Floating kelps in Patagonian Fjords: an important vehicle for rafting invertebrates and its relevance for biogeography

In this review, we present the current state of biodiversity knowledge for the class Staurozoa (Cnidaria), including richness estimates, geographical and bathymetric distributions, substrate use, feeding, behavior, life cycle, and conservation. Based on non-parametric, statistical incidence estimators, the global inventory of 50 known and accepted species of stalked jellyfishes might be regarded as close to complete, but we discuss possible bias related to the lower research effort applied in the Southern Hemisphere. Most of the species occur at mid-latitudes, presenting a distributional pattern that disagrees with the classic pattern of diversity (higher richness near the Equator). Specimens are frequently found on algae, but they have also been reported attached to rocks, seagrasses, shells, mud, sand, coral/gorgonian, sea cucumber, and serpulid tube. Most of the species are found in the intertidal and shallow subtidal regions, but species of Lucernaria have been reported at more than 3000 m deep. Amphipods and copepods are the prey items most frequently reported, and stauromedusae have been observed being actively preyed upon by nudibranch mollusks and pycnogonids. Apparently, stalked jellyfishes have a high sensitivity to anthropic impacts in the environment, and promotion of the class, one of the least studied among Cnidaria, is perhaps the best possible conservation strategy.

Small mobile crustaceans are abundant on seaweeds. Many of these crustaceans rapidly abandon their host if it is detached from the seafloor and floats towards the surface, but the trigger for this ‘bailout’ behaviour is unknown. We tested 2 potential cues, i.e. rapid change in light and rapid change in water pressure, using >1 mm epifauna on the brown seaweed Carpophyllum plumosum as a model system. Bailout occurred in response to reduced water pressure, but not to changing light, as (1) bailout occurred at similar rates in light and dark, (2) bailout occurred on the seafloor when water pressure was reduced within a transparent chamber by the equivalent of ~0.5 m depth or more, and (3) little bailout occurred when water pressure was held constant within the chamber while seaweeds were raised to the surface. Increase in pressure (simulating sinking) did not induce bailout. The rate of bailout increased with increasing magnitude of pressure reduction but was not influenced greatly by the rate of change of pressure within the range tested (up to an equivalent of 0.4 m depth s-1). The use of pressure rather than light as a cue for bailout is consistent with the need for seaweed-associated crustaceans to rapidly abandon a detached host and relocate to suitable habitat during both day and night.

Dispersal on floating seaweeds depends on availability, viability, and trajectories of the rafts. In the southern hemisphere, the bull kelp Durvillaea antarctica is one of the most common floating seaweeds, but phylogeographic studies had shown low connectivity between populations from continental Chile, which could be due to limitations in local supply and dispersal of floating kelps. To test this hypothesis, the spatiotemporal dynamics of kelp strandings were examined in four biogeographic districts along the Chilean coast (28°-42°S). We determined the biomass and demography of stranded individuals on 33 beaches for three subsequent years (2013, 2014, 2015) to examine whether rafting is restricted to certain districts and seasons (winter or summer). Stranded kelps were found on all beaches. Most kelps had only one stipe (one individual), although we also frequently found coalesced holdfasts with mature males and females, which would facilitate successful rafting dispersal, gamete release, and reproduction upon arrival. High biomasses of stranded kelps occurred in the northern-central (30°S-33°S) and southernmost districts (37°S-42°S), and lower biomasses in the northernmost (28°S-30°S) and southern-central districts (33°S-37°S). The highest percentages and sizes of epibionts (Lepas spp.), indicative of prolonged floating periods, were found on stranded kelps in the northernmost and southernmost districts. Based on these results, we conclude that rafting dispersal can vary regionally, being more common in the northernmost and southernmost districts,depending on intrinsic (seaweed biology) and extrinsic factors (shore morphology and oceanography) that affect local supply of kelps and regional hydrodynamics.

Long-term persistence of the floating bull kelp Durvillaea antarctica from the South-East Pacific: Potential contribution to local and transoceanic connectivity

Abstract Ecological communities are jointly structured by dispersal, density-independent responses to environmental conditions and density-dependent biotic interactions. Metacommunity ecology provides a framework for understanding how these processes combine to determine community composition among local sites that are regionally connected through dispersal. In 17 temperate seagrass meadows along the British Columbia coast, we tested the hypothesis that eelgrass (Zostera marinaL.) epifaunal invertebrate assemblages are influenced by local environmental conditions, but that high dispersal rates at larger spatial scales dampen effects of environmental differences. We used hierarchical joint species distribution modelling to understand the contribution of environmental conditions, spatial distance between meadows, and species co-occurrences to epifaunal invertebrate abundance and distribution across the region. We found that patterns of taxonomic compositional similarity among meadows were inconsistent with dispersal limitation and meadows in the same region were often no more similar to each other than meadows over 1000 km away. Abiotic environmental conditions (temperature, dissolved oxygen) explained a small fraction of variation in taxonomic abundances patterns across the region. We found novel co-occurrence patterns among taxa that could not be explained by shared responses to environmental gradients, suggesting the possibility that interspecific interactions influence seagrass invertebrate abundance and distribution. Our results add to mounting evidence that suggests that the biodiversity and ecosystem functions provided by seagrass meadows reflect ecological processes occurring both within meadows and across seascapes, and suggest that management of eelgrass habitat for biodiversity may be most effective when both local and regional processes are considered.

ABSTRACT The ecology and biogeography of the invertebrate epifauna related to floating Sargassum in the East China Sea were studied. Floating algal rafts composed of only Sargassum horneri (Turner) C. Agardh were collected in the East China Sea in March 2012 when the rafts were distributed not only on the continental shelf but also, unusually, south and east of the Kuroshio Current in the Pacific Ocean. In total, 71 samples collected at 11 stations were studied. Of the 11 invertebrate taxa recognized, harpacticoid copepods were most abundant, followed by malacostracans, and cirripedes. The densities of these three taxa were higher than those of other invertebrates. The number of invertebrate taxa was not correlated with the algal wet weight. There were more invertebrate taxa in the coastal area around Amami-Oshima Island than in other areas. The trajectories of drifting buoys indicated that the algal rafts collected in this area floated longer than those in other areas, allowing the colonization of invertebrates from the surrounding water.

Abstract AimMarine habitats and their dynamics are difficult to systematically monitor, particularly those in remote locations. This is the case with the sub‐Antarctic ecosystem of the giant kelp Macrocystis pyrifera, which was already noted by Charles Darwin in his accounts on the Voyage of the Beagle and recorded on the nautical charts made during that expedition. We combined these and other nautical charts from the 19th and early 20th centuries with surveys conducted in the 1970s and 1980s and satellite detection algorithms from 1984 to 2019, to analyse kelp distribution through time and the factors that correlate with it.LocationMarine ecoregions of Channels and Fjords of Southern Chile, Falkland Islands (Malvinas), and the island of South Georgia.TaxonMacrocystis pyrifera.MethodsWe characterised 309 giant kelp forests by their coastal geospatial attributes. Statistically significant variables were included in a conditional inference tree to predict kelp forest size. Sea surface temperature (SST) records were analysed to confirm temperature ranges over the last four decades. Nautical charts, historical surveys, aerial photogrammetry, unmanned aerial vehicle (UAV) surveys and satellite imagery were overlaid to assess spatial distribution of kelp forest canopies, spanning the period 1829–2020.ResultsConsidering the extensive natural and human caused changes over the last two centuries, this diverse kelp ecosystem is remarkably persistent. We found that the ocean currents and wave exposure, combined with the geomorphological settings of the coastline are the most critical factors predicting the extent of the kelp forests.Main conclusionsWe have described the long‐term ecological persistence of the kelp forests in this vastly under‐studied region that offers a conceptual biogeographical model supporting the global importance proposed by Charles Darwin 200 years ago (Darwin, 1845). In the current context of global change, the need for conservation of this persistent and well‐preserved marine ecosystem has never been more important.

The invertebrate fauna associated with floating Sargassum horneri (Turner) C. Agardh, 1820 in the East China Sea was investigated in terms of ecology and biogeography. Floating algal rafts consisting of only S. horneri were collected at 16 stations in the East China Sea during a cruise of research vessel (R/V) Tansei-Maru in February, 2011. A total of 53 rafts were obtained for faunal investigations at 14 of the 16 stations. In addition to fish eggs, 10 invertebrate taxa were found on the floating algae. Of the collected phytal animals, harpacticoid copepods were most abundant in terms of number (80%), followed by cirripedes (15%), amphipods (4%), and others (1%). The faunal diversity on the algae was correlated to the algal weight. The faunal diversity differed significantly between stations and was highest in the northernmost part of the East China Sea. Density and abundance of animals were highest in areas close to the Kuroshio Current. Considering the direction of flow of the Kuroshio Current and the density of cirripedes immigrating from the surrounding water onto the floating algae, most of the floating S. horneri possibly originated in the southern part of the East China Sea. On the other hand, the Sargassum rafts collected in the northernmost part of the East China Sea could have originated from the coast of eastern China. Most of the animals found on the floating Sargassum were pelagic taxa that complete their life cycles on the rafts in situ. Typical coastal animals, including gammaridean and caprellidean amphipods, tanaidaceans, nematodes, gastropods, polychaetes, halacarid mites, bryozoans, and hydrozoans, were also obtained from the floating Sargassum , but only occasionally and their densities were quite low. These facts suggest that dispersal events among benthic habitats via Sargassum rafts are relatively rare.

Marine invasions are well-recognized as a worldwide threat to biodiversity and cause for tremendous economic damage. Fundamental aspects in invasion ecology are not yet fully understood, as there is neither a clear definition of invasive species nor their characteristics. Likewise, regulations to tackle marine invasions are fragmentary and either restricted to specific regions or certain aspects of the invasion process. Nonetheless, marine anthropogenic vectors (e.g., vessel fouling, ballast water, aquaculture, marine static structures, floating debris, and human-mediated climate change) are well described. The most important distribution vector for marine non-indigenous species is the shipping sector, composed by vessel fouling and ballast water discharge. Ship traffic is a constantly growing sector, as not only ship sizes are increasing, but also remote environments such as the polar regions are becoming accessible for commercial use. To mitigate invasions, it is necessary to evaluate species’ capability to invade a certain habitat, as well as the risk of a region of becoming invaded. On an ecological level, this may be achieved by Ecological Niche Modelling based on environmental data. In combination with quantitative vector data, sophisticated species distribution models may be developed. Especially the ever-increasing amount of available data allows for comprehensive modelling approaches to predict marine invasions and provide valuable information for policy makers. For this article, we reviewed available literature to provide brief insights into the backgrounds and regulations of major marine vectors, as well as species distribution modelling. Finally, we present some state-of-the-art modelling approaches based on ecological and vector data, beneficial for realistic risk assessments.

Many seaweed species (primary rafters) float at the sea surface and travel with marine currents after detachment from benthic habitats. Various studies have confirmed that dispersal via floating sporophytes and/or gametophytes influences the phylogeography and genetic population structure of these buoyant seaweeds. In addition, non-buoyant seaweeds (secondary rafters) that grow attached to or intermingled with these primary floaters may also become dispersed by rafting on their floating hosts. Here, we examine reports of non-buoyant seaweed species associated with buoyant seaweeds and discuss potential consequences for their phylogeography and/or genetic population structure. We found that mostly red and brown algae have been reported with floating seaweed rafts, most of them growing as epiphytes and some as obligate parasites (e.g. endophytes) that travel with their hosts. Molecular evidence suggests dispersal associated with primary floaters in 16 non-buoyant seaweeds, although colonization of distant sites could also have occurred via other floating substrata such as wood, buoys, and other man-made materials. Transoceanic dispersal has been inferred for non-buoyant seaweeds (for example, Gracilaria chilensis and Capreolia implexa) based on low levels of genetic structure and shared haplotypes among populations separated over vast distances of open ocean (e.g. New Zealand–Chile). Some non-buoyant species suspected or shown to be dispersed by rafting are from intertidal habitats, and these algae can resist physiologically stressful conditions during long trips at the sea surface. However, subtidal and low intertidal non-buoyant species have higher potential to be transported because they cohabit with common raft-forming kelps, often growing on them as epiphytes. We conclude that buoyant seaweeds play an important role in driving the phylogeography, evolution, connectivity and distribution of non-buoyant associated seaweeds. Dispersal of non-buoyant seaweeds via these floating seaweeds may have been underestimated in the past.