Implications of changing Caribbean coral reefs on Diadema antillarum larvae settlement

Abstract

Tropical western Atlantic reefs have gradually shifted from being dominated by corals to being mainly covered by macroalgae. The mass-mortality of the sea urchin Diadema antillarum in the 80s and the slow to non-existent recovery exacerbated this shift. Chemical cues associated with these reefs are expected to have shifted too with potential negative effects on larval recruitment, possibly limiting recovery of important species like D. antillarum. In this study, we tested the effects of naturally derived biofilm and macroalgae species native to Caribbean coral reefs on the settlement rate of cultured D. antillarum larvae in two separate experiments. Crustose coralline algae (CCA) were included in both experiments, making it possible to compare settlement rates from both experiments. A biofilm of one week old yielded significantly lower settlement rates compared to two, four, and six weeks old biofilm and the highest settlement rate was found for CCA with over 62% of total larvae. All six tested macroalgae species resulted in settled larvae, with little significant difference between algal species, partly due to a high variation in settlement rates within treatments. Sargassum fluitans induced the highest settlement rate with 33%, which was not significantly different from CCA with 29%. We conclude that dominant macroalgae species likely to be encountered by D. antillarum on shifted reefs are no major constraint to settlement. Our findings increase the understanding of alternative stable state settlement dynamics for a keystone coral reef herbivore.

Assessing the influence of macroalgae and micropredation on the early life success of the echinoid Diadema africanum

“…In our study, only 'La Jaca' exhibited su cient levels of recruitment to allow us to detect a clear pattern of avoidance by early D. africanum recruits in treatments containing Lobophora schneideri. A similar result was recently observed in laboratory assays performed by Wijers et al (2024), who also found lower settlement rates of the Western Atlantic sibling species Diadema antillarum on Lobophora variegata tiles in comparison with other algae treatments such as CCA, Sargassum uitans, and Padina pavonica. These observations can be explained by previous studies showing that the widespread algae genus Lobophora can produce multiple secondary metabolites, such as 2,4-dibromophenol (Vieira et al 2017), which has been found to inhibit the metamorphosis of several species of sea urchins (Agatsuma et al 2008;Li et al 2011).…”

“…This settlement phase encompasses all the processes necessary for the transition from pelagic to benthic life, including substrate selection and metamorphosis. Certain algal species, such as CCAs, are able to have a great in uence on the sea urchin settlement phase, by providing competent larvae with appropriate cues to initiate settlement (Pearce and Scheibling, 1990;Huggett et al 2006;Swanson et al 2006;Dworjanyn and Pirozzi, 2008;Pilnick et al 2023;Wijers et al 2024). Conversely, although lesser studied, some algal species are able to constrain sea urchin settlement success due to the release of toxic compounds (Norris and Fernical, 1982;Agatsuma et al 2008;Li et al 2011;Wijers et al 2024).…”

“…Certain algal species, such as CCAs, are able to have a great in uence on the sea urchin settlement phase, by providing competent larvae with appropriate cues to initiate settlement (Pearce and Scheibling, 1990;Huggett et al 2006;Swanson et al 2006;Dworjanyn and Pirozzi, 2008;Pilnick et al 2023;Wijers et al 2024). Conversely, although lesser studied, some algal species are able to constrain sea urchin settlement success due to the release of toxic compounds (Norris and Fernical, 1982;Agatsuma et al 2008;Li et al 2011;Wijers et al 2024). Once settlement and metamorphosis are completed, early sea urchin recruits still have a long path before reaching adulthood, becoming less vulnerable to predation and acquiring the capacity to reproduce in the process.…”

Restoration of herbivory on Caribbean coral reefs: are fishes, urchins, or crabs the solution?

“…Many species of sea urchins respond to bare substrate or those with new biofilms as cues for settlement (Bak, 1985;Pearce and Scheibling, 1991;Rahim et al, 2004;Williams et al, 2011), but the spatiotemporal patterns of settlement by D. antillarum vary appreciably among locations (Bak, 1985;Miller et al, 2009;Vermeij et al, 2010;Williams et al, 2010Williams et al, , 2011Feehan et al, 2019;Maldonado-Sańchez et al, 2019;Hylkema et al, 2022a). Recent laboratory experiments by Pilnick et al (2023) and Wijers et al (2024) revealed a lack of settlement by competent D. antillarum larvae exposed to a sterile seawater control, but settlement rates increased to ~50% in response to two types of calcareous macroalgae (Pilnick et al, 2023;Wijers et al, 2024) and to natural biofilms (Wijers et al, 2024). If settlement hotspots are identified it is possible to collect high numbers of D. antillarum settlers during certain months of the year (Williams et al, 2010(Williams et al, , 2011Hylkema et al, 2022a;Klokman and Hylkema, 2024).…”