Sea urchin

Nederlands below:

Diadema sea-urchins play a vital role in maintaining a balanced coral reef ecosystem and their restoration is essential to assist recovery of the degraded coral reefs around Saba and St. Eustatius. A collaborative effort between University of Applied Sciences Van Hall Larenstein, WUR, STENAPA, CNSI and NIOZ studied settlement rates of sea-urchin larvae. The new findings provide insight into why the Diadema population has not been restored since the massive die-off in the mid 1980’s and are important for developing and implementing effective sea urchin restoration projects.

Diadema sea urcin settler (~0.5 mm) on a bio ball, one of the settlement substrates studied. Credit: Alwin Hylkema

Long-spined sea urchins (Diadema antillarum) play a critical role in maintaining healthy coral reefs. They help sustain the delicate balance within the reef by grazing on algae, which are the main competitors of corals. Unfortunately, in the mid-1980s, a disease swept through the Caribbean, wiping out nearly the entire sea urchin population. Soon after, reports of rapid algae growth were documented throughout the Caribbean. The algae occupy all available space, preventing coral recruitment and limiting the coral reef’s ability to recover from other disturbances. So far, recovery ofD. antillarum populations has been very slow to nonexistent. In the few areas where sea urchins were able to naturally recover, these trends were reversed, highlighting the importance of restoring this keystone species.

Studying sea-urchin settlement

A collaborative study between Van Hall Larenstein, Wageningen University and Research (WUR), STENAPA, Caribbean Netherlands Science Institute (CNSI) and NIOZ Royal Netherlands Institute for Sea Research worked to unravel the mystery behind the slow recovery rate of these vital species. As both juveniles and adults are absent from most reefs, it is likely that the bottle-neck in D. antillarum recovery occurs in the first phase of the life cycle. Sea urchins start their life as larvae and remain the water column for 30-60 days, until they find a suitable place to settle. Understanding settlement rates and patterns will help understand the slow recovery and guide recovery efforts moving forward.

Effectiveness of settlement collectors

Diadema sea urchin juvenile on a settlement panel, one of the studied settlement collectors. Credit: Alwin Hylkema

Settlement collectors were set up in five different locations around the island of St. Eustatius. Between April and December 2019, researchers carefully monitored and analyzed the settlement rates by counting each individual sea urchin larvae which settled on the collectors. Over these 10 months, more than 890 sea urchin settlers were collected. Furthermore, this study compared different materials to identify the most suitable settlement collector for Diadema sea urchins. These materials included panels, artificial turf, bio balls, frayed ropes and a doormat collector. Results showed both the bio ball and doormat collectors were most effective, especially when placed mid-water column.

The mystery behind the slow recovery

Despite high settlement densities on several locations, no juvenile or adult sea urchins were observed on the reefs around the settlement collectors. This hints that there are likely other reasons these populations have been unable to recover around the island. Furthermore, previously conducted reef surveys from 2017 through 2019 found less than 0.01 sea urchins per square meter, far below pre-die-off densities. The fact that this study proves there are sea urchin larvae in the water paired with the lack of juveniles on reefs could be an indication of high predation pressures on settlers on the reef.

Future Populations

One solution which may aid in the restoration of this species would be to collect settlers from the reef and raise them in land-based nurseries. This would protect them from predation early in life and allow them to grow to adequate size before being returned to the reef. In order for this to be economically feasible, high numbers of settlers would need to be collected. Studies such as this can help the islands design and implement effective methods to restore Diadema sea urchins, which can help coral reefs to recover from disturbances naturally.

To learn more, you can read the full article on the DCBD by using the link below.

See also the recent item in Atlas (NPO2) in Dutch- starts at 22 min:

https://www.npostart.nl/atlas/02-02-2022/VPWON_1335373

More info in the Dutch Caribbean Biodiversity Database

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Belangrijke nieuwe inzichten voor het herstel van zee-egels

Diadema-zee-egels spelen een belangrijke rol binnen een gezond koraalrifecosysteem en hun herstel is essentieel voor het herstel van de aangetaste koraalriffen rondom Saba en St. Eustatius. Hogeschool Van Hall Larenstein, WUR, STENAPA, CNSI en NIOZ onderzochten de vestigingssnelheid van zee-egellarven. De nieuwe bevindingen geven inzicht in waarom de Diadema-populatie niet is hersteld sinds de massale sterfte in het midden van de jaren tachtig en zijn belangrijk voor het ontwikkelen en uitvoeren van effectieve herstelprojecten voor zee-egels.

De zee-egel (Diadema antillarum) speelt een cruciale rol bij het in stand houden van gezonde koraalriffen. Ze helpen het evenwicht in het rif te behouden door te grazen op algen, de belangrijkste concurrenten van koralen. Helaas raasde halverwege de jaren tachtig een ziekte door het Caribisch gebied, waarbij bijna de hele zee-egelpopulatie werd uitgeroeid. Kort daarna werd in het hele Caribisch gebied toename in algengroei waargenomen. De algen nemen alle beschikbare ruimte in beslag, waardoor nieuwe koralen zich moeilijk kunnen vestigen en het vermogen van het koraalrif om te herstellen van andere verstoringen wordt beperkt. Tot op heden is het herstel van D. antillarum-populaties erg traag tot niet-bestaand. In de weinige gebieden waar zee-egels zich op natuurlijke wijze wel konden herstellen, werden deze trends omgekeerd, wat het belang van het herstel van deze soort benadrukt.

Bestuderen van zee-egels

Diadema zee-egel (~0,5 mm) gevestigd op een biobal, een van de onderzochte vestigingssubstraten. Credit: A. Hylkema

Een gezamenlijke studie tussen Van Hall Larenstein, Wageningen University and Research (WUR), STENAPA, Caribisch Nederlands Instituut voor Wetenschappelijk Onderzoek (CNSI) en NIOZ Koninklijk Nederlands Instituut voor Onderzoek der Zee heeft het mysterie achter het trage herstel van deze vitale soorten ontrafeld. Aangezien zowel juvenielen als volwassenen afwezig zijn op de meeste riffen, is het waarschijnlijk dat het knelpunt bij het herstel van D. antillarum optreedt in de eerste fase van de levenscyclus. Zee-egels beginnen hun leven als larve en blijven 30-60 dagen in de waterkolom, totdat ze een geschikte plek hebben gevonden om zich te vestigen. Het bestuderen van deze eerste levensfase zal helpen bij inzicht te krijgen in het langzame herstel en helpen bij herstel projecten.

Verzamelen van zee-egellarven

Diadema zee-egel juveniel op een vestigingspaneel, een van de bestudeerde vestigingsverzamelaars. Credit A. Hylkema

Op vijf verschillende locaties rond het eiland Sint Eustatius werden verzamelaars geplaatst waar zee-egellarven zich op kunnen vestigen. Tussen april en december 2019 hebben onderzoekers de vestigingspercentages nauwkeurig gevolgd en geanalyseerd door elke individuele zee-egellarven te tellen die zich op de verzamelaars vestigden. In deze 10 maanden werden meer dan 890 zee-egelkolonisten verzameld. Bovendien vergeleek deze studie verschillende materialen om de meest geschikte verzamelaar voor Diadema-zee-egels te identificeren. Deze materialen waren onder andere panelen, kunstgras, bioballen, gerafelde touwen en deurmatten. De resultaten toonden aan dat zowel de biobal ​​als de deurmatverzamelaars het meest effectief waren, vooral wanneer ze midden in de waterkolom werden geplaatst.

Het mysterie achter het langzame herstel

Ondanks de hoge vestigingsdichtheid op verschillende locaties, werden er geen juveniele of volwassen zee-egels waargenomen op de riffen rond de verzamelaars. Dit wijst erop dat er waarschijnlijk andere redenen zijn waarom deze populaties zich rond het eiland niet hebben kunnen herstellen. Bovendien vonden onderzoekers bij eerder uitgevoerde rifonderzoeken in 2017 -2019 minder dan 0,01 zee-egels per vierkante meter, ver beneden de dichtheden voor de massale sterfte. Het feit dat deze studie aantoont dat er zee-egellarven in het water zijn in combinatie met het gebrek aan juvenielen op de riffen, kan een indicatie zijn van hoge predatiedruk op de zee-egels op het rif.

Toekomstige zee-egel populaties

Een oplossing die kan zou kunnen helpen bij het herstel van deze soort zou zijn om gevestigde jonge zee-egels van het rif te verzamelen en ze op te kweken in aquaria op het land. Dit zou hen op jonge leeftijd tegen predatie beschermen en hen in staat stellen om voldoende groot te worden voordat ze naar het rif worden teruggebracht. Om dit economisch haalbaar te maken, zouden grote aantallen jonge zee-egels moeten worden verzameld. Studies zoals deze kunnen de eilanden helpen bij het ontwerpen en implementeren van effectieve methoden om Diadema-zee-egels te herstellen, wat koraalriffen kan helpen om op natuurlijke wijze te herstellen van verstoringen.

Lees meer over over deze studie in Dutch Caribbean Biodiversity Database

Of zie het recente item in Atlas (NPO2) – start bij 22 min:

https://www.npostart.nl/atlas/02-02-2022/VPWON_1335373

More info in the Dutch Caribbean Biodiversity Database

Published in BioNews 51

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 sea urchin Diadema antillarum is an important herbivore in coral reef ecosystems of the Caribbean. The Caribbean is still recovering from a 1983 mass mortality event that decimated D. antillarum throughout the Caribbean, reducing populations to <3% of former levels (Lessios et al. 2001). By feeding on algae, D. antillarum helps prevent excessive algal growth and is beneficial to juvenile corals (Edmunds and Carpenter 2001). The urchin die off may have precipitated a phase shift in coral reef communities in the Caribbean from coral to algal dominated ecosystems. However, at high densities D. antillarum can have a negative effect, reducing the number of coral recruits (Sammarco 1980; Mumby et al. 2006)

In light of the potential of ecosystem level consequences of D. antillarum population; the purpose of this study was to document the densities in Bonaire to determine if they are over or underpopulated. The range of D. antillarum expected in an area at equilibrium was found to be equal to or greater than 1 m-2 and less than 16 m-2 (Mumby et al. 2006). Using transects, the densities of D. antillarum were counted on the leeward side of Bonaire, Netherlands Antilles at 20 sites. Urchins were found to be extremely patchy and underpopulated (< 1 urchin m-2) for all but one site. Karpata’s population of D. antillarum was within the normal parameters (1< x < 16 urchins m-2). No sites sampled had densities that would indicate overpopulation of D. antillarum (> 16 urchins m-2). 

This student research was retrieved from Physis: Journal of Marine Science I (Fall 2006)19: 38-44 from CIEE Bonaire.

Recent studies have indicated that levels of solar ultraviolet radiation (UVR) are increasing globally as ozone is depleted. Ultraviolet radiation often has negative effects on organism survival; in sea urchins, UVR has been found to reduce sperm motility (reducing fertilization success) and to increase the occurrence of abnormal embryonic development (decreasing embryonic survival). Several species of sea urchins cover themselves with rubble; this study examined how this behavior was affected by UVR in situ. Three types of treatment boxes were placed over Tripneustes ventricosus specimens: a control box, a UVR-blocking box, and an opaque box. Specimens were photographed before and after treatment and percent change in rubble cover was calculated for each individual. The mean change in percent rubble cover presented significant reduction under opaque conditions relative to other treatment conditions (p = 0.005, p = 0.010). There was no significant difference between the latter two treatment groups (p = 0.980). The data suggest that covering behavior in T. ventricosus is not a response specific to UVR, but is a response to light. Further studies of how UVR affects T. ventricosus and as well as how other species cover themselves in response to sunlight and UVR are needed to understand the benefits of covering behavior in elevated UVR climate conditions. Further study on the costs and benefits of covering behavior as well as studies on the covering and sheltering behaviors of other sea urchins is needed to gain a full understanding of how increasing UVR will affect sea urchins.

This student research was retrieved from Physis: Journal of Marine Science XVIII (Fall 2015)19: 27-33 from CIEE Bonaire.

In the microbial loop, heterotrophic bacteria utilize dissolved organic matter (DOM) as an energy source. DOM becomes remineralized into inorganic material and nutrients available for primary production. As the amount of nutrients increase, the abundance of each trophic level increases, which is known as the bottom-up effect. This study investigates the effect of the increasing human population density on an intertidal community along the waterfront of Kralendijk, Bonaire. DOM, fecal indicator bacteria (enterococci, Escherichia coli, and coliform bacteria), nutrients (nitrate, phosphate, and ammonia), primary producers (percent cover of macroalgae), and herbivorous consumers (density of sea urchins) were sampled. There was no pattern between the variables and the increase of the adjacent population density. Factors such as rainfall, changes over time, tides, and herbivore grazing may have influenced the results. When graphed over time, rainfall impacted the concentrations of nutrients and fecal indicators. Nitrate, ammonia, and coliform bacteria increased, while phosphate, enterococci, and E. coli decreased. Concentrations of ammonia were found to exceed the threshold for a healthy coral reef ecosystem (6x). No correlation was found between DOM and heterotrophic bacteria, although concentrations of E. coli and nutrients were high at one site. This intertidal ecosystem does not appear to be influenced by bottom-up controls, as there was neither a correlation found between the percent cover macroalgae and nutrients or the density of sea urchins. The site with the highest percent cover of macroalage had the lowest density of urchins and vice versa.

This student research was retrieved from Physis: Journal of Marine Science XIX (Spring 2016)19: 91-99 from CIEE Bonaire.

Bonaire’s coral reefs remain among the healthiest in the Caribbean. Although the island’s reefs have suffered bleaching disturbances similar to those plaguing reefs throughout the Caribbean, they uniquely show signs of recovery. Here we highlight key findings from our March 2015 biennial coral reef monitoring expedition. We put the findings in the context of both the trends recorded since 2003 when we began our regular monitoring and the most recent research related to the factors controlling the structure and functioning of healthy coral reef ecosystems. 

The sea urchin Diadema antillarum commonly occurs on Caribbean reefs in densities sufficiently high to influence characteristics such as community composition and reef growth. We observed an outbreak of mass mortality in this species reducing population densities by 98 to 100 %. Mortality spread from the Curacao harbour mouth along the coast, most rapidly advancing in the down-current direction. Our calculations show a pronounced effect on the carbonate budget of the reef. Recovery of Diadema populations may be facilitated by parthenogenesis. There is continuing recruitment on affected reefs, a possible location of the parent population being up-current unaffected reefs of Bonaire.

Abstract:

Recruitment of the sea urchin Diadema antillarum philippi, 1845 was studied on artificial recruitment panels along the leeward coast of the island of Curaçao, southern Caribbean. data were compared with historical data from the same coast that were collected before (1982–1983) and after (1984) the Caribbean-wide mass mortality of Diadema in October 1983. Average recruitment rates observed in 2005 were equal to 2.2 times lower compared to those observed before the D. antillarum die-off (1982 and 1983), but 56.5 times higher than those observed after the die-off in 1984. The increase in recruitment rates between 1984 and 2005 was 5–51 times greater than the increase in abundance of adult individuals over the same period. This suggests that despite the largely recovered recruitment rates of this important reef herbivore, unknown sources of high post-settlement mortality currently prevent a similar recovery of its adult population.