Alwin Hylkema

The massive die-off of hte sea urchin Diademan antillarum in 1983-1984 is one of the main reasons for low coral recuirtmenat and little coral recovery in the Caribbean. As the natural recovery of D. antillarum is slow to non-existent, multiple restoration studies have been attempted. There are currently three different approaches to obtain individuals for restocking: the translocation of wild-collected juveniles or adults, lab-reared juveniles cultured from wild=collected settlers, or lab-reared juveniles cultured from gametes. All three methods are costly and can only be applied on a relatively small scale.  We propose a fourth, new, approach, which we term assisted natural recovery (ANR) of D. antillarum populations. 

https://books.google.com/books?hl=en&lr=&id=ZNjpEAAAQBAJ&oi=fnd&pg=PA10&...

Project description

Time: July 2023- July 2025

Project leaders: VHL (Alwin Hylkema), SCF (Kai Wulf), STENAPA (Erik Boman), WUR (Dolfi Debrot)

Project summary

Many Caribbean reefs have shifted from coral-dominated to algal-dominated ecosystems. The high algae cover reduces coral recruitment, making the reef unable to recover from other disturbances and resulting in flatter reefs with lower biodiversity. One of the reasons for the proliferation of algae is a mass die-off of the herbivorous sea urchin Diadema antillarum in the early 1980s. Natural recovery of Diadema populations is slow to non-existent, making active restoration of this important grazer a top priority in Caribbean coral reef management, especially since Diadema densities were reduced by another mass mortality event in 2022. The marine park organizations of Saba and St. Eustatius want to restore Diadema populations by restocking cultured individuals. However, important knowledge gaps need to be addressed before large numbers of Diadema can be restocked on the reef. Current culture methods can only produce a limited number of competent larvae. In addition, only 8% of the settlers survive and after restocking, survival on the reef is low as well. In the RAAK PRO Diadema II project, the bottlenecks in Diadema culture will be addressed by comparing larval survival across multiple culture methods and investigating the relation between larval size and post-settlement survival. Growing-out juveniles at sea is likely to help prepare them for life in the wild, while restocking at an optimal size might also increase survival. Finally, a thorough restocking site selection based on high shelter availability and settlement rates will increase the long-term Diadema densities. The acquired knowledge and developed practices will be verified in a larger scale restocking experiment involving at least 5000 Diadema urchins. By restoring Diadema populations through restocking, macroalgae will be more intensively removed and corals will have a chance to settle and to survive, increasing the ability of the reef to cope with other stressors.

A B S T R A C T

Fish assemblages of different types of artificial reefs can differ greatly in abundance, biomass and composition, with some reef types harboring over five times more herbivores than others. It is assumed that higher herbivorous fish abundance results in a higher grazing intensity, affecting the benthic community by means of enhanced coral recruitment, survival and growth. Territorial fish species might affect this process by chasing away other fish, especially herbivores. In this study we compared the fish assemblage, territorial behavior and grazing intensity by fish on two artificial reef types: reef balls and layered cakes, differing greatly in their fish assemblage during early colonization. In addition, the effect of artificial reef type on benthic development and coral recruitment, survival and growth, was investigated. Although layered cakes initially harbored higher herbivorous fish biomass, this effect was lost during consecutive monitoring events. This seems to be the result of the higher territorial fish abundance around the layered cakes where almost four times more chasing behavior was recorded compared to the reef balls. This resulted in a more than five times lower fish grazing intensity compared to the reef-ball plots. Although macroalgae were effectively controlled at both reefs, the grazing intensity did not differ enough to cause large enough structural changes in benthic cover for higher coral recruitment, survival or growth. The high turf algae cover, combined with increasing crustose coralline algae and sponge cover likely explained reduced coral development. We recommend further research on how to achieve higher grazing rates for improved coral development on artificial reefs, for example by facilitating invertebrate herbivore. 

The 1983-1984 die-off of the long-spined sea urchin Diadema antillarum stands out as a catastrophic marine event because of its detrimental effectson Caribbean coral reefs. Without the grazing activities of this key herbivore, turf and macroalgae became the dominant benthic group, inhibiting coral recruitment and compromising coral reef recovery from other disturbances. In the decades that followed, recovery of D. antillarum populations was slow to non-existent. In late January 2022, a new mass mortality of D. antillarum was first observed in the U.S. Virgin Islands. We documented the spread and extent of this new die-off using an online survey. Infected individuals were closely monitored in the lab to record signs of illness, while a large population on Saba, Dutch Caribbean, was surveyed weekly before and during mortality to determine the lethality of this event. Within four months the die-off was distributed over 1,300 km from north to south and 2,500 km east to west. Whereas the 1983-1984 die-off advanced mostly with the currents, the 2022 event has appeared far more quickly in geographically distant areas. First die-off observations in each jurisdiction were often close to harbor areas, which, together with their rapid appearance, suggests that anthropogenic factors may have contributed to the spread of the causative agent. The signs of illness in sick D. antillarum were very similar to those recorded during the 1983-1984 die-off: lack of tube feet control, slow spine reaction followed by their loss, and necrosis of the epidermis were observed in both lab and wild urchins. Affected populations succumbed fast; within a month of the first signs of illness, a closely monitored population at Saba, Dutch Caribbean, had decreased from 4.05 individuals per m2 to 0.05 individuals per m2. Lethality can therefore be as high as 99%. The full extent of the 2022 D. antillarum die-off event is not currently known. The slower spread in the summer of 2022 might indicate that the die-off is coming to a (temporary) standstill. If this is the case, some populations will remain unaffected and potentially supply larvae to downstream areas and augment natural recovery processes. In addition, several D. antillarum rehabilitation approaches have been developed in the past decade and some are ready for large scale implementation. However, active conservation and restoration should not distract from the primary goal of identifying a cause and, if possible, implementing actions to decrease the likelihood of future D. antillarum die-off events.

This powerpoint walks through the recently published position paper entitled Nature based Solutions’: essential to safeguard the Dutch Caribbean against the consequences of climate change.

A B S T R A C T
The long spined sea urchin Diadema antillarum was an abundant grazer on Caribbean coral reefs, until
1983–1984, when densities were reduced by ~98% during a region wide die-off. Since then, there has been very
little natural recovery of the species and interest is growing in applying aquaculture as a tool for population
enhancement. In this study we optimized a new shaker bottle cultivation method for D. antillarum. The method
was tested in a series of experiments by culturing D. antillarum from egg to juvenile in the Netherlands as well as
the USA. Larvae were cultured in standard 1-L glass reagent bottles, suspended by gentle constant movement on
an orbital shaking table and fed with either the microalgae Rhodomonas lens or Rhodomonas salina. Effects on
larval growth and survival were evaluated for different microalgal feeding concentrations, larval densities, and
culture temperatures. Larval density and growth were measured twice a week over a period of up to 56 days.
Larvae grew significantly faster on a higher feeding concentration up to 90,000 Rhodomonas sp. cells mL

The massive die-off of the sea urchin Diadema antillarum in 1983–1984 is one the main reasons for low coral recruitment and little coral recovery in the Caribbean. As the natural recovery of D. antillarum is slow to non-existent, multiple restoration studies have been attempted. There are currently three different approaches to obtain individuals for restocking: the translocation of wild-collected juveniles or adults, lab-reared juveniles cultured from wild-collected settlers, or lab-reared juveniles cultured from gametes. All three methods are costly and can only be applied on a relatively small scale. We here propose a fourth, new, approach, which we term assisted natural recovery (ANR) of D. antillarum populations. ANR, a concept already applied in terrestrial restoration to restore forests and grasslands, can accelerate succession by removing barriers to natural recovery. In this study, performed on the Dutch Caribbean island of Saba, suitable settlement substrate was provided in the form of bio ball streamers that were attached to the reef shortly before the settlement season. At the end of the experiment, reefs with streamers had significantly higher D. antillarum recruit densities than control reefs without additional settlement substrate, indicating that the lack of settlement substrate is an important factor constraining natural recovery. However, D. antillarum recruit abundance was low compared to the measured settlement rates, possibly due to low post-settlement survival. The size distribution of recruits showed that recruits almost never became larger than 20 mm, which is likely due to predation. We conclude that, next to low settlement availability, low post-settlement survival and high predation on recruits also constrain the natural recovery of D. antillarum populations on Saba. To improve the survival of settlers till adults, we propose to 1) reduce predation on settlers by using bio balls or other substrates that can provide shelter to larger individuals and 2) optimize the reef habitat by removing macroalgae, either manually or by facilitating other herbivores. To improve the survival of recruits, we suggest to 1) choose sites with a known lower predation density or 2) protect recruits with a corral around the reef underneath the streamers. The combination of these measures could improve prospects for ANR, and we expect this new approach can contribute to the recovery of D. antillarum populations in the future.

On October 22st, 2021, the first ever recorded Diadema sea urchins in the Caribbean were cultured on Saba. Diadema sea urchins are important grazers and can facilitate corals by reducing their competition with algae. By culturing them, researchers from University of Applied Sciences Van Hall Larenstein set an important step in restocking sea urchins on Saba’s coral reefs.

Long-spined urchins (Diadema antillarum). Photo credit: Hans Leijnse

The long spined black sea urchin, also known as Diadema, has been absent on most coral reefs throughout the Caribbean, since 99% of all populations died to an unknow disease in the early 80s. Before the die-off these sea urchins were the major herbivore in the Caribbean, by scraping off and eating the seaweeds. After the mass mortality, Diadema sea urchin populations never really recovered and most reefs nowadays are dominated by macroalgae. Restoring Diadema populations is therefore seen as key priority in Caribbean coral reef management.

Culturing Diadema and releasing them in the wild would speed up the recovery of this keystone species. Unfortunately, the culture of Diadema is very hard, due to the sensitive nature of the larvae. It has been tried several times, especially in Florida, but despite some successes, a consistent method was never developed. In 2020 a new culture method was developed in The Netherlands by researchers from University of Applied Sciences Van Hall Larenstein, making it possible to consistently culture Diadema from tiny larvae to juvenile sea urchins.

In July of this year the research team moved their culture efforts from the cold and not so tropical Netherlands to the Dutch Caribbean island of Saba, in an attempt to culture juveniles near the reefs where they are much needed. With help of the Saba Conservation Foundation, the first larvae have successfully been cultured and settled, resulting in the first 19 Caribbean cultured Diadema sea urchin juveniles! The first 19 cultured Diadema are raised in captivity until they are big enough for release in the wild, where they can graze away the algae that are smothering the corals and prevent new corals from settling.

First 19 cultured Diadema sea urchins in the Caribbean. Squares are 1×1 mm. Source: Tom Wijers and Alwin Hylkema

The next step is to upscale cultivation, with currently over 3000 larvae being cultured. If this approach is proven to be effective on Saba, it can be copied throughout the Caribbean. By removing their most important competitors, Diadema sea urchins can help coral reefs to cope with other stressors like climate change and pollution.

More information: https://www.vhluniversity.com/research/research-projects/diadema

Or contact researchers Tom Wijers (tom.wijers@hvhl.nl) or Alwin Hylkema (alwin.hylkema@hvhl.nl)

https://www.dcbd.nl/document/new-diadema-project-aims-restore-sea-urchin...

Article published in BioNews 50

The massive die-off of the herbivorous sea urchin Diadema antillarum in 1983 and 1984 resulted in phase shifts on Caribbean coral reefs, where macroalgae replaced coral as the most dominant benthic group. Since then, D. antillarum recovery has been slow to non-existent on most reefs. Studying settlement rates can provide insight into the mechanisms constraining the recovery of D. antillarum, while efficient settlement collectors can be used to identify locations with high settlement rates and to collect settlers for restoration practices. The aim of this study was to compare pre and post die-off settlement rates and to determine possible settlement peaks in the Eastern Caribbean island of St. Eustatius. Additionally, we aimed to determine the effectiveness and reproducibility of five different settlement collectors for D. antillarum. D. antillarum settlement around St. Eustatius was highest in May, June and August and low during the rest of the study. Before the die-off, settlement recorded for Curaçao was high throughout the year and was characterized by multiple settlement peaks. Even though peak settlement rates in this study were in the same order of magnitude as in Curaçao before the die-off, overall yearly settlement rates around St. Eustatius were still lower. As no juvenile or adult D. antillarum were observed on the reefs around the settlement collectors, it is likely that other factors are hindering the recovery of the island's D. antillarum populations. Of all five materials tested, bio ball collectors were the most effective and reproducible method to monitor D. antillarum settlement. Panels yielded the least numbers of settlers, which can partly be explained by their position close to the seabed. Settler collection was higher in mid-water layers compared to close to the bottom and maximized when strings of bio balls were used instead of clumps. We recommend research into the feasibility of aiding D. antillarum recovery by providing suitable settlement substrate during the peak of the settlement season and adequate shelter to increase post-settlement survival of settlers. The bio ball collectors could serve as a suitable settlement substrate for this new approach of assisted natural recovery.

The native herpetofauna of the Lesser Antillean island of Saba (13 km2; 17.63°N, -63.24°W) includes one snake, Alsophis rufiventris, and four species of lizards, Anolis sabanus, Iguana melanoderma, Sphaerodactylus sabanus, and Thecadatylus rapicauda (Powell et al. 2015). Here, we report the establishment of both Gymnophthalmus underwoo-di Grant, 1958 and Indotyphlops braminus (Daudin, 1803) on the island.