Introduce larvae directly onto natural or artificial reefs to encourage settlement

Overall effectiveness category Awaiting assessment
Number of studies: 3
Share Tweet Email

View assessment score

How is the evidence assessed?

Effectiveness

not assessed
Certainty

not assessed
Harms

not assessed

Calculating overall effectiveness category

Background information and definitions

Introducing larvae directly onto natural or artificial reefs can be used with the aim of enhancing the natural settlement process to restore coral species on a reef. Wild coral spawning events often result in ‘slicks’ containing billions of egg/sperm bundles (Heyward et al. 2002). These are collected and allowed to transform into larvae in a nursery (either in-situ or ex-situ) before being released directly onto the natural or artificial reef (Heyward et al. 2002; Edwards et al. 2015). The release process usually involves covering an area of reef to prevent the larvae dispersing then introducing larvae through a hose or syringe into the covered area. The reef remains covered for approximately 24 hours to enable the larvae to settle onto the substrate.

This action describes introducing larvae onto a reef and, once settled, allowing the larvae to develop without further intervention. Studies describing cultivating corals in in-situ or ex-situ nurseries are covered in Cultivate coral larvae in an artificial nursery located in a natural habitat; and Cultivate corals in an ex-situ nursery. Studies describing transplanting nursery-grown coral onto natural or artificial substrates or wild-grown corals onto natural or artificial substrates are covered in Transplant nursery-grown corals onto natural substrate; Transplant wild-grown corals onto natural substrate; Transplant wild-grown coral onto natural substrate; and Transplant wild-grown coral onto artificial substrate.

^{Edwards A.J., Guest J.R., Heyward A.J., Villanueva R.D., Baria M.V., Bollozos I.S.F. & Golbuu Y. (2015) Direct seeding of mass-cultured coral larvae is not an effective option for reef rehabilitation. Marine Ecology Progress Series, 525, 105-116. https://doi.org/10.3354/meps11171}

^{Heyward A.J., Smith L.D., Rees M. & Field S.N. (2002). Enhancement of coral recruitment by in situ mass culture of coral larvae. Marine Ecology Progress Series, 230, 113–118. http://www.jstor.org/stable/24865098}

Study locations

Key messages

Three studies evaluated the effects of releasing larvae directly onto natural or artificial reefs to encourage settlement. One study was in each of Australia, the USA, and Palau.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (3 STUDIES)

Abundance (3 studies): Two of three replicated studies (including one controlled) in Australia, the USA, and Palau, found that settlement density was higher when larvae were released directly onto tiles on the reef compared to naturally settling on tiles or the reefnearby, whereas another study found no difference between settlement rate of larvae released directly onto the different natural substrates. One of the studies found that settlement rate was higher when larvae were released onto the reef at a slower rate.
Survival (1 studies): One replicated study in the USA found no difference in survival for larvae released directly onto different natural substrates. found that 44% of stony coral spat (settled larvae) that were released directly onto the reef survived.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

A replicated, controlled study in 1997 at an in-situ nursery and natural coral reef site in Coral Bay, Australia (Heyward et al. 2002), reported that reefs re-seeded using stony coral larvae cultivated in an ex-situ nursery then pumped directly onto artificial settlement tiles on the reef had a higher settlement rate than larvae naturally settling on tiles placed on a nearby reef. In addition, pumping larvae over a longer time led to a higher settlement rate than larvae pumped onto the tiles quickly. Six weeks after re-seeding, there was an average of 236 coral spat (settled larvae)/tile (range 80–384) in areas where direct re-seeding took place over 12 hours, 23/tile (range 0.17–68) in areas re-seeded for 20 minutes, and 0.27/tile in areas with natural settlement. Data were not statistically tested. In March 1997, approximately four million egg/sperm bundles (gametes) were collected from a stony coral (Acroporid family) spawning slick on the ocean surface and transferred to four in-situ settlement ponds (~one million/pond) and cultured for seven days. Six terracotta settlement tiles (110 × 110 × 10 mm) were attached to the substrate at each of four sites around Coral Bay using a 10 cm roofing nail and PVC spacer pipe (4 cm diameter, 2 cm long). A mesh (1.8 × 1.0 m, 200 µ) was placed over the tiles at two sites and cultured larvae was pumped from the pond into one meshed area for 20 minutes and another for 12 hours using a hose. The other two sites were left to be naturally settled. All tiles were retrieved after six weeks and the number of coral spat on each counted.
Study and other actions tested
Referenced paper
Heyward A. J., Smith L. D., Rees M. & Field S. N. (2002) Enhancement of coral recruitment by in situ mass culture of coral larvae. Marine Ecology Progress Series, 230, 113-118.
A replicated study in 2006 at a coral reef in Biscayne National Park, Florida, USA (Cooper et al. 2014) found that after larvae were released directly onto three natural substrates on a reef, there was no difference in settlement rate or survival of brooding coral Porites asteroids spat (settled larvae) growing on the different substrates. Two to three days after release, there was no difference in the proportion of released larvae that settled on different substrate types (6.1–8.9 % settled on coralline crustose algae; 4.7–8.8 % on turf algae, and 7.9–10.1 % on a mixture of coralline crustose algae, turf algae and bare substrate). Overall survival after five months was <1% and there was no difference in survival rate between spat on coralline crustose, turf, or mixed substrates (data reported as interval survival rate). In April and May 2006, larvae were collected from 20 wild-grown adult Porites asteroides colonies (see paper for methods). Ten seeding pods (see paper for design) were attached to individual 10 × 10 cm plots on each of three substrates (crustose coralline algae, turf algae, and mixed crustose coralline algae, turf algae and bare substrate), 4 m deep. Coral larvae were injected into each seeding pod (approximately 100/pod in April, 75/pod in May). Once pods were removed after two (April) or three (May) days, coral spat settlement patterns were mapped and survival recorded using a fluorescence technique (see original paper). Monitoring was carried out at irregularly spaced intervals for five months.
Study and other actions tested
Referenced paper
Cooper W.T., Lirman D., VanGroningen M.P., Parkinson J.E., Herlan J. & McManus J.W. (2014) Assessing techniques to enhance early post-settlement survival of corals in situ for reef restoration. Bulletin of Marine Science, 90, 651-664.
A replicated, study in 2007–2008 at Iou Lukes reef, Palau (Edwards et al. 2015), found that using nursery-cultivated stony coral Acropora digitata larvae to directly ‘seed’ artificial reef structures initially led to a higher density of Acropora spp. coral spat (settled larvae) on the structures compared to natural settlement, but there was no difference in density of coral spat over time. Average coral spat density after five weeks was significantly higher on settlement tiles seeded with larvae (205/0.1m²) than unseeded tiles (52). However, after 30 weeks, stony coral density on seeded tiles had declined significantly (60/0.1m²), and there was no longer a statistically significant difference compared to unseeded tiles (33/0.1m²). In January 2007, fourteen concrete/limestone ‘pallet-balls’ (1.2 × 0.9 m) were placed 3–5 m apart, 5–8 m deep on the seafloor adjacent to a natural reef. Fibre cement settlement tiles (10 × 10 × 0.6 cm) were attached to each ball in mid-January 2008 (4 tiles/ball). In April 2008, a tent with an inner 250 × 250 µm mesh was placed over each of seven randomly selected pallet balls, and 40,000–260,000 nursery-cultured stony coral larvae were poured onto each pallet-ball (density 54.6–459.8/0.1m²). Tents remained for 24 hours. Coral density was recorded on tiles retrieved five and 30 weeks after wild-growing coral colonies had spawned.
Study and other actions tested
Referenced paper
Edwards A., Guest J.R., Heyward A., Villanueva R., Baria M., Bollozos I. & Golbuu Y. (2015) Direct seeding of mass-cultured coral larvae is not an effective option for reef rehabilitation. Marine Ecology Progress Series, 525, 105-116.

Please cite as:

Thornton A., Morgan, W.H., Bladon E.K., Smith R.K. & Sutherland W.J. (2024) Coral Conservation: Global evidence for the effects of actions. Conservation Evidence Series Synopsis. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

This Action forms part of the Action Synopsis:

Coral Conservation

Related Actions

Actions	Effectiveness	Studies	Category
Play reef ‘soundscapes’ to enhance settlement of coral larvae Action Link	No evidence found (no assessment)	0	Synopsis Link
Use electrical current to stimulate coral growth Action Link	Awaiting assessment	4	Synopsis Link
Introduce larvae directly onto natural or artificial reefs to encourage settlement Action Link	Awaiting assessment	3	Synopsis Link