Chemotactic effects of nutrients on spores of the kelps Macrocytis pyrifera and Pterygophora california

Fertile Macrocystis pyrifera (L.) C. Ag. and Pterygophora californica Rupr. were collected in California, USA in 1987 to 1988. Spores of the kelps exhibited both positive and negative chemotaxis to a variety of chemical nutrients. Chemotaxis was measured by counting the number of spores that swam into flattened capillary tubes with the chemical relative to the number that swam into control tubes. Video-motion-analysis also showed that P. californica spores swam towards a nitrogen source more often than they swam away. Similar chemotactic effects were observed in both 2 and 8 h-old preparations. M. pyrifera spores swam towards nitrate, ammonium (1 to 90 M), glycine, aspartate iron (1 m), boron, cobalt, and manganese. Negative chemotaxis was elicited by ammonium (1 000 M) and iron (45 M). Neither phosphate nor zinc had significant effects. P. californica spores were attracted by nitrate, ammonium, phosphate, and boron. Negative chemotactic effects were recorded with iron (45 M) and manganese. Iron (1 M), cobalt, and zinc had no effect. It is suggested that chemotactic behavior is an adaptation which allows the kelp spores to find and settle in microhabitats suitable for gamatophytic growth and reproduction.     

Effects of sediments on the development of Macrocystis pyrifera gametophytes

Rocky ocean bottoms are covered from time to time with small amounts of fine sediments. This material may interfere with the development of germling stages of Macrocystis pyrifera. Tests were performed by introducing sediments before and after spores were dispersed in culture dishes. 10 mg cm^-2 of sediment, enough to occlude the surface, prevents spore attachment, greatly reducing the probability of survival. Smothering of established germlings was severe at 108 mg sediment cm^-2. Water motion further reduced spore success where sediments were present, probably because of abrasive scour. Mechanisms of natural and human interference with M. pyrifera reproduction are suggested.     

Nutrient-limited growth of juvenile kelp,Macrocystis pyrifera,during the 1982–1984 “El Niño” in southern California

The relative growth rates of juvenile Macrocystis pyrifera in southern California kelp forests were substantially reduced during the El Niño of 1982–1984. The lower growth rates were correlated with increased temperature and decreased nitrogen availability. Fertilization of juvenile plants with slow-release nitrogen-phosphorus fertilizer increased their growth rates to levels previously observed when temperatures were low and nutrient levels were high. The limitation in growth of M. pyrifera by levels of available nutrients during El Niño was in contrast to the usual limitation of growth by irradiance during non-El Niño years. Thus, there was a shift in the relative importance of factors controlling growth of juvenile M. pyrifera during El Niño.     

Sediment load and timing of sedimentation affect spore establishment in Macrocystis pyrifera and Undaria pinnatifida

Although the frequency and magnitude of sedimentation often varies across coastal landscapes creating patches with different mean sediment loads, duration of sedimentation and rates of sediment resuspension, few studies have documented the emergent effects of spatio-temporal variability in sedimentation. Here, we conducted two laboratory experiments to evaluate such effects on the establishment of Macrocystis pyrifera and Undaria pinnatifida spores. In the first experiment, spore establishment was significantly affected by sediment load (the effective dose required for a 40 % reduction in establishment ranged between 16 and 60 mg sediment l^?1) and sediment regime (relative sedimentation occurring before spore settlement, ~3 times more sediment was required for 20 % reduction in spore establishment when sedimentation occurred after spore settlement). The second experiment demonstrated that the effects of sediment depended on sediment load (spore establishment was 2–4 times greater when sediment load was 200 mg l^?1 relative to 400 mg l^?1), variability in sedimentation (spore establishment was 1.36 times greater with variable than fixed sediment loads), repeated pulses of sedimentation (pulsed sedimentation decreased spore establishment by 59–91 % relative to a single sedimentation event) and timing of sedimentation relative to spore settlement (sedimentation before spore settlement decreased establishment by 51–95 % relative to sedimentation after spore settlement). These results have important implications for ecologists and resource managers attempting to predict the consequences of sedimentation, suggesting that it is not only important to consider sediment load, but also fine-scale temporal variability in sedimentation relative to key life-history events of the impacted organisms.     

Intraspecific competition in<Emphasis Type="Italic"> Fucus serratus</Emphasis> and<Emphasis Type="Italic"> F</Emphasis>.<Emphasis Type="Italic"> evanescens</Emphasis> (Phaeophyceae: Fucales) germlings: effects of settlement density,nutrient concentration,and temperature

Survival and growth of early post-settlement stages are critical for the development of seaweed populations. Fucoid germlings commonly settle in dense monospecific aggregates, where intraspecific competition and environmental variables (e.g. nutrient concentration and temperature) may affect survival and growth. Using factorial experiments, we determined the effects of settlement density (~10, ~50 and ~250 germlings cm^–2), nutrient enrichment (from ~10 to ~40 µM N and from ~0.5 to ~2.5 µM P), and temperature (7°C and 17°C) on Fucus serratus and F. evanescens germlings in laboratory cultures over 3 months. Settlement density, nutrient concentration and temperature interactively affected growth of germlings, and the magnitude of this interaction varied between the two species. This represents the first record of such factorial interactions in Fucus spp. germlings. Intraspecific competition, estimated as the relative reduction in germling growth and survival from low to high densities, increased with decreasing nutrient concentration and increasing temperature in both species. While temperature and nutrient concentration had little effect on germling size distributions, size inequality and skewness generally increased with germling density, indicating that a few large individuals gained dominance and suppressed many smaller ones at high density. Self-thinning increased with settlement density and depended on nutrient concentration and species at high density. At high density, self-thinning increased with decreasing nutrient levels in F. evanescens, but not in F. serratus. At low density, nutrient enrichment increased germling growth in F. evanescens, but not in F. serratus, whereas growth in both species was stimulated by nutrient enrichment at higher densities. These results suggest that germling growth and self-thinning are more sensitive to variation in nutrient concentration in F. evanescens than in F. serratus. The potential implications of our findings for the understanding of eutrophication-related abundance changes in both species in southern Norway are discussed.Communicated by L. Hagerman, Helsingør     

Expansion of a central California kelp forest following the mass mortality of sea urchins

The mass mortality by disease of a localized population of sea urchins, Strongylocencrotus franciscanus, on the seaward side of a kelp forest was followed by the rapid seaward expansion of 4 species of brown algae, Macrocystis pyrifera, Laminaria dentigera, Pterygophora california, and to a lesser extent, Nereocystis leutkeana. One other brown alga, Cystoseira osmundacea, failed to become established in the newly available area. Competition among M. pyrifera, L. dentigera, P. californica, and N. Leutkeana apparently was severe, and within 1 year after the demise of the sea urchins, M. pyrifera formed a dense, nearly monospecific stand. Experimental removal of M. pyrifera demonstrated that the canopy of these plants limited light penetration to levels below that necessary for the growth and survival of other brown and red algae.     

The effect of water movement,temperature and salinity on abundance and reproductive patterns of<Emphasis Type="Italic"> Macrocystis</Emphasis> spp. (Phaeophyta) at different latitudes in Chile

This study describes the density variation and phenology of Macrocystis integrifolia and M. pyrifera populations from northern and southern Chile, respectively. Samples of both species were taken in wave-exposed and wave-protected areas. In addition, spore production, germination and early growth rate of sporophytes of each population was studied at monthly intervals under three temperature and salinity regimes. Results indicate that M. integrifolia from northern Chile presents perennial plants with a mean density of three individuals per 0.25 m² throughout the year and that it reproduces mainly during spring and winter. Although, M. pyrifera in exposed areas of southern Chile also have a perennial-type life strategy, they are able to reproduce all year round. In contrast, M. pyrifera populations in protected areas of southern Chile show a clear annual cycle, with high recruitment during late winter and fertile sporophytes in summer and autumn, although the populations become completely decimated thereafter. The effect of temperature and salinity on M. integrifolia shows that it is independent of water movement, but requires low temperatures and high salinities for the release of zoospores, germination and early sporophyte growth. This pattern differs from that of M. pyrifera in southern Chile, which has a broader tolerance range for salinity and temperature than does M. integrifolia. However, in southern Chile wave-protected populations showed higher spore release and germination at 15°C and 18°C, whereas sporophyte growth responded better at the lowest temperature tested (8°C). In general, these results are contrary to those expected, since a seasonal reproductive pattern was observed in M. integrifolia inhabiting a less seasonally variable environment. In exposed sites of southern Chile, plants showed greater tolerance and continuous reproduction throughout the year, despite the greater environmental variability. Finally, population dynamics of protected kelps in southern Chile shows an annual pattern, which is contrary to the expected perennial strategy shown by exposed populations.Communicated by P.W. Sammarco, Chauvin     

Severe storm disturbances and reversal of community structure in a southern California kelp forest

Regular observations made over a period of 5 yr in four permanent transects provided data on plant, sea urchin, and fish densities which indicate that two unusually severe winter storms in 1980 (Storm I) and 1983 (Storm II) had different effects on a southern California kelp-forest community. Storm I removed all canopies of the giant kelp Macrocystis pyrifera, but spared most understory kelps, mainly Pterygophora californica. Hence, the previously large accumulation of detached drift kelp, mostly M. pyrifera, disappeared. Denied their preferred diet of drift kelp, the sea urchins Strongylocentrotus franciscanus and S. purpuratus then emerged from shelters to find alternative food. Without effective predators, they consumed most living plants, including the surviving understory kelps. This weakened the important detritus-based food chain, as indicated indirectly by declining abundances of algal turf and fish (Embiotocidae) that eat small animals living in turf. In 1983, Storm II reversed the process by eliminating exposed urchins, while clearing rock surfaces for widespread kelp settlement and growth. By summer 1984, the kelp grew to maturity to form extensive canopies despite elevated water temperatures during summer and fall of 1983. Thus, severe storms may have vastly different effects on community structure, depending on the state of the community before the disturbance.     

Life-form population responses of a marine planktonic diatom,Chaetoceros pseudocurvisetus,to oligotrophication in regionally upwelled water

Life-form population responses of a centric planktonic diatom,Chaetoceros pseudocurvisetus Mangin, were investigated in summer 1986 and 1988 in the Izu Islands, Japan, in regionally upwelled water where nutrient concentration changed from favorable to unfavorable conditions for active growth and reproduction (oligotrophication). Two types of life form were observed: vegetative cells of healthy and unhealthy looking conditions and resting spores. The observed life-form responses were experimentally evaluated along with a depletion of limiting nutrients. The algal population ceased vegetative growth and initiated resting spore formation with a disappearance of limiting nitrate, and this life-form response seemed to be triggered by the decrease of cellular nitrogen content below a certain level. Since a large amount of silicon was required for the resting spore formation, a part of vegetative cells were unable to form resting spores and formed unhealthy looking vegetative cells under insufficient concentrations of silicic acid. Percentage shares of the resting spores in the population were linearly related to the amounts of available silicic acid. Vegetative cells which did not form resting spores showed greater mortality than resting spores by attack of bacteria and protozoa; however, vegetative cells could respond quickly to possible nutrient replenishment.     

Neutral lipids in macroalgal spores and their role in swimming

We followed changes in the neutral lipid content of actively swimming zoospores of the palm kelp Pterygophora californica in a laboratory experiment to investigate the degree to which spore swimming is fueled by endogenous lipid reserves. The neutral lipid content of individual zoospores during the experiment was measured by flow cytometry using Nile Red, a fluorescent stain that is specific for neutral lipid. Results showed that photosynthesis greatly influenced lipid consumption during zoospore swimming. We found no detectable change in the neutral lipid content of zoospores after 30 h of swimming under conditions where light was near the optimum for photosynthesis. By contrast, neutral lipid declined by ≃43% over 30 h in zoospores kept in the dark. To evaluate whether lipid reserves are generally related to spore motility in macroalgae, we surveyed spore lipid-content and composition in species with motile spores and non-motile spores using thin-layer chromatography (TLC), and flame-ionization detection (FID). We observed substantial differences in lipid content and composition among the 20 species examined. Spores high in total lipid (as estimated by the ratio of lipid:carbon) generally had a large amount of neutral lipid; motile spores had significantly more lipid and a significantly larger neutral lipid fraction than non-motile spores. The kelps as a group had the highest total lipid content and the largest neutral lipid fraction, while non-motile spores of red algae were generally low in total lipid and in the proportional abundance of neutral lipid. Phospholipids accounted for more than half of all lipid in 14 of the 20 species examined, while neutral lipid accounted for the majority of lipid in all five species of kelp examined. Triacylglycerols, which function primarily in energy storage, were the primary form of neutral lipid in all but one species of kelp (Agarum fimbriatum), whereas free fatty acids were the dominant form of neutral lipid in most red algae and in brown algae that had a small neutral lipid fraction. Our results are largely consistent with the hypothesis that macroalgae use endogenous lipid reserves to fuel spore-swimming. The small amounts of triacylglycerols observed in the motile spores of several species of brown and green algae, however, indicate that the amount of lipid reserves needed to fuel spore-swimming may be influenced by a variety of factors including swimming behavior, photosynthetic efficiency, and the light environment inhabited by spores. Received: 8 September 1998 / Accepted: 8 January 1999     

Resting spore formation and biochemical composition of the marine planktonic diatom Chaetoceros pseudocurvisetus in culture: ecological significance of decreased nucleotide content and activation of the xanthophyll cycle by resting spore formation

The biochemical composition of vegetative cells and resting spores of Chaetoceros pseudocurvisetus Mangin was compared in cultures under various nutrient and light conditions. The cellular content of major nucleotides such as AMP, ADP, ATP and UTP decreased in the order: vegetative cells, nutrient-starved (vegetative) cells and resting spores, indicating that the general metabolism of resting spores is relatively inactive. ADP-glucose was only abundant in nutrient-starved vegetative cells, suggesting metabolic imbalance in these cells. The chl a–specific fluorescence yield of vegetative cells grown under all culture conditions was low, but very high in resting spores. The ratios of the cellular contents of diadinoxanthin to chl a and of diatoxanthin to chl a were higher in resting spores and nutrient-starved vegetative cells than in nutrient-replete vegetative cells. The diadinoxanthin–diatoxanthin xanthophyll cycle was active in resting spores; the xanthophyll cycle was synchronized with a 14 h light:10 h dark photoperiod. Also, the ratios of cellular content of diadinoxanthin and diatoxanthin to cellular content of chl a in resting spores were relatively high in high irradiance, and decreased gradually in conditions of darkness over long culture periods. Under conditions of strong light and high temperature, most resting spores survived more than 40 d while nutrient-starved vegetative cells died within 33 d. These results suggest that resting spore formation is a strategy for enhancing protection and lowering metabolic rate for survival. These physiological changes accompanying spore formation enable resting spores not only to overwinter but also to “oversummer” in the coastal euphotic layer. Received: 23 March 1999 / Accepted: 11 August 1999     

Tidal height,rather than habitat selection for conspecifics,controls settlement in mussels

Settlement is a major determinant of intertidal populations. However, the energy costs of lost larvae are very high. Accordingly, arrival and attachment on suitable substrata are essential requirements for species’ survival. On the intertidal, the presence of cues left by adult or juvenile conspecifics could be vital for the successful establishment of larvae arriving on the shore. Two mussel species, the indigenous Perna perna and the invasive Mytilus galloprovincialis, co-occur on the lower eulittoral zone on the south coast of South Africa. P. perna dominates the low and M. galloprovincialis the high mussel zones, with co-existence in the mid mussel zone. This study tested the hypothesis of settlement selectivity for conspecifics in these two mussel species, to understand whether the final adult distribution of mussels on the shores is determined by active behavioural and chemical mechanisms. Preferential selection by larvae for conspecifics was tested in the field during the peak settlement period in 2004 in natural mussel beds across zones and through manipulative experiments in the mid-zone where the species co-exist. On natural beds, settlement was determined by counts of settlers attached over 48 h onto artificial collectors. Collectors were placed on beds of P. perna and M. galloprovincialis present at both high- and low-adult densities, as well as in mixed beds. On such natural beds, settlers of both species consistently favored low-zone P. perna beds. Settlement patterns over 24 h onto experimentally created mussel patches consisting of P. perna, M. galloprovincialis or the two species combined beds, set in the mixed zone, did not conform with the results of the natural beds study: settlers of both species settled with no discrimination among different patches. The results indicate that mussels, which are sedentary, lack attraction to conspecifics at settlement. This highlights the importance of tidal height in setting settlement rates, and of post-settlement events in shaping populations of these broadcast spawners.     

Delayed recovery of giant kelp near its southern range limit in the North Pacific following El Niño

The northern distribution limit of giant kelp, Macrocystis pyrifera (Agardh), along the Pacific coast of North America is relatively stable near Point Año Nuevo, California, but its southern limit has varied over hundreds of kilometers along the Baja California peninsula during the past 20 years. The factors that drive this variability in the southern limit were examined by: (1) quantifying the abundances of M. pyrifera and the subsurface stipitate kelp, Eisenia arborea (Areschoug), near M. pyriferas then southern limit at Punta San Roque (27°15N; 114°42W) on ten occasions between August 1997 and June 2002 (prior to and following the 1997/1998 El Niño Southern Oscillation); (2) initiating a field manipulation at Punta San Roque in June 2000 to examine competition between these two kelp species; and (3) surveying 20 additional sites near Punta San Hipólito (27°01N; 114°00W) in June 2002 to determine whether M. pyrifera populations had recovered to their pre-1982/1983 El Niño southern range limit after ~20 years of absence. These observations and the experimental results suggest that M. pyrifera is removed from its southern limit by the high temperatures and large waves associated with El Niños, while E. arborea is able to survive and recruit in high densities and thereby delay or even prevent M. pyriferas recovery. An inverse relationship was observed between these two species over the 5-year study. The field experiment showed that at both 4 months and 1 year after areas occupied by E. arborea were cleared, substantial recruitment of M. pyrifera occurred, while recruitment was negligible in uncleared areas. This study suggests that the southern range limit of M. pyrifera is determined by increased mortality and recruitment failure following the warm temperatures and low nutrient conditions associated with El Niños, limited substrate availability, and by competition with the more persistent populations of E. arborea.Communicated by J.P. Grassle, New Brunswick     

Settlement times of blue crab (Callinectes sapidus) megalopae during flood-tide transport

Settlement by blue crab (Callinectes sapidus Rathbun) megalopae on artificial settlement substrates was monitored relative to tidal currents throughout ten nights from July to September 1997 in which the phase relationship between tides and the light dark cycle differed. Most megalopae were in intermolt, and the total number settling to collectors sampled at hourly intervals was greater than totals on collectors immersed all night. Maximum settlement occurred at slack water before ebb tide (SBE), with a smaller peak at slack water before flood tide (SBF). These results support the hypothesis that during flood-tide transport (FTT) blue crab megalopae remain swimming during flood tide at night in response to water turbulence and settle in response to the decline in turbulence occurring near SBE. Settlement peaks near SBF can be explained by a behavioral response of megalopae to increasing salinity at the beginning of flood tide, which results in an ascent response lasting only a few minutes. Depth maintenance in the water column is not maintained at SBF because of low water turbulence. Since light inhibits swimming and upward movement into the water column, settlement, and, presumably, transport were reduced when SBE occurred near the times of sunrise and sunset. Collectively, these results suggest that the phase relationship between the tide and light: dark cycles affects FTT, the timing of settlement, and behaviors associated with habitat selection. Published online: 9 August 2002     

Regulation of algal community development in a Macrocystis pyrifera forest

The effects of small and large-scale roughness, overstory development, competition for space with sessile animals, and grazing on algal community development in a subtidal Macrocystis pyrifera forest were examined using specially prepared concrete blocks as substrata. Variation in small-scale roughness (crevices and grooves in the order of 0.1 to 3 mm width and depth) had no significant effects on community composition. However, M. pyrifera colonization, algal diversity, and sessile animal biomass were higher near the upper horizontal edges of blocks and concrete prisms. This “edge” effect may result from a combination of increased spore and larval settlement and enhanced growth of plants and animals associated with the turbulent eddies formed around these obstructions. Natural and experimentally produced variations in the algal overstory demonstrated that the presence of an overstory can reduce algal diversity and cover beneath. Caging experiments suggest that predatory fishes and sea-stars indirectly affect the algal community by removing sessile animals (primarily bryozoans) which compete with the algae for space. The exclusion of grazers resulted in increased growth of Gigartina spp. Selective grazing on this genus may account for its reduced abundance in the study area.     

Survival and recovery of resting spores and resting cells of the marine planktonic diatom Chaetoceros pseudocurvisetus under fluctuating nitrate conditions

Of the two resting life-forms of the planktonic diatom Chaetoceros pseudocurvisetus Mangin formed during periods of nitrate depletion, resting spores survived at least 1 month after spore formation at 24 °C, while resting cells survived only for about 10 d at the same temperature. Under nitrogen limitation, resting cells exhibited higher specific death rates than resting spores at temperatures ranging from 5 to 30 °C. After nitrogen replenishment, resting spores required a certain lag period of about 1 d to initiate vegetative growth at levels of nitrate supply from 0.5 to 20 M, while resting cells initiated vegetative growth almost immediately. Resting spores exhibited an intracellular accumulation of the supplied nitrate during germination and initial vegetative growth. The resting cells, however, exhibited more active vegetative growth, closely coupled with the uptake of the supplied nitrate. The resting spores and resting cells appear to play different roles in the maintenance of populations under nutrient fluctuations depending on the interval length between nutrient fluxes in natural waters. Received: 27 April 1998 / Accepted: 1 March 1999     

Production and role of hyaline hairs in Ceramium rubrum

Light intensity within the range of 40 to 210 E · m^-2 · s^-1 had relatively little effect on the abundance, length or distribution of Ceramium rubrum unicellular, hyaline hairs. External NH ₄ ⁺ concentrations less than 0.5 M stimulated hair initiation and growth in apical regions of the thalli. Ammonium concentrations in excess of 20 M inhibited hair formation on all regions of the thalli. Ammonium uptake velocities of plants with hairs were approximately twice those of plants without hairs. These hairs may be adaptive to take advantage of intermittent bursts of nutrients by increasing the plant's surface area and, hence, the number of nutrient uptake sites.Woods Hole Oceanographic Institution Contribution No. 5526     

Spatio-temporal patterns in the genetic structure of recently settled blue mussels (<Emphasis Type="Italic">Mytilus</Emphasis> spp.) across a hybrid zone

Previous studies of a hybrid zone between the mussels Mytilus edulis Linnaeus and M. galloprovincialis Lamarck have not resolved the relative importance of the genetic composition of settling larval cohorts versus post-settlement selection in determining the distribution of the parental species and their hybrids. In the present study, recently settled mussels (spat) were collected from 20 sites in southwest England throughout the summer and fall (May–October) in 1998 and 1999. This study investigated the spatio-temporal patterns of settlement and genetics of mussel spat by genetically identifying M. edulis, M. galloprovincialis and their hybrids using the diagnostic PCR marker Glu-5. Settlement was observed earlier in populations of M. edulis than in populations of M. galloprovincialis. Settlement occurred in hybrid populations at times intermediate to and overlapping with both of the parental populations. Temporal genetic variation within years was rare at most sites, while there was some variation between the two years. Spatial genetic variation, however, was common among spat settling within the hybrid populations and matched that observed in small, sub-adults at the same sites. No consistent directional changes in allele frequency were observed over the course of several weeks after settlement. These data suggest that the observed spatial variation in the adult populations is the result of spatial variation in settling larval cohorts and not of either temporal genetic variation or of selection soon after settlement.Communicated by J.P. Grassle, New Brunswick     

Dispersal of faunal and floral propagules associated with drifting Macrocystis pyrifera plants

The potential of drifting Macrocystis pyrifera kelp for transporting associated animals and plants long distances around the southern oceans was assessed by anchoring kelp holdfasts off the Tasmanian coast in 1985, monitoring the turnover of organisms, and relating species survival to water-transport times and species geographic distributions. Although most of the common animal species and approximately half of the plant species associated with Tasmanian M. pyrifera holdfasts were still present on kelp holdfasts after 191 d at sea, very few of these species have been recorded from New Zealand. It therefore seems unlikely that M. pyrifera plants with intact holdfasts are presently drifting to New Zealand. Drifting kelps probably become negatively buoyant in the Tasman Sea because dissolved nitrate concentrations are insufficient for normal plant growth. Moreover, even if some kelp plants do drift to New Zealand it is possible that their holdfasts rapidly disintegrate in the open ocean because of the abundance of the boring isopods Phycolimnoria spp. in Tasmanian holdfasts. In contrast to the restricted distributions of Tasmanian holdfast-inhabiting species, most of the identified species collected from M. pyrifera holdfasts at subantarctic Macquarie Island also occurred 5 000 km west at Kerguelen Island. Because of the extensive ranges of many subantarctic species, the good probability of survival of epifaunal species on drifting kelps, and the high surface-water nitrate concentrations and low holdfast-densities of Phycolimnoria spp. in the higher latitudes, it is likely that M. pyrifera-mediated transport of faunal and floral propagules has recently occurred, and is probably presently occurring, in subantarctic waters.