Measuring fertilization success of broadcast-spawning marine invertebrates within seagrass meadows

Seagrass meadows as ocean acidification refugia for sea urchin larvae

Foundation species have been widely documented to provide suitable habitats for other species by ameliorating stressful environmental conditions. Nonetheless, their role in rescuing stress-sensitive species from adverse conditions due to climate change remains often unexplored. Here, we performed a mesocosm experiment to assess whether the seagrass, Posidonia oceanica, through its photosynthetic activity, could mitigate the negative effects of ocean acidification on larval development and growth of the calcifying sea urchin, Paracentrotus lividus. Sea urchin larvae at early and late developmental stages that are generally associated to benthic habitats, were grown in aquaria with or without P. oceanica plants, under ambient or low pH conditions predicted by the end of the century under the worst climate scenario (RCP8.5). The percentage of abnormal larvae and their total body length under different experimental conditions were assessed on early- (i.e., pluteus; 72 h post-fertilization) and final-developmental stages (i.e., echinopluteus; 30 days post-fertilization), respectively. The presence of P. oceanica increased mean daily pH values of ∼0.1 and ∼0.15 units at ambient and low pH conditions, respectively, compared with tanks without plants. When grown at low pH in association with P. oceanica, plutei showed a ∼23 % reduction of malformations and echinoplutei a ∼34 % increase in total body length, respectively, compared with larvae developing in tanks without plants. Our results suggest that P. oceanica, by increasing pH and altering seawater carbonate chemistry through its metabolic activity, could buffer the negative effects of ocean acidification on calcifying organisms and could, thus, represent a tool against climate-driven loss of biodiversity.

Potential impacts of climate change on the distribution of echinoderms in the Yellow Sea and East China Sea

To detect potential impacts of climate change on the distribution of common echinoderm species in the Yellow Sea (YS) and East China Sea (ECS), species distribution models (SDMs) were applied. Ensemble SDMs were constructed and were in good model performance for six of the eight selected common echinoderm species. Under future climate scenarios, the brittle stars Ophiopholis mirabilis, Amphioplus depressus and the sea cucumber Protankyra bidentata were projected to expand in the southwestern areas of the YS, the ECS, and the coastal areas of the YS and ECS, respectively; the brittle stars Stegophiura sladeni, Amphiura digitula and Amphiura vadicola will likely contract their ranges in the south distribution areas and expand in the north, showing a northward movement trend. Temperature was the most important environmental variable influencing the distribution of the latter three echinoderms. Our findings will improve our understanding of the impacts of climate change on marine species distributions.

Tracking larvae with molecular markers reveals high relatedness and early seasonal recruitment success in a partially spawning marine bivalve

The partial synchronized spawning strategy adopted by some marine invertebrate broadcast-spawners can lead to the production of many distinct pools of larvae within a single reproductive cycle. Following the fate of these larval groups from birth to settlement with molecular markers might shed light on mechanisms regulating their population recruitment. Larvae and recruits of Mya arenaria, a partially spawning marine bivalve, were monitored and collected over 13 consecutive weeks during an entire reproductive cycle. Each sampled individual (n = 218) was sorted according to size (early veligers, late veligers, post-larval recruits) and genotyped at seven microsatellite loci for comparisons among samples and with adult reference samples (n = 270). While traditional differentiation statistics (e.g., pairwise Δ(ST), allelic richness) suggested the absence of sweepstakes reproductive success, the level of relatedness found within and among larvae and recruit samples suggested otherwise. Four samples out of ten were observed to have higher within-sample relatedness values than randomly expected, including the very first group of early veligers produced in the season (E1) and the last group of post-larvae who survived recruitment (P10). E1 individuals were also found to be more related than randomly expected to individuals of more than 80% of all other samples including the last surviving recruits (P8 and P10). These results suggest that the first larvae produced in the season were the most successful to survive recruitment. Results also show direct evidence for larval retention and demonstrate for the first time larval and post-larval kin aggregation in a marine bivalve.

Effects of oscillatory flow on fertilization in the green sea urchin Strongylocentrotus droebachiensis

Broadcast spawning invertebrates that live in shallow, high-energy coastal habitats are subjected to oscillatory water motion that creates unsteady flow fields above the surface of animals. The frequency of the oscillatory fluctuations is driven by the wave period, which will influence the stability of local flow structures and may affect fertilization processes. Using an oscillatory water tunnel, we quantified the percentage of eggs fertilized on or near spawning green sea urchins, Strongylocentrotus droebachiensis. Eggs were sampled in the water column, wake eddy, substratum and aboral surface under a range of different periods (T = 4.5 – 12.7 s) and velocities of oscillatory flow. The root-mean-square wave velocity (rms(u_w)) was a good predictor of fertilization in oscillatory flow, although the root-mean-square of total velocity (rms(u)), which incorporates all the components of flow (current, wave and turbulence), also provided significant predictions. The percentage of eggs fertilized varied between 50 – 85% at low flows (rms(u_w) <0.02 m s⁻¹), depending on the location sampled, but declined to below 10% for most locations at higher rms(u_w). The water column was an important location for fertilization with a relative contribution greater than that of the aboral surface, especially at medium and high rms(u_w) categories. We conclude that gametes can be successfully fertilized on or near the parent under a range of oscillatory flow conditions.