Fg. 1 Crinoids of Bonaire
Reef Expeditions research now front page featured article for BioNews
Where have all the crinoids gone?
David L. Meyer, Professor of Geology Emeritus, University of Cincinnati, Cincinnati, OH 45221
James A. Vogel, CEO, Reef Expeditions, 7766 Stonehill Drive, Cincinnati, OH 45255
Colorful feather stars (comatulid crinoids, Phylum Echinodermata) were plentiful on the reefs of Curaçao and Bonaire in the late 1960s when DM was seeking accessible localities for study of the ecology and distribution of these echinoderms that are modern representatives of a group that first appears in the fossil record over 450 million years ago (Fig. 1). Curaçao and Bonaire became principal study sites where the abundance and depth distribution of several species could be documented using SCUBA. In 1996 DM was alarmed to find that the most common species, Davidaster rubiginosa and D. discoidea (both formerly included in the genus Nemaster with its type species, N. grandis, also found in Curaçao and Bonaire) were much less common than in previous years. From 2000 – 2010, DM with student help resurveyed sites on the leeward reefs of both islands where previous quantitative censuses existed and confirmed the initial impression that crinoids were down, including virtual disappearance of the once most-common species, D. discoidea (Meyer et al. 2008). Exactly when this decline began, as well as its cause, remains undetermined, but it was sometime after 1989 and before 1996. Thermal stress that caused a severe coral bleaching event in 1995 throughout the southern and western Caribbean (Nagelkerken, 2006) might have been the culprit. Considering that crinoids were down on both islands – Curaçao, with a much higher human population, industry, and coastal development, and Bonaire, with fewer people, almost no industry except tourism, and some of the best-protected coral reefs in the world, this decline was alarming and prompted a resurvey of other Caribbean sites where other quantitative transects had been conducted.
In 2013 and 2014, resurveys of reef sites at Discovery Bay on the north coast of Jamaica first surveyed by DM in 1968 showed that both feather star species, D. rubiginosa and D. discoidea are still common, with D. rubiginosa at least as abundant as in 1968 and D. discoidea increased to about 6 times its abundance in 1968. This result was surprising, because the Jamaican reefs have low coral cover, having suffered many impacts from die-off of Diadema, hurricanes, coral disease, bleaching events, and chronic overfishing. It is paradoxical that feather stars are declining on the reefs of Curaçao and Bonaire, where coral cover is the highest in the Caribbean (Jackson et al., 2014). Currently, several hypotheses to account not only for the crinoid decline in Curaçao and Bonaire but also for the increase of the same species in Jamaica are being considered (Meyer, 2015).
The most recent resurveys of crinoids in Curaçao and Bonaire were in 2010 and 2000, respectively. Therefore an opportunity for an updated look at crinoid populations in Bonaire in 2015 was most welcome. A recently formed non-profit organization, Reef Expeditions, led by JV, brought a group of 15 divers to Bonaire in June, 2015, where they visited 9 divesites where crinoids had been found previously by DM. Working under a research permit from the Bonaire Marine Park, they ran transects along the depth contours where the species D. rubiginosa and D. discoidea usually occur. The only crinoids found at any of these sites were a few individuals of Nemaster grandis, which occurs somewhat deeper than D. rubiginosa and D. discoidea. Among the sites examined were Karpata and the Habitat reef. This recent survey suggests that the downward trend in numbers of the most common crinoids (Meyer, 2008) has continued to the point at which both species are apparently absent from sites previously inhabited. It is our intention to revisit these sites and others on future trips to Bonaire and Curaçao, but in the meantime, we would like to hear from any readers who have reliable records or photos of any crinoids seen on these reefs.
Populations could recover if there are any healthy individuals remaining. However, reduced population sizes may be unable to generate sufficient gametes to achieve enough successful fertilizations that would produce larvae. The same problem, known as the “Allee Effect”, may be part of the reason why the echinoid Diadema antillarum has been so slow to repopulate the reefs following the 1983 mass mortality. We do not know whether crinoid larvae could survive transport by currents from other islands away from Curaçao and Bonaire. (Thus far, no other reef areas in the Caribbean have shown a similar decline in these crinoid species.) Crinoid larvae are generally not regarded as “long-distance” larvae as are those of some other echinoderms. Thus there may be a heavy dependence on maintaining continuity of local populations through “self-seeding”. If crinoid populations on reefs of Curaçao and Bonaire have declined to that critical level, then it may be a long time before we again can enjoy their graceful beauty, and the reef ecosystem will have lost components of the filter-feeding food web.
Jackson, J.B.C., Donovan, M.K., Cramer. K.L., & Lam, V.V. (eds). 2014. Status and Trends of Caribbean Coral Reefs: 1970-2012. Global Coral Reef Monitoring Network, IUCN, Gland, Switzerland.
Meyer, D.L., E.A. Dame, & P.B. Lask. 2008. Decline of crinoids on the reefs of Curaçao and Bonaire, Netherlands Antilles. 11th International Coral Reef Symposium, Proceedings. Ft. Lauderdale, Florida. 875-879.
Meyer, D.L. 2015. Crinoid paradox. Reef Encounter (Newsletter of the Society for Reef Studies) 30(2):24-27.
Nagelkerken, I. (2006) Relationship between anthropogenic impacts and bleaching-associated tissue mortality of corals in Curaçao (Netherlands Antilles). Rev Biol Trop 54:31-43.
Figure 1: Two individuals of Davidaster rubiginosa, Bonaire, 1970. Photo by D. Meyer.