Comparison of antifouling properties of native and invasive Sargassum (Fucales,Phaeophyceae) species

The invasiveness of algal species can be facilitated by chemo-ecological traits that allow the establishment of invasive species in a highly competitive environment. Anti-bacterial, anti-quorum sensing, anti-diatom and anti-larval properties of the invasive brown macroalga Sargassum muticum and three native Sargassum species from Oman waters were compared in laboratory and field experiments to assess whether these traits have the potential to facilitate the invasion process. Only the extract of S. muticum inhibited bacterial growth of four marine bacterial strains and quorum sensing in the reporter strain Chromobacterium violaceum CV017. Settlement, growth and survival of the diatom Cylindrotheca closterium and larvae of the bryozoan Bugula neritina were significantly inhibited by all Sargassum extracts in laboratory experiments. However, crude extracts of S. muticum had the strongest antifouling effect. Natural tissue-level concentrations of S. muticum extract reduced diatom density to about 20% compared with the controls. Larval mortality increased by 80–90% compared with controls with S. muticum extract diluted to one-third natural levels. Significant anti-diatom activity of S. muticum was confirmed in the field experiments with Sargassum extracts embedded in a phytagel matrix. Comparison of non-polar compounds by gas chromatography–mass spectrometry demonstrated that S. muticum extracts had overall fewer secondary metabolites but more species-unique compounds than extracts of native Sargassum spp. The greater antifouling defence of invasive vs. native Sargassum species indicates a selective trait that may contribute to the invasion success of S. muticum.     

Impacts of the invasive alga Sargassum muticum on ecosystem functioning and food web structure

Biological invasions have the potential to cause severe alterations to the biodiversity of natural ecosystems. At the same time, variation in the diversity and composition of native communities may have an important influence on the impact of invasions. Here, effects of the invasive Japanese wireweed, Sargassum muticum, were tested across a range of native marine algal assemblages using a combined additive and substitutive design. The invasive alga significantly reduced primary production, an important component of ecosystem functioning, and increased connectance, a key property of the food webs associated with the algal resources. These impacts were mediated by changes in the proportions of intermediate and top species, as well as apparent reductions in faunal species richness and diversity. Some key alterations to faunal species composition (including the arrival of generalist species associated with S. muticum) may have been important in determining these patterns. Overall results suggest that S. muticum not only directly impeded the native algal community, but that these effects extended indirectly to the native fauna and therefore caused major changes throughout the ecosystem.     

Faunal differences between the invasive brown macroalga Sargassum muticum and competing native macroalgae

Interactions between macroalgae and their associated fauna are of great interest for marine invasions, because fauna may increase the biotic resistance of a system and macroalgal invasions may cause shifts in faunal composition. We tested for differences in faunal community structure between a macroalgal invader, Sargassum muticum, and several native macroalgae in intertidal pools on both the west and south coast of Portugal. On each coast, we compared the faunal diversity and composition associated with the invader with that of the competing native macroalga(e). On the west coast, the diversity of the fauna associated with S. muticum was equal to or lower than with the native competitor, Cystoseira humilis. Fauna composition differed between S. muticum and C. humilis at both locations, but within each species, no differences between locations were detected. In contrast, the fauna diversity on S. muticum of the south coast varied among locations. S. muticum fauna differed from the fauna of all native macroalgae at one location, but only from three out of seven native macroalgae at the other location. Discriminating fauna species did not show a consistent pattern towards higher or lower abundances in S. muticum compared to most native macroalgae, and species-specific contributions were small. Differences in fauna community also depended on the identity of the native macroalga. In conclusion, the fauna associated with S. muticum differs from many native brown macroalgae, but these differences were not consistent as they depend both on the native macroalgal species and on location. This invader does not seem to have a severe negative impact on local macroalgae-associated fauna.     

IMPACT OF THE INVASIVE SEAWEED SARGASSUM MUTICUM (PHAEOPHYTA) ON AN INTERTIDAL MACROALGAL ASSEMBLAGE1

The impact of the invasive seaweed Sargassum muticum (Yendo) Fensholt on a low intertidal macroalgal assemblage was assessed at a semiexposed rocky shore in northern Spain between 2002 and 2004. Sargassum muticum plants were removed from the mature macroalgal assemblage and from those occurring along the successional process of the assemblage. Biomass, richness, diversity, and percentage cover of macroalgae in experimental plots were compared with unmanipulated controls. The effect of S. muticum removal on the macroalgal assemblage more than 2 years after the beginning of the experiment was negligible. Moreover, no differences between treatments were detected in the general patterns of succession. Only significant differences in S. muticum abundance were detected between treatments at the end of the experiment. We suggest that the low abundance of S. muticum at this intertidal level and its pseudoperennial life cycle may limit competition with native macroalgae. However, long‐term removal experiments may be a more indicator of the impact of S. muticum at the upper limit of its vertical distribution.     

<Emphasis Type="Italic">Sargassum muticum</Emphasis> (Yendo) Fensholt in Ireland: an invasive species on the move

The invasive marine macroalga Sargassum muticum was recorded from Kilmore Quay, Co. Waterford, for the first time in the Republic of Ireland in 2001. It was found at several other places in that year and in subsequent years. An intensive survey was carried out from April 2003 to November 2003 to map the geographical distribution and spread of this invasive brown macroalga, and data on distribution were collected in subsequent years through 2006. This study indicated that S. muticum most probably arrived in the early or mid 1990s and has spread all around the Irish coastline colonising Co. Donegal for the first time in 2006. The results indicate that spreading is facilitated by boating and perhaps via shellfish transport. A rough rate of spread of 2–3 km year⁻¹ has been calculated within one bay and for the Irish coastline of about 54 km year⁻¹. Observations showed that S. muticum has been found growing in seagrass beds and in rock pools, which might have serious consequences for the biodiversity in rock pools and for the protective status of seagrass habitats. It is recommended that monitoring of S. muticum should continue, especially in areas of high amenity value and economic importance in order to observe possible effects on local flora and fauna, aquaculture, the seaweed industry and tourism.     

Biomass dynamics of exotic Sargassum muticum and native Halidrys siliquosa in Limfjorden,Denmark—Implications of species replacements on turnover rates

The expansion of Sargassum muticum in the Danish estuary Limfjorden between 1984 and 1997 was followed by a decrease in abundance of native perennial macroalgae such as Halidrys siliquosa. Although commonly associated with the expansion of exotic species, it is unknown whether such structural changes affect ecosystem properties such as the production and turnover of organic matter and associated nutrients. We hypothesized that S. muticum possesses ‘ephemeral’ traits relative to the species it has replaced, potentially leading to faster and more variable turnover of organic matter. The biomass dynamics of S. muticum and H. siliquosa was therefore compared in order to assess the potential effects of the expansion of Sargassum. The biomass of Sargassum was highly variable among seasons while that of Halidrys remained almost constant over the year. Sargassum grew faster than Halidrys and other perennial algae and the annual productivity was therefore high (P/B = 12 year⁻¹) and exceeded that of Halidrys (P/B = 5 year⁻¹) and most probably also that of other perennial algae in the system. The major grazer on macroalgae in Limfjorden, the sea urchin Psammechinus miliaris, preferred Sargassum to Halidrys, but estimated losses due to grazing were negligible for both species and most of the production may therefore enter the detritus pool. Detritus from Sargassum decomposed faster and more completely than detritus from Halidrys and other slow-growing perennial macrophytes. High productivity and fast decomposition suggest that the increasing dominance of S. muticum have increased turnover of organic matter and associated nutrients in Limfjorden and we suggest that the ecological effects of the invasion to some extent resemble those imposed by increasing dominance of ephemeral algae following eutrophication.     

Spatio-temporal distribution patterns of the invasive macroalga Sargassum muticum within a Danish Sargassum-bed

Sargassum muticum was first observed in Scandinavia in Limfjorden (Denmark) in 1984, where it is now the most abundant and conspicuous macroalga. Despite the ecological importance of Sargassum, few studies have described seasonal patterns within Scandinavian Sargassum beds. We quantified the dynamics of macroalgae among years and seasons along a depth transect through a typical Sargassum bed in Limfjorden. The annual investigations (summer transects 1989–1999) showed a gradual increase in the dominance of Sargassum, especially at the 2–4-m depth interval. Significant seasonal dynamics in macroalgal abundance and assemblage structure were observed in this depth interval; the mean cover of Sargassum varied from ca. 5% (autumn and winter) to 25% (mid-summer). In comparison, encrusting algae had high and relatively stable covers throughout the year (ca. 20%). Other perennial macroalgae had low mean covers (<2%) characterized by a few patches of higher abundances. Except from a spring bloom, filamentous algae had low covers throughout the year. Within this relatively uniform bed, Sargassum abundance was positively related to boulders >10 cm in diameter and species richness was negatively correlated to depth and stones <10 cm in diameter, and non-correlated to other algal form-groups or grazer densities. Thus, in Limfjorden, the distribution of Sargassum is determined by large- (>6 m) and small-scale (<1 m) depth differences where low light limits Sargassum at depth, physical disturbance and sediment stress limits Sargasum in shallow waters, and the presence of stable boulder substratum facilitate Sargassum. Competition for space from other macroalgae and herbivory are probably of minor importance.     

Epiphyte and herbivore interactions impact recruitment in a marine subtidal system

Marine assemblages are influenced by the rate and timing of species settlement and recruitment. Both abiotic factors and biotic interactions can impact recruitment rates of marine species in a variety of systems. However, the impacts of species which recruit at the same time upon each other are less well understood. We investigated the relationship between the recruitment of Lacuna vincta, a small (<6 mm shell diameter) marine snail, and two species of algal epiphytes, the native Ceramium virgatum and the invasive Neosiphonia harveyi, in the shallow subtidal zone of Narragansett Bay, Rhode Island. All three species exhibit peak recruitment densities during two months in the summer. We found that the presence of algal epiphytes facilitates the recruitment rate of L. vincta, regardless of the epiphyte species composition. We also found a positive relationship between the number of L. vincta present and epiphyte recruitment, which is disproportionately driven by higher recruitment of N. harveyi than C. virgatum. Understanding recruitment dynamics and interactions is vital to effectively mitigate the effects of and adaptations to changes due to the establishment of non-native species.     

Stressor intensity determines antagonistic interactions between species invasion and multiple stressor effects on ecosystem functioning

Biological invasions, nutrient enrichment and ocean warming are known to threaten biodiversity and ecosystem functioning. The independent effects of these ecological stressors are well studied, however, we lack understanding of their cumulative effects, which may be additive, antagonistic or synergistic. For example, the impacts of biological invasions are often determined by environmental context, which suggests that the effects of invasive species may vary with other stressors such as pollution or climate change. This study examined the effects of an invasive seaweed (Sargassum muticum) on the structure and functioning of a synthetic macroalgal assemblage and tested explicitly whether these effects varied with nutrient enrichment and ocean warming. Overall, the presence of S. muticum increased assemblage productivity rates and warming altered algal assemblage structure, which was characterised by a decrease in kelp and an increase in ephemeral green algae. The effects of S. muticum on total algal biomass accumulation, however, varied with nutrient enrichment and warming, producing antagonistic cumulative effects on total algal biomass accumulation. These findings show that the nature of stressor interactions may vary with stressor intensity and among response variables, which leads to less predictable consequences for the structure and functioning of communities.     

Genetic structure analysis of natural <Emphasis Type="Italic">Sargassum muticum</Emphasis> (Fucales,Phaeophyta) populations using RAPD and ISSR markers

Sargassum muticum is important in maintaining the structure and function of littoral ecosystems, and is used in aquaculture and alginate production, however, little is known about its population genetic attributes. In this study, random amplified polymorphic DNA (RAPD) and inter-simple sequence repeat (ISSR) markers were used to investigate the genetic structure of four populations of S. muticum and one outgroup of S. fusiforme (Harv.) Setchell from Shandong peninsula of China. The selected 24 RAPD primers and 19 ISSR primers amplified 164 loci and 122 loci, respectively. Estimates of genetic diversity with different indicators (P%, percentage of polymorphic loci; H, the expected heterozygosity; I, Shannon’s information index) revealed low or moderate level of genetic variations within each S. muticum population, and a high level of genetic differentiations were determined with pairwise unbiased genetic distance (D) and fixation index (F _ST ) among the populations. The Mantel test showed that two types of matrices of D and F _ST were highly correlated whether from RAPD (r = 0.9706, P = 0.009) or ISSR data (r = 0.9161, P = 0.009). Analysis of molecular variance (AMOVA) was conducted to apportion the variations among and within the S. muticum populations. It indicated that variations among populations were higher than those within populations, being 55.82% verse 44.18% by RAPD and 55.21% verse 44.79% by ISSR, respectively. Furthermore, the Mantel test suggested that genetic differentiations among populations were related to the geographical distances (r > 0.6), namely, conformed to the IBD (isolation by distance) model, as expected from UPGMA (unweighted pair group method with arithmetic averages) cluster analysis. On the whole, the high genetic structuring among the four S. muticum populations along the distant locations was clearly indicated in RAPD and ISSR analyses (r > 0.9, P < 0.05) in our study.     

Use of RAPD in differentiation of selected species of <Emphasis Type="Italic">Sargassum</Emphasis> (Sargassaceae,Phaeophyta)

Sargassum C. Agardh is one of the most common but little understood genera of Phaeophyta in Malaysia. The difficulty in species delineation is due to morphological plasticity. A combination of morphology and random amplified polymorphic DNA (RAPD) studies of selected Sargassum species was carried out to have a better understanding of the taxonomy. Primer OPA13 was found to be good for discriminating between Sargassum species. Sargassum binderi was shown to be different from S. oligocystum (S_D>0.5 = 14.11%), indicating the importance of the vesicle and receptacle in species differentiation. S. baccularia was clearly separated out from S. polycystum and S. stolonifolium using primer OPA13. RAPD analysis showed that the presence of the stolon is an important character for separating S. baccularia (no stolon) from S. polycystum (stolon) and S. stolonifolium (stolon). Presented at the 6th Meeting of the Asian Pacific Society of Applied Phycology, Manila, Philippines.     

LOW GENETIC VARIABILITY OF SARGASSUM MUTICUM (PHAEOPHYCEAE) REVEALED BY A GLOBAL ANALYSIS OF NATIVE AND INTRODUCED POPULATIONS1

Sargassum muticum (Yendo) Fensholt is one of the most well‐known invasive species in the world. There have, however, been few genetic investigations on both its introduced and native populations. There are also some questions about the taxonomic status of this species. This study is the first to assess the genetic diversity of S. muticum on a global scale, by utilizing one marker each from the extranuclear genomes, namely, plastidial RUBISCO and mitochondrial TrnW_I spacers, as well as the nuclear internal transcribed spacer 2 (ITS2). Based on the markers investigated, both the invasive as well as the native populations of this species appeared very homogenous, when compared with other invasive and brown macroalgae. No variation in ITS2 and RUBISCO spacer was revealed in S. muticum populations, including those from its native ranges in Asia and the introduced ranges in Europe and North America. Two TrnW_I spacer haplotypes with a fixed two‐nucleotide difference were found between the populations of eastern Japan and the other 15 populations examined. This study confirms that there is no cryptic diversity in the introduced range of this species. All the materials collected globally are indeed S. muticum. Results depicting the distribution range of the two TrnW_I spacer haplotypes also support the earlier suggestion that the source of the introduced S. muticum populations is most likely western and central Japan (Seto Inland Sea), where the germlings of S. muticum were likely to have been transported with the Pacific oysters previously introduced for farming in Canada, UK, and France in earlier years.     

An invasive crab alters interaction webs in a marine community

Over the last decade, the non-native, filter-feeding crab Petrolisthes armatus invaded oyster reefs of the South Atlantic Bight at densities of thousands m⁻². Mesocosm and field experiments demonstrated that P. armatus at ∼10–75% of mean summer densities: (1) suppressed growth of small oysters, biomass of benthic microalgae, and recruitment of native mud crabs, (2) enhanced oyster, mussel, and total bivalve recruitment, macroalgal cover, and survivorship of predatory oyster drills, but (3) did not affect native taxonomic richness. Laboratory feeding assays, field tethering experiments, and population changes in field and mesocosm experiments suggest that P. armatus is a preferred prey for native mud crabs and other consumers, thus relieving predation on native species and enhancing recruitment or survival of bivalves and oyster drills. In contrast, the invasive crab can consume crustacean larvae and via this feeding may suppress recruitment of native mud crabs. Our findings should be conservative given the low densities of P. armatus seeded into experimental plots and our inability to run longer-term experiments due to controls rapidly being colonized by non-native crabs recruiting from the plankton. Invasive crabs commonly impact native communities via predation, but community impacts of this invasive crab may be as much due to its role as a preferred prey of native consumers as to its predation on native prey. Given that oysters are foundation species for shallow reefs in the South Atlantic Bight, the long-term effects of this invasion could be considerable.     

<Emphasis Type="Italic">Lacuna vincta</Emphasis> (Mollusca,Neotaenioglossa) herbivory on juvenile and adult <Emphasis Type="Italic">Nereocystis luetkeana</Emphasis> (Heterokontophyta,Laminariales)

Herbivory can be an important factor structuring coastal algal communities. Herbivores may preferentially graze particular algal species or tissue types. Mesograzers, despite their small size, can critically weaken kelp thalli and impact entire kelp beds. We propose that when kelp beds are composed of several kelp cohorts, mesograzers will selectively choose to inhabit younger plants and grazing activities will have a greater impact on younger plants. This study investigated the effects of grazing by the littorinid gastropod, Lacuna vincta, on different age classes of the bull kelp, Nereocystis luetkeana by (1) testing food preference of L. vincta on juvenile, first-year adult, and second-year adult Nereocystis blades in the laboratory, (2) determining substrate (blades of different ages) preference of L. vincta in the laboratory, and by (3) estimating in-situ herbivore abundances and densities on juvenile and adult Nereocystis. Results demonstrated that grazing by L. vincta produced greater damage on juvenile than older Nereocystis tissues. Although L. vincta did not select juvenile versus older kelps as substrate in the laboratory, in situ surveys showed that differences existed between age classes with higher L. vincta densities on juvenile than adult kelp. We conclude that at a local scale, L. vincta can be an important structuring factor in Nereocystis populations due to its high density and grazing ability. Handling editor: K. Martens     

LONG‐TERM CHANGES IN THE STRUCTURE OF INTERTIDAL ASSEMBLAGES AFTER INVASION BY SARGASSUM MUTICUM (PHAEOPHYTA)1

The macroalgal assemblages at the low intertidal zone were studied at three localities on the north coast of Spain between 1977 and 2002. Two of these localities were invaded at the end of the 1980s by the brown seaweed Sargassum muticum (Yendo) Fensholt (Phaeophyta, Sargassaceae), whereas the third locality remained free of the invader. In 2002, distinct algal assemblages were noticed in invaded and noninvaded localities. No major changes were detected in the noninvaded locality. Apart from the obvious presence of S. muticum, the changes observed in the invaded localities included a significant reduction in abundance of the previous dominant species (the red alga Gelidium spinosum (S. G. Gmelin) P. C. Silva) as well as an increased number of species and diversity, increased primary productivity, and variations in the seasonal abundance patterns of some species. We speculate that the arrival of S. muticum had a negative effect on the dominant native G. spinosum, probably related to competition for light. This resulted in indirect positive effects on other species of the assemblage (such as Bifurcaria bifurcata R. Ross). Other small epiphytic opportunistic species might also have been benefited from the presence of S. muticum, because the invader has a rich associated epiphytic assemblage.     

Dominance of an invasive earthworm in native and non-native grassland ecosystems

More attention is currently being focused on earthworm invasions; however, in many ecosystems the relative abundance of native and invasive earthworm species is unknown. We characterized earthworm populations of two grassland types within the Palouse region: native prairie remnants and Conservation Reserve Program (CRP) set asides planted with exotic grasses. The earthworm community in both grassland types was completely dominated by the exotic-invasive Aporrectodea trapezoides. Only one individual of a native species, Driloleirus americanus (the giant Palouse earthworm), was found in a prairie remnant. No differences were found between prairie remnants and CRP sites for mean earthworm density (24–106 individuals m⁻²) or fresh weight (12–45 g m⁻²). Our results suggest that the combined effects of land-use change, habitat fragmentation and competitive interactions have resulted in the decimation of native earthworm populations and dominance of invasive earthworms in native and non-native grasslands of the Palouse region.     

Epibiota communities of the introduced and indigenous macroalgal relatives<Emphasis Type="Italic"> Sargassum muticum</Emphasis> and<Emphasis Type="Italic"> Halidrys siliquosa</Emphasis> in Limfjorden (Denmark)

Sargassum muticum (Phaeophyceae, Fucales) has recently been introduced to Limfjorden (Denmark) where its closest relative is the indigenous Halidrys siliquosa. Previous studies have demonstrated large quantitative (canopy biomass) and qualitative (canopy persistence) differences in the habitat available to epibiota within the canopies of these two macroalgae. We therefore hypothesised that these algae would support different epibiota communities and tested this by sampling the epibiota of S. muticum and H. siliquosa on seven occasions throughout 1997 by enclosing entire thalli in mesh bags. We found 53 epibiota taxa and, with only one exception, they were all recorded on both host species. Species richness and abundance of epibiota exhibited clear seasonal variation on both host species, although epibiota biomass was seasonally constant on H. siliquosa but not on S. muticum. These patterns were consistent with the different life histories of the host species. There was a weakly negative correlation between thallus size and epibiota biomass for both host species. When taking species-specific seasonal variation in thallus size into consideration, S. muticum and H. siliquosa were found to support significantly different epibiota biomasses. Multivariate analyses showed that epibiota community structure was different, although highly overlapping, between the two species, whereas there was an almost parallel temporal development in epibiota community structure. We conclude that it is unlikely that the introduction of S. muticum to Limfjorden has caused major changes in local epibiota community structure. However, the standing stock of epibiota is likely to have increased.Communicated by H.-D. Franke     

Anti-microfouling Activity of Lipidic Metabolites from the Invasive Brown Alga Sargassum muticum (Yendo) Fensholt

The purification of the chloroform extract from the brown invasive macroalga Sargassum muticum, through a series of chromatographic separations, yielded 12 fractions that were tested against strains of bacteria, microalgae, and fungi involved in marine biofilm formation. The chemical composition of four (a, c, g, and k) out of the six fractions that exhibited anti-microfouling activity was investigated. Fraction a contained saturated and unsaturated linear hydrocarbons (C₁₂–C₂₇). Arachidonic acid was identified as the major metabolite in fraction c whereas fraction g contained mainly palmitic, linolenic, and palmitoleic acids. Fraction k was submitted to further purification yielding the fraction kAcaF1e that was composed of galactoglycerolipids, active against the growth of two of the four bacterial strains (Shewanella putrefaciens and Polaribacter irgensii) and all tested fungi. These promising results, in particular the isolation and the activity of galactoglycerolipids, attest the potential of the huge biomass of S. muticum as a source of new environmentally friendly antifouling compounds.     

GRAZING IMPACT OF,AND INDIRECT INTERACTIONS BETWEEN MESOGRAZERS ASSOCIATED WITH KELP (LAMINARIA DIGITATA)1

Little is known about the indirect effects of nonlethal grazing impacts in mesograzer–seaweed interactions. Using laboratory experiments, the effect of grazing by the seasonally abundant kelp‐associated gastropod Lacuna vincta on subsequent kelp consumption by one kelp‐associated (Idotea granulosa) and one nonassociated species of isopod (I. emarginata) was determined. Measurements of the toughness and elemental composition of different parts of the sporophyte of Laminaria digitata (Huds.) J. V. Lamour., as well as grazer‐induced changes in the palatability of the blade, were conducted to explore possible mechanisms of indirect effects. In situ grazing pressure was the highest between July and September, with the blade being the preferred part of the kelp sporophyte, despite missing differences in the elemental composition among kelp parts. The laboratory experiments supported our hypotheses in that kelp consumption by both species of isopods was lower on intact than on L. vincta–damaged areas of the blade. This pattern was not caused by grazing‐induced changes in blade palatability. Instead, the observed increase in isopod consumption following grazing by L. vincta resulted more likely from the combined effects of a reduction in the toughness of L. vincta–damaged kelp blades and some unknown gastropod cue(s). These results suggest that kelp‐associated and nonassociated mesograzers may benefit from the nonlethal grazing impact of L. vincta due to changes in physical traits of the seaweed. Thus, the nonlethal grazing impact by one species of mesograzer can positively modify the trophic interactions between kelp and other potential competitors, suggesting that the interactions among mesograzers might be more complex than previously assumed.     

Habitat selection and host location by symbiotic shrimps associated with Sargassum communities: The role of chemical and visual cues

One of the most fundamental aspects of symbioses is how the symbionts establish the association. The Sargassum community is comprised of a unique and diverse assemblage of symbiotic organisms, most of which are likely seeking refuge. Host location of the Sargassum habitat by two of the most common invertebrate symbionts, Latreutes fucorum and Leander tenuicornis, was investigated. Based on our trials, neither shrimp species used chemical cues, in the absence of visual cues. Habitat selection of both shrimp species was also examined under a variety of conditions. Our results show that L. fucorum and L. tenuicornis can distinguish visually between different structures and habitats. Specifically, they selected live Sargassum over artificial Sargassum. Moreover, L. fucorum selected Sargassum natans over Sargassum fluitans when only visual cues were available, whereas large-sized L. tenuicornis selected S. fluitans under the same conditions. Conversely, habitat selection trials combining both chemical and visual cues resulted in no preference between the two Sargassum species by either shrimp species. Additional ecological factors that might influence habitat location and selection are discussed, and the importance of Sargassum in terms of pelagic fisheries management.