Tom Wijers

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

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

At Van Hall Larenstein University of Applied Sciences (HVHL) in Leeuwarden, the sea urchin Diadema antillarum has been cultivated to help restore the coral reefs around Saba and St. Eustatius (Caribbean Netherlands). The first young urchins bred in Leeuwarden were released on March 24th to the Rotterdam Zoo (Diergaarde Blijdorp). The ultimate goal is to also breed this species on Saba in order to give the sea urchin populations there a helping hand. These sea urchins keep algae growth under control, giving corals more room to grow. During this project, researchers worked closely with students from the Coastal and Marine Management program.

Repopulation of sea urchins for reef conservation

Diadema antillarum sea urchins were the main grazers of Caribbean coral reefs until over 95% of sea urchins were killed by an unknown disease in 1983. Without sea urchins grazing, algae became the dominant group on the coral reef, outcompeting coral. Today, nearly 40 years after their mass death, sea urchins have still not recovered. HVHL is working with the RAAK PRO Diadema project (2019-2023) along with project partners for the restoration of this species on Saba and St. Eustatius (Caribbean Netherlands).

©Tom Wijers

Long awaited breeding method

For the past 40 years, researchers have been trying to breed Diadema in captivity, but unfortunately have only had limited success. Breeding as been found to be very difficult. Larvae of this type of sea urchin float along sea currents for the first 50 days of their life and are sensitive to water quality and nutrient availability. However, in 2020 researchers and students from HVHL in Leeuwarden managed to develop a method for stable and consistent breeding of young Diadema.

Rotterdam Zoo

It is difficult to transport these animals on a large scale to Saba or St. Eustatius, so the first group of young urchins will find a nice new home in the Rotterdam Zoo starting on March 24th. The next step will be to breed urchins on Saba so that they can be released into the wild, strengthening the populations and helping to restore the coral reef.

More Information

For more information about this project, visit the HVHL website. For a video about this project, please visit our YouTube channel. Follow the project on Facebook for updates.

Article published in BioNews 42