Habitat heterogeneity promotes the coexistence of exotic seaweeds

Keep It Simple: Improving the Ex Situ Culture of Cystoseira s.l. to Restore Macroalgal Forests

Brown algae from genus Cystoseira s.l. form dense underwater forests that represent the most productive areas in the Mediterranean Sea. Due to the combined effects of global and local stressors such as climate change, urbanization, and herbivore outbreaks, there has been a severe decline in brown algal forests in the Mediterranean Sea. Natural recovery of depleted sites is unlikely due to the low dispersal capacity of these species, and efficient techniques to restore such habitats are needed. In this context, the aims of our study were (1) to improve and simplify the current ex situ laboratory protocol for the cultivation of Gongolaria barbata by testing the feasibility of some cost-effective and time-efficient techniques on two donor sites of G. barbata and (2) to evaluate the survival and growth of young thalli during the laboratory phase and during the most critical five months after out-planting. Specifically, the following ex situ cultivation methods were tested: (A) cultivation on clay tiles in mesocosms with culture water prepared by three different procedures (a) filtered seawater with a 0.22 μm filter membrane, (b) filtered seawater with a 0.7 μm filter membrane (GF), and (c) UV-sterilized water, and (B) cultivation on clay tiles in open laboratory systems. After two weeks, all thalli were fixed to plastic lantern net baskets suspended at a depth of 2 m in the coastal sea (hybrid method), and the algal success was monitored in relation to the different donor sites and cultivation protocol. The satisfactory results of this study indicate that UV-sterilized water is suitable for the cultivation of G. barbata in mesocosm, which significantly reduces the cost of the laboratory phase. This opens the possibility of numerous and frequent algal cultures during the reproductive period of the species. Additionally, if the young thalli remain in the lantern net baskets for an extended period of several months, they can grow significantly in the marine environment without being exposed to pressure from herbivorous fish.

The role of environmental conditions in regulating long-term dynamics of an invasive seaweed

The mechanisms underpinning long-term dynamics and viability of invader populations in the receiving environment remain largely unknown. We tested the hypothesis that temporal variations in the abundance of a well-established invasive seaweed, Caulerpa cylindracea, in the NW Mediterranean, could be regulated by inter-annual fluctuations in environmental conditions. Abundance data of C. cylindracea, sampled repeatedly between 2005 and 2020 at the peak of its growing season (late summer/early fall), were related to interannual variations in seasonal seawater temperature, wind speed and rainfall recorded during different growth phases of the alga, in both subtidal and intertidal habitats. In both habitats, higher peak of C. cylindracea cover was associated with lower seawater temperature in spring and summer, when the seaweed exits the winter resting phase and starts a period of active growth. In addition, the peak abundance of subtidal C. cylindracea was positively associated with higher autumn wind speed intensity and spring daily total precipitation. Our study reveals the importance of seasonal and interannual variation of abiotic factors in shaping temporal patterns of abundance of C. cylindracea, in both subtidal and intertidal habitats. Identifying the factors underpinning invasive population temporal dynamics and viability is essential to predict the time and conditions under which an invader can thrive, and thus guide management strategies aimed to containing invasions under current and future climates.

Different environmental variables predict distribution and cover of the introduced red seaweed Eucheuma denticulatum in two geographical locations

In this study we examined abiotic and biotic factors that could potentially influence the presence of a non-indigenous seaweed, Eucheuma denticulatum, in two locations, one outside (Kane’ohe Bay, Hawai’i, USA) and one within (Mafia Island, Tanzania) its natural geographical range. We hypothesized that the availability of hard substrate and the amount of wave exposure would explain distribution patterns, and that higher abundance of herbivorous fishes in Tanzania would exert stronger top–down control than in Hawai’i. To address these hypotheses, we surveyed E. denticulatum in sites subjected to different environmental conditions and used generalized linear mixed models (GLMM) to identify predictors of E. denticulatum presence. We also estimated grazing intensity on E. denticulatum by surveying the type and the amount of grazing scars. Finally, we used molecular tools to distinguish between indigenous and non-indigenous strains of E. denticulatum on Mafia Island. In Kane’ohe Bay, the likelihood of finding E. denticulatum increased with wave exposure, whereas on Mafia Island, the likelihood increased with cover of coral rubble, and decreased with distance from areas of introduction (AOI), but this decrease was less pronounced in the presence of coral rubble. Grazing intensity was higher in Kane’ohe Bay than on Mafia Island. However, we still suggest that efforts to reduce non-indigenous E. denticulatum should include protection of important herbivores in both sites because of the high levels of grazing close to AOI. Moreover, we recommend that areas with hard substrate and high structural complexity should be avoided when farming non-indigenous strains of E. denticulatum.

Dynamics of recruitment and establishment of the invasive seaweed Codium fragile within an eelgrass habitat

Knowledge of the potential distribution (i.e. abundance and spatial extent) of an invasive species is important to estimating its potential impacts on recipient communities. Most previous studies have focused on the potential spatial extent of invasive species populations at regional scales, but little is known on how species successfully recruit and establish at more local scales. In this study, we examined how recruitment of the green alga Codium fragile ssp. fragile (hereafter Codium) can vary spatially and the environmental factors associated with Codium establishment in eelgrass (Zostera marina) beds. Standardized recruitment blocks (65 blocks in a 720 × 240 m² grid) were used to monitor the number of Codium recruits, juveniles and adults over 2 years. Environmental factors (depth, relative water flow, light and temperature) and attributes of the surrounding macrophyte assemblage (eelgrass density, eelgrass length, Codium biomass) were also measured. Recruitment occurred on all blocks or nearby artificial structures (i.e. buoys) and mainly originated from button stages (i.e. female gametes or utricles). Contrary to other studies, the abundance of Codium (recruits, juveniles and adults) was best predicted by the density of the native canopy-forming species, Z. marina, which highlights a positive interaction between native and non-native canopy-forming species. Seasonal variation in recruitment was observed; it was lower during the summer. Recruitment did not show any distinct spatial pattern (e.g. gradient or patch), but the same spatial pattern of recruitment was observed every sampling date, suggesting that there are "hotspots" for recruitment. In general, the total number of Codium fronds observed on a block at the end of the experiment was positively correlated with the cumulative number of recruits. However, recruitment occurred on some blocks but recruits never grew, suggesting that some environmental factors limit Codium distribution and abundance in eelgrass beds. Overall, the assessment of Codium recruitment over 2 years showed that the colonization of suitable locations by Codium within seagrass beds may take several years and that some factors may not only limit, but also inhibit Codium expansion within eelgrass beds.

1H NMR Spectroscopy and MVA Analysis of Diplodus sargus Eating the Exotic Pest Caulerpa cylindracea

The green alga Caulerpa cylindracea is a non-autochthonous and invasive species that is severely affecting the native communities in the Mediterranean Sea. Recent researches show that the native edible fish Diplodus sargus actively feeds on this alga and cellular and physiological alterations have been related to the novel alimentary habits. The complex effects of such a trophic exposure to the invasive pest are still poorly understood. Here we report on the metabolic profiles of plasma from D. sargus individuals exposed to C. cylindracea along the southern Italian coast, using ¹H NMR spectroscopy and multivariate analysis (Principal Component Analysis, PCA, Orthogonal Partial Least Square, PLS, and Orthogonal Partial Least Square Discriminant Analysis, OPLS-DA). Fish were sampled in two seasonal periods from three different locations, each characterized by a different degree of algal abundance. The levels of the algal bisindole alkaloid caulerpin, which is accumulated in the fish tissues, was used as an indicator of the trophic exposure to the seaweed and related to the plasma metabolic profiles. The profiles appeared clearly influenced by the sampling period beside the content of caulerpin, while the analyses also supported a moderate alteration of lipid and choline metabolism related to the Caulerpa-based diet.

Dispersal and spatial heterogeneity: single species

A recent result for a reaction-diffusion equation is that a population diffusing at any rate in an environment in which resources vary spatially will reach a higher total equilibrium biomass than the population in an environment in which the same total resources are distributed homogeneously. This has so far been proven by Lou for the case in which the reaction term has only one parameter, m(x), varying with spatial location x, which serves as both the intrinsic growth rate coefficient and carrying capacity of the population. However, this striking result seems rather limited when applies to real populations. In order to make the model more relevant for ecologists, we consider a logistic reaction term, with two parameters, r (x) for intrinsic growth rate, and K(x) for carrying capacity. When r (x) and K(x) are proportional, the logistic equation takes a particularly simple form, and the earlier result still holds. In this paper we have established the result for the more general case of a positive correlation between r (x) and K(x) when dispersal rate is small. We review natural and laboratory systems to which these results are relevant and discuss the implications of the results to population theory and conservation ecology.

Could molecular effects of Caulerpa racemosa metabolites modulate the impact on fish populations of Diplodus sargus?

The green alga Caulerpa racemosa is a non-native, invasive species in the Mediterranean, and an important stressor for several native organisms. The algal capacity to produce secondary metabolites has been suggested to modulate success of the C. racemosa invasion, although many of potentially involved biological pathways still remain unexplored. In this respect, the aim of the present study was to investigate some molecular and cellular effects in the white seabream Diplodus sargus, an ecologically key species, which included the alien C. racemosa in its diet. Organisms were sampled in 2 seasonal periods from 3 locations of Southern Italy, each characterized by different levels of algal abundance. The level of caulerpin, one of the main secondary algal metabolites, in fish tissues has been used as an indicator of the trophic exposure to the seaweed and related with molecular and cellular responses. Chemical analyses indicated that fish from invaded sites can accumulate caulerpin, with liver concentrations ranging from a few up to hundreds of μg/g. Biomarkers analyses revealed only limited alterations of the main antioxidant defences, such as glutathione reductase and levels of glutathione; on the other hand, increased enzymatic activities of cytochrome P450, glutathione S-transferases and acyl CoA oxidase, as well as enhanced gene transcription for peroxisome proliferator-activated receptor alpha, cytochrome P4501A and vitellogenin 1 were observed in fish more exposed to C. racemosa as indicated by liver concentrations of caulerpin higher than 50 μg/g. Despite a direct molecular relationship with this algal metabolite could not be established, our results suggest that a C. racemosa enriched diet can modulate biotransformation and fatty acids metabolism of D. sargus. Assessing whether similar effects represent short- or long-term effects will be of crucial importance to understand consequences on the general health status and reproductive performance of exposed key fish species in the Mediterranean region.