaries influences invertebrate communities of mudflats that are subsidized by detritus from submerged aquatic vegetation. Two months after experimental enrichment of sediments with high (60?g dry weight per 0.25?m² plot) or low (30?g dry weight) quantities of either non-native C. taxifolia or native Posidonia australis or Zostera capricorni detritus, there were positive effects of detrital addition on invertebrate abundance that occurred irrespective of the resource added. By 4?months after addition, however, detritus from invasive C. taxifolia had produced effects on benthic communities that could not be replicated by detritus from either of the native seagrasses. Plots receiving the high loading of C. taxifolia detritus contained fewer invertebrates than plots of the other treatments, perhaps due to the induction of sediment hypoxia. The pattern, however, reversed at low detrital loading, with the plots receiving 30?g of C. taxifolia containing more invertebrates and more taxa than the other plots, presumably due to the greater resource availability for detritivores. Our results demonstrate that replacement of native seagrass with invasive algal detritus can have large impacts on sediment-dwelling communities.     

Sublethal effects on reproduction in native fauna: are females more vulnerable to biological invasion?

Although invasive species are a major threat to survivorship of native species, we know little about their sublethal effects. In soft-sediment marine systems, mat-forming invasive species often have positive effects, facilitating recruitment and enhancing the diversity and abundance of native invertebrates. However, because mat-forming invasive species change the habitat in which they invade, and benthic invertebrates are sensitive to environmental disturbance, important sublethal effects on native species may exist. Using a model marine system we show that the widespread mat-forming invasive alga Caulerpa taxifolia (Vahl) C. Agardh has strong negative effects on the reproductive traits of a native bivalve Anadara trapezia (Deshayes, 1840) (e.g. timing of reproductive development and spawning, and follicle and gamete production) even though the invader has positive effects on recruitment. Moreover, gender specific responses occurred and indicated that females were more susceptible to invasion than males. Our results indicate that sublethal effects of an invasive species on reproductive traits will have severe consequences for fitness of the native species.     

Dinucleotide microsatellite markers in the genus <Emphasis Type="Italic">Caulerpa</Emphasis>

Caulerpa spp. are clonal green marine algae which often act as invasive species when growing outside their native biogeographical borders. Over the two past decades, Caulerpa taxifolia has spread along the Mediterranean coast, presently occurring at 70 sites and covering nearly 3,000 ha of subtidal area. New genetic markers (microsatellites) have been developed to assess clonal structure and genetic diversity of recently established populations of the invasive species C. taxifolia and Caulerpa racemosa in comparison with populations of the native Caulerpa prolifera in the Mediterranean. Our results show that nine polymorphic markers have been developed for C. prolifera, seven for C. taxifolia, and three for C. racemosa. Genetic diversity in Caulerpa was assessed in two geographical scales: one at a population scale where 40 thalli units were collected from C. prolifera in Cala d’Or, Mallorca, Spain, and another at a species scale, where 30 sample units were analyzed for C. prolifera, 24 for C. taxifolia, and 24 for C. racemosa from different sites in the Mediterranean, Atlantic, and Pacific Ocean. Number of alleles, expected heterozygosity, and marker amplification success are provided in each case.     

Impact of Caulerpa taxifolia colonization on the littoral ichthyofauna of North-Western Mediterranean sea: preliminary results

After its accidental introduction in the French waters of the Mediterranean, the green seaweed Caulerpa taxifolia is now spreading along the coast of the Alpes-Maritimes, invading various biotopes (rock, sand and Posidonia oceanica seagrass beds). During the spring and fall of 1992, we evaluated the impact of C. taxifolia on the infralittoral ichthyofauna of Cap Martin (Menton) and compared invaded and reference sites, in shallow (3–8 m) and deep (11–25 m) stations. Caulerpa taxifolia apparently does not have any effect on the composition and global species richness of the ichthyofauna. Therefore, no exclusion phenomenon has been observed and fish behavior has remained normal (feeding habits, reproduction, recruitment). However, fish density and biomass are slightly lower in the colonized sites. Negative regression was observed, in shallow sites during the spring season, between the percentage of substrate invaded by C. taxifolia and the average density of fish assemblages (slope= –0.13, r=0.26, p<0.01). Despite these observations, C. taxifolia meadows seem to be a favorable environment for the recruitment of some species of Labridae (Coris julis, Symphodus ocellatus), Sparidae (Diplodus annularis) and Serranidae (Serranus cabrilla) in the fall.The relative importance of C. taxifolia as environmental parameter and of other habitat characteristics (structural complexity, substrate slope, hydrodynamism, depth, fishing pressure) is discussed to explain these differences. The results of this first study indicate that there exists no simple relation between the presence of C. taxifolia and fish assemblages.     

Habitat preference of three common fishes for seagrass, <Emphasis Type="Italic">Caulerpa taxifolia</Emphasis>, and unvegetated substrate in Moreton Bay,Australia

A decrease in seagrass cover and a commensurate increase in Caulerpa taxifolia distribution in Moreton Bay have prompted concern for the impact that habitat change may have on faunal communities. Therefore, it is important to understand the patterns of habitat use. We examined habitat selection of three common seagrass species: double-ended pipefish (Syngnathoides biaculeatus), eastern trumpeter (Pelates quadrilineatus), and fan-bellied leatherjacket (Monacanthus chinensis) using a mesocosm experiment. Fish were given three possible habitat pairings (1) seagrass and C. taxifolia, (2) seagrass and unvegetated, and (3) C. taxifolia and unvegetated. Observation trials were conducted during the day and night over two days. In all trials, fish preferred vegetated habitat (seagrass or C. taxifolia) over unvegetated habitat (sand). In seagrass and C. taxifolia trials, all species preferred seagrass significantly over C. taxifolia. Habitat use patterns did not differ between day and night trials. Caulerpa taxifolia provides a valuable structured habitat in the absence of seagrass; however, it is unclear if C. taxifolia meadows provide other resource benefits to fishes beyond that of shelter.     

Antioxidant response and caulerpenyne production of the alien Caulerpa taxifolia (Vahl) epiphytized by the invasive algae Lophocladia lallemandii (Montagne)

Invasive algae alter the structure and function of ecosystems. The red algae Lophocladia lallemandii grows in the West Mediterranean epiphyting the green algae Caulerpa taxifolia, another invasive alga. Our aim was to determine whether the invasive algae Lophocladia, recently introduced in the West Mediterranean, induces oxidative stress and an antioxidant response in Caulerpa taxifolia. We measured the caulerpenyne production, the activities of antioxidant enzymes and the levels of markers of lipid peroxidation in Caulerpa taxifolia. Caulerpenyne concentration was significantly higher in Caulerpa taxifolia epiphytized by Lophocladia. End-products of lipid peroxidation -malondialdehyde (MDA) and 4-hydroxinonenal (4-HNE)- were significantly increased in Caulerpa taxifolia samples from the station with Lophocladia. Antioxidant enzyme activities -catalase and superoxide dismutase (SOD)-, as well as H₂O₂ production significantly increased in the Lophocladia station compared to the control station. The activities of catalase and SOD determined in Lophocladia lallemandii showed no significant differences between Lophocladia living alone and Lophocladia epiphytating Caulerpa taxifolia. Caulerpa taxifolia epiphytized by Lophocladia lallemandii responded by increasing the caulerpenyne and H₂O₂ production and the antioxidant enzymes activities as a defensive mechanism against the new invasive algae.     

Density and assemblage influence the nature of the species richness–productivity relationship in Australian dry sclerophyll forest species

The relationship between species richness and productivity is important from both a basic, theoretical perspective and also because it has important ramifications for applied ecology including ecosystem restoration and the design of carbon offset plantings. While a more species‐rich community is often believed to be more productive than a species‐poor community, findings from observational and experimental studies differ and our understanding of the relationship comes largely from grasslands. Consequently, we aimed to determine for the first time the nature of the species richness–productivity relationship in a southern‐hemisphere dry sclerophyll ecosystem. We investigated the impact of species richness on productivity, plant density and mean plant biomass at three sowing densities in three species assemblages. Eucalyptus globulus, Acacia mearnsii and Allocasuarina verticillata were each grown as monocultures and included in every subsequent level of species richness, forming three distinct species assemblages. Communities were grown in a glasshouse pot experiment for four months, then harvested and above‐ground biomass measured. We found no general species richness–productivity relationship in the communities studied. There were no overall increases in productivity as species richness increased and in fact in most cases the productivity of communities with 4 and 8 species was lower than monocultures of the dominants. Importantly, density influenced the way richness affected productivity and this effect was dependent upon assemblage, indicating that species identity is a key determinant of productivity. These results demonstrate important ecological principles in a previously untested system. A key outcome of this experiment is that density alters the relationship between species richness and initial productivity in assemblages of Australian dry sclerophyll species.     

A FORENSIC AND PHYLOGENETIC SURVEY OF CAULERPA SPECIES (CAULERPALES,CHLOROPHYTA) FROM THE FLORIDA COAST,LOCAL AQUARIUM SHOPS,AND E‐COMMERCE: ESTABLISHING A PROACTIVE BASELINE FOR EARLY DETECTION1

Baseline genotypes were established for 256 individuals of Caulerpa collected from 27 field locations in Florida (including the Keys), the Bahamas, US Virgin Islands, and Honduras, nearly doubling the number of available GenBank sequences. On the basis of sequences from the nuclear rDNA‐ITS 1+2 and the chloroplast tufA regions, the phylogeny of Caulerpa was reassessed and the presence of invasive strains was determined. Surveys in central Florida and southern California of >100 saltwater aquarium shops and 90 internet sites revealed that >50% sold Caulerpa. Of the 14 Caulerpa species encountered, Caulerpa racemosa was the most common, followed by Caulerpa sertularioides, Caulerpa prolifera, Caulerpa mexicana, and Caulerpa serrulata. None of the >180 field‐collected individuals (representing 13 species) was the invasive strain of Caulerpa taxifolia or C. racemosa. With one exception (a sample of C. racemosa from a shop in southern California belonged to the invasive Clade III strain), no invasive strains were found in saltwater aquarium stores in Florida or on any of the internet sites. Although these results are encouraging, we recommend a ban on the sale of all Caulerpa species (including “live rock”) because: morphological identification of Caulerpa species is unreliable (>12% misidentification rate) and invasive strains can only be identified by their aligned DNA sequences, and because the potential capacity for invasive behavior in other Caulerpa species is far from clear. The addition of the Florida region to the genetic data base for Caulerpa provides a valuable proactive resource for invasion biologists as well as researchers interested in the evolution and speciation of Caulerpa.     

Macrobenthic assemblages of newly introduced <Emphasis Type="Italic">Caulerpa taxifolia</Emphasis> from the Eastern Mediterranean coast of Turkey

Caulerpa taxifolia is one of the most important and best-studied alien species in the Mediterranean Sea. The present study reveals the macrobenthic assemblages associated with C. taxifolia from the region. We found 26 species from Polychaeta, 31 species from Crustacea, 22 species from Mollusca and 5 species from Echinodermata. In conclusion, C. taxifolia in İskenderun Bay can be considered an ecosystem engineer that modifies local habitats and also enhances biodiversity.     

优势种去除对崇明东滩盐沼湿地生态系统的影响

盐沼生态系统环境梯度明显,物种组成较简单,是研究生物多样性与生态系统功能关系的理想对象。本研究以崇明东滩盐沼湿地为研究区域,研究优势种去除对植物群落结构以及底栖动物群落的影响。结果表明:(1)去除处理仅对植物群落分株密度有极显著效应(P0.01)。去除组和对照组物种组成差异随时间增加而减小,处理效应逐渐减弱。(2)去除组底栖动物密度均低于对照组,但差异不显著。(3)盐沼植物群落特征与底栖动物群落有密切关系,植物密度、冠层高度与底栖动物密度相关性极显著。去除优势种后,植物群落分株密度升高,群落内剩余物种占比有所上升,次优势种对群落的补偿效应具有较大贡献;而底栖动物群落密度下降,其生物量和多样性指数的变化趋势与密度并不一致。上述结果表明生物多样性变化影响了盐沼湿地生态系统植物群落和底栖动物群落结构,进而可能影响物质循环和能量流动过程。     

Genetic polymorphism in Caulerpa taxifolia (Ulvophyceae) chloroplast DNA revealed by a PCR‐based assay of the invasive Mediterranean strain

Abstract An invasive, cold‐tolerant strain of the tropical green alga Caulerpa taxifolia was introduced recently in the Mediterranean Sea and along the Californian coast. We screened 50 aquarium and open‐sea C. taxifolia specimens for the presence/absence of an intron located in the rbcL gene of chloroplast DNA. We also reanalysed a total of 229 sequences of the Internal Transcribed Spacer (ITS) of ribosomal DNA, combining previously published sequences from different studies with 68 new sequences to complement rbcL data. The introduced Mediterranean strain was found to be characterized by the absence of the rbcL intron and by the occurrence of a particular monomorphic ITS type. A PCR assay based on rbcL gene was developed to detect new introductions of the invasive strain of C. taxifolia. This rapid and inexpensive test could be useful to assist environment managers in the preservation of coastal marine ecosystems.     

Native species behaviour mitigates the impact of habitat-forming invasive seaweed

Habitat-forming invasive species cause large, novel changes to the abiotic environment. These changes may elicit important behavioural responses in native fauna, yet little is known about mechanisms driving this behaviour and how such trait-mediated responses influence the fitness of native species. Low dissolved oxygen is a key abiotic change created by the habitat-forming invasive seaweed, Caulerpa taxifolia, which influences an important behavioural response (burrowing depth) in the native infaunal bivalve Anadara trapezia. In Caulerpa-colonised areas, Anadara often emerged completely from the sediment, and we experimentally demonstrate that water column hypoxia beneath the Caulerpa canopy is the mechanism instigating this “pop-up” behaviour. Importantly, pop-up in Caulerpa allowed similar survivorship to that in unvegetated sediment; however, when we prevented Anadara from popping-up, they suffered >50% mortality in just 1 month. Our findings not only highlight the substantial environmental alteration by Caulerpa, but also an important role for the behaviour of native species in mitigating the effects of habitat-forming invasive species.     

Community diversity outweighs effect of warming on plant colonization

Abiotic environmental change, local species extinctions and colonization of new species often co‐occur. Whether species colonization is driven by changes in abiotic conditions or reduced biotic resistance will affect community functional composition and ecosystem management. We use a grassland experiment to disentangle effects of climate warming and community diversity on plant species colonization. Community diversity had dramatic impacts on the biomass, richness and traits of plant colonists. Three times as many species colonized the monocultures than the high diversity 17 species communities (~30 vs. 10 species), and colonists collectively produced 10 times as much biomass in the monocultures than the high diversity communities (~30 vs. 3 g/m²). Colonists with resource‐acquisitive strategies (high specific leaf area, light seeds, short heights) accrued more biomass in low diversity communities, whereas species with conservative strategies accrued most biomass in high diversity communities. Communities with higher biomass of resident C4 grasses were more resistant to colonization by legume, nonlegume forb and C3 grass colonists, but not by C4 grass colonists. Compared with effects of diversity, 6 years of 3°C‐above‐ambient temperatures had little impact on plant colonization. Warmed subplots had ~3 fewer colonist species than ambient subplots and selected for heavier seeded colonists. They also showed diversity‐dependent changes in biomass of C3 grass colonists, which decreased under low diversity and increased under high diversity. Our findings suggest that species colonization is more strongly affected by biotic resistance from residents than 3°C of climate warming. If these results were extended to invasive species management, preserving community diversity should help limit plant invasion, even under climate warming.     

Nutrient availability in the sediment and the reciprocal effects between the native seagrass Cymodocea nodosa and the introduced rhizophytic alga Caulerpa taxifolia

Two reciprocal experiments testing for the effects of nutrient addition in the sediment and competitive interactions between the native seagrass Cymodocea nodosa (Ucria) Ascherson and the introduced alga Caulerpa taxifolia (Vahl) C. Agardh were performed. This study was conducted for 13 months (August 1995 until September 1996) in a bay on the south coast of Elba Island (Italy). Each experiment consisted of the manipulation of the level of nutrients (addition vs. control) and the manipulation of the neighbours (presence vs. removal). Response variables were blade density and size for one experiment and shoot density and leaf length of seagrass in the other. Results indicated that the presence of Caulerpa taxifolia did not affect significantly Cymodocea nodosa shoot density and the increased nutrient availability in the sediment did not alter this pattern. Neither the removal of the canopy of the seagrass nor the fertilization of the sediment has influenced significantly the density of the alga. Both species, where co-occurring, show larger size than where the neighbour is removed. Hence, results of this study suggest that the two species on the long term are likely to coexist and that the high nutrient supply of the sediment would not enhance the probability of success neither of the seagrass nor of the alga. Predictions made on the basis of short-term results, that high nutrient loads of the substratum would have represented an even more suitable condition for C. taxifolia to colonize C. nodosa beds and that on the long-term the alga has a high probability of success, did not occur.     

But next time?: Unsuccessful Establishment of the Mediterranean Strain of the Green Seaweed Caulerpa taxifolia in the Sea of Japan

We conducted a survey of 63 public aquariums in Japan by means of a questionnaire asking if a green seaweed, Caulerpa taxifolia, was cultured or exhibited in aquariums under the network of the public aquarium association of Japan in 1997. Of 51 aquariums, 16 cultured or exhibited C. taxifolia. Most of the public aquariums possessing C. taxifolia purchased them from aquarium shops or received them from another public aquarium as a donation. Notojima aquarium reported temporal establishment of C. taxifolia in the Sea of Japan between the summer and autumn of 1992 and 1993. C. taxifolia was released into the sea from a 1 m³ cage that was submerged in an open pool. Two colonies with diameters less than 2 m were found near the mouth of a water outflow pipe in both years. Molecular analysis of the ITS rDNA gene of the aquarium strain of Notojima clarified that the seaweed is the same as the aquarium-Mediterranean and Californian invasive strain. Unsuccessful establishment of the invasive strain of C. taxifolia in the Sea of Japan is probably due to water temperatures in winter that are lethal for C. taxifolia. C. taxifolia remains present in many public and private aquariums. If C. taxifoliawere to be released in more temperate waters of Japan, there would be a high risk of establishment and, thus, impact on the marine ecosystem. This suggests that the trade, culture and exhibition of C. taxifolia should be strongly avoided to reduce the chances of accidental release of this harmful species.     

An invasive wetland grass primes deep soil carbon pools

Understanding the processes that control deep soil carbon (C) dynamics and accumulation is of key importance, given the relevance of soil organic matter (SOM) as a vast C pool and climate change buffer. Methodological constraints of measuring SOM decomposition in the field prevent the addressing of real‐time rhizosphere effects that regulate nutrient cycling and SOM decomposition. An invasive lineage of Phragmites australis roots deeper than native vegetation (Schoenoplectus americanus and Spartina patens) in coastal marshes of North America and has potential to dramatically alter C cycling and accumulation in these ecosystems. To evaluate the effect of deep rooting on SOM decomposition we designed a mesocosm experiment that differentiates between plant‐derived, surface SOM‐derived (0–40 cm, active root zone of native marsh vegetation), and deep SOM‐derived mineralization (40–80 cm, below active root zone of native vegetation). We found invasive P. australis allocated the highest proportion of roots in deeper soils, differing significantly from the native vegetation in root : shoot ratio and belowground biomass allocation. About half of the CO₂ produced came from plant tissue mineralization in invasive and native communities; the rest of the CO₂ was produced from SOM mineralization (priming). Under P. australis, 35% of the CO₂ was produced from deep SOM priming and 9% from surface SOM. In the native community, 9% was produced from deep SOM priming and 44% from surface SOM. SOM priming in the native community was proportional to belowground biomass, while P. australis showed much higher priming with less belowground biomass. If P. australis deep rooting favors the decomposition of deep‐buried SOM accumulated under native vegetation, P. australis invasion into a wetland could fundamentally change SOM dynamics and lead to the loss of the C pool that was previously sequestered at depth under the native vegetation, thereby altering the function of a wetland as a long‐term C sink.     

Soil pathogen communities associated with native and non‐native Phragmites australis populations in freshwater wetlands

Soil pathogens are believed to be major contributors to negative plant–soil feedbacks that regulate plant community dynamics and plant invasions. While the theoretical basis for pathogen regulation of plant communities is well established within the plant–soil feedback framework, direct experimental evidence for pathogen community responses to plants has been limited, often relying largely on indirect evidence based on above‐ground plant responses. As a result, specific soil pathogen responses accompanying above‐ground plant community dynamics are largely unknown. Here, we examine the oomycete pathogens in soils conditioned by established populations of native noninvasive and non‐native invasive haplotypes of Phragmites australis (European common reed). Our aim was to assess whether populations of invasive plants harbor unique communities of pathogens that differ from those associated with noninvasive populations and whether the distribution of taxa within these communities may help to explain invasive success. We compared the composition and abundance of pathogenic and saprobic oomycete species over a 2‐year period. Despite a diversity of oomycete taxa detected in soils from both native and non‐native populations, pathogen communities from both invaded and noninvaded soils were dominated by species of Pythium. Pathogen species that contributed the most to the differences observed between invaded and noninvaded soils were distributed between invaded and noninvaded soils. However, the specific taxa in invaded soils responsible for community differences were distinct from those in noninvaded soils that contributed to community differences. Our results indicate that, despite the phylogenetic relatedness of native and non‐native P. australis haplotypes, pathogen communities associated with the dominant non‐native haplotype are distinct from those of the rare native haplotype. Pathogen taxa that dominate either noninvaded or invaded soils suggest different potential mechanisms of invasion facilitation. These findings are consistent with the hypothesis that non‐native plant species that dominate landscapes may “cultivate” a different soil pathogen community to their rhizosphere than those of rarer native species.     

MEDITERRANEAN CAULERPA TAXIFOLIA AND C. MEXICANA (CHLOROPHYTA) ARE NOT CONSPECIFIC

In 1984, Caulerpa taxifolia (Vahl) C. Agardh was reported along the coast of Monaco. Over the past decade it has spread along 60 km of the Mediterranean coastline and presently represents a potential risk to biodiversity. Several explanations have been advanced regarding the presence of C. taxifolia in the Mediterranean. One hypothesis maintains that the alga was introduced accidentally into the sea at Monaco, where it has been used as a decorative alga in aquaria. Caulerpa taxifolia has not been reported in earlier marine floras of the Mediterranean, and its sudden appearance has suggested that it may be a recent introduction. Another hypothesis proposes that C. taxifolia and Caulerpa mexicana Sonder ex Kützing are morphological variants of one another and hence conspecific taxa. Caulerpa mexicana has been found in the eastern Mediterranean since at least 1941. In order to establish the taxonomic identities of these taxa, individuals from five populations of C. taxifolia and four populations of C. mexicana were collected from within and outside of the Mediterranean. Comparative DNA sequence analysis of the nuclear ribosomal cistron, including the 3′-end of the 18S, ITS1, 5.8S, and ITS2 regions, show clear phylogenetic separation of the two taxa using parsimony and maximum likelihood analyses. Separation is maintained whether the analyses are based on just the more conserved 18S data or just the fast- evolving spacers. The two species are thus not conspecific. For specimens of uncertain identity (i.e. taxifolia–mexicana intermediates), a PCR diagnostic amplification can easily be performed because the ITS1 in C. taxifolia is 36 nucleotides shorter than the ITS1 in C. mexicana. Whether or not C. taxifolia has been present for a longer period of time in the marine flora, either as a cryptic endemic species or as the result of one or more introductions, represents an additional hypothesis that will require identification of biogeographic populations from throughout the world, as well as a population-level study of the Mediterranean region.     

Caulerpa taxifolia in seagrass meadows: killer or opportunistic weed?

Seagrass habitats are being lost throughout the world and the invasive alga C. taxifolia has often been implicated in seagrass declines. Although C. taxifolia can impact a variety of species, evidence for its effects on seagrasses is largely correlative. This study combined observational studies and manipulative experiments done over many years to test hypotheses about effects of C. taxifolia on two Australian seagrasses, namely Posidonia australis and Zostera capricorni. Results indicated that C. taxifolia is not having adverse impacts on the coverage of these seagrasses in the sites studied. Rather, C. taxifolia appears to be an opportunist, persisting longer and its coverage being greater in previously non-vegetated sediments than amongst seagrasses. C. taxifolia co-existed with P. australis and did not cause reductions in the cover of the seagrass. Outcomes of experimental manipulations of C. taxifolia amongst Z. capriconi were less clear due to losses of Z. capriconi in all plots, regardless of the presence of C. taxifolia. It was possible that C. taxifolia may have enhanced the decline in canopy cover of Z. capricorni, but the presence of alga did not alter the final fate of Z. capricorni. There was also no evidence that long-term areal coverage of P. australis or Z. capriconi has been affected by the introduction of C. taxifolia in the embayments studied. A review of literature on effects of species of Caulerpa on seagrasses provided limited experimental evidence for negative impacts of this genus on seagrass abundance.