RESEARCH IN MARINE SCIENCE

Healthy coral fragment growing on a Reefscapers coral frame (Sheraton Maldives)

Coral spawning patterns of Acropora
across two Maldivian reef ecosystems

Monfared MAA, Sheridan K, Dixon SP, Gledhill M, Le Berre T. 2023. Coral spawning patterns of Acropora across two Maldivian reef ecosystems. PeerJ 11:e16315 https://doi.org/10.7717/peerj.16315

Abstract

Understanding patterns in coral reproductive biology at local and regional scales is crucial to elucidate our knowledge of characteristics that regulate populations and communities. The lack of published data on coral spawning patterns in the Maldives hinders our understanding of coral reproductive biology and limits our ability to assess shifts in reproductive phenology over time.

Here we document baseline environmental cues, spawning patterns, exact timings and oocyte development of restored and wild Acropora, inhabiting shallow water reefs, across two Maldivian atolls. A total of 1,200 colonies were recorded spawning across the two sites between October 2021 and April 2023. These colonies represent 22 species of Acropora, with coral spawning observed over an extended period of eight months. This research details exact spawning times of multi-specific spawning, asynchronous spawning and ‘split spawning’ of Acropora, across multiple lunar phases; and highlights the need to consider restored colonies when discussing the sexual reproductive patterns of Maldivian Acropora in the future.

Overall, corals spawned earlier in North Male Atoll compared with Baa Atoll. Earlier spawning events were significantly correlated with lower tide depths, wind speeds, daily precipitation and higher sea surface temperatures which helped explain inter-atoll, inter-annual, and intra-annual variations in spawning day. This study contributes to understanding sexual reproductive cycles of Acropora in the Maldives; knowledge that is vital for effective management of a critically endangered ecosystem in a changing climate.

Stages of sexual reproduction in Acropora, Maldives (Reefscapers PEERJ paper 2023)
Stages of sexual reproduction in Acropora, Maldives.

(A–C) Development of oocyte colouration from white (immature; (A)), to pale (B), to pigmented (mature; (C)) in A. millepora. These classifications were used to inform field monitoring for spawning. Photographs by Margaux A.A. Monfared.
(D) Observed gamete ‘setting’ in A. secale prior to spawning. The presence of ‘setting’ as shown in this image is what observers were looking for during nightly surveys. Photograph by Kate Sheridan.
(E) Photograph to show moment of gamete bundle release in A. rosaria, defined as the spawning time per colony. Photograph by Simon P. Dixon.
(F) A colony of A. gemmifera spawning egg-sperm bundles. In situ observations suggested these oocytes were white-pale. Photograph by Kate Sheridan.

Conclusion

Our research details for the first-time exact spawning times of Acropora across two Maldivian atolls elucidating patterns of multi-specific spawning, asynchronous spawning and ‘split spawning’ across multiple lunar phases.

It is clear the Maldives experiences two distinct spawning seasons throughout the year, but spawning events can occur over an extended period of eight months.

Inter-atoll variations in spawning day are likely influenced by local environmental factors, however further research into coral reproductive patterns of multiple study sites within atolls and across the Maldives needs to be conducted to ascertain regional disparities and seasonal variations.

The preliminary findings of spawning patterns based on reef type in this study demonstrate the need to consider restored colonies when discussing the sexual reproductive patterns of Maldivian Acropora in the future.

Insights into Coral Spawning Patterns
on Maldivian Reef Ecosystems

Introduction

Coral reproduction is a fundamental process retaining coral reef functions and structure (PDF: Baird et al., 2015). Understanding coral reproduction timings is crucial to elucidate our knowledge on characteristics regulating populations and communities, particularly through recruitment rates and ecosystem connectivity (Kool, Moilanen & Treml, 2013; and Done, Gilmour & Fisher, 2015). This has important implications on coral reef management and conservation (Richmond, 1997).

Little remains documented about the extent of spawning synchronicity on the world’s seventh largest coral reef ecosystem, comprising 3.14 % of global reefs, here in the Maldives (Dhunya, Huang & Aslam, 2017).

This lack of publicised data on histological sampling to distinguish the onset of gametogenesis in-situ to amalgamate regional spawning patterns hinders our understanding of coral reproductive biology at a regional scale and limits our ability to assess shifts in reproductive phenology over time.

Coral reproductions - stages of broadcast spawning

Methodology & Results

We examined baseline environmental cues, spawning patterns, timings and gametogenic cycles of Acroporidae in naturally occurring wild colonies and asexually propagated restored colonies. We analysed shallow water reefs across Landaa Giraavaru (Baa Atoll), Kuda Huraa (Kaafu Atoll), and Sheraton Full Moon (Kaafu Atoll).

Differences in tide height across 4 days in November 2021, with time of spawning overlayed for 3 species of Acroporidae in Baa atoll.

Reefscapers corals spawning Maldives tides

The difference in day of species spawning is highly significant between atolls in Autumn (p<0.001) and significantly different in Spring (p<0.05). For both seasons Kaafu Atoll spawns before Baa Atoll. This is linked to significantly different environmental conditions at each latitude: low tide (p<0.001), wind speed (p<0.0) and tidal depth (p<0.001).

Reefscapers coral spawning Maldives Marine Savers results

The number of coral colonies per species spawning during Autumn 2021 in Kaafu and Baa Atolls.

Reefscapers coral spawning Maldives Marine Savers results

The number of coral colonies per species spawning during Spring 2022 in Kaafu and Baa Atolls.

Key Findings & Future Implications

    • 18 species of Acroporidae observed spawning and one species of Goniastrea
    • Two spawning cycles occur each year during spring and autumn
    • Multi-specific synchronous spawning and split spawning observed
    • Kaafu Atoll coral species typically spawn before Baa Atoll
    • Environmental cues identified that coincide with spawning windows
    • Species spawning variation across atolls and seasons

Coral bleaching decreases reproductive potential of survivors (Leinbach et al., 2021), reduces gamete numbers (Ward, Harrison & Hoegh-guldberg, 2002) and leads to a long-term impact on reproduction over multiple spawning periods (Levitan et al., 2014).
As the persistence of Maldivian reef ecosystems rely on natural recruitment through reproduction, it is imperative we understand reproductive rates to predict population recovery following disturbance. This information will help us to understand regional spawning patterns and shifts in reproductive phenology over time.

Researchers: Margaux Monfared, Simon Dixon, Amelie Carraut, Kate Sheridan, Matthew Gledhill, Thomas Le Berre & Alejandra Woolrich

Acknowledgements: Thanks to the whole Reefscapers team, and to our Resort colleagues at both Four Seasons and Sheraton for their support.

Presentation of our work at the Maldives Marine Symposium 2022
(arranged by the MMRI)

What is Coral Spawning?
Enjoy our short documentary ‘New Beginnings’

PUBLISHED RESEARCH PAPERS

Evaluation of sea turtle morbidity and mortality within the Indian Ocean from 12 years of data shows high prevalence of ghost net entanglement

Author(s): Katrina Himpson, Simon Dixon, Thomas Le Berre
PLOS ONE – Publication date: 09 August 2023 > https://doi.org/10.1371/journal.pone.0289167

Abstract

  • Anthropogenic activities can negatively affect sea turtle populations. Quantifying the effect of human actions on these threatened species can help guide management strategies to reduce adverse impacts.
  • Assessments require extensive effort and resources and as such have not been carried out in many areas of important sea turtle habitat, including the Republic of the Maldives (Maldives).
  • Here, we utilise 12 years of data (2010–2022) collected from marine turtle stranding and rehabilitation cases from across the Maldives to identify the key threats in this region.
  • Olive ridley turtles were found stranded or injured most frequently (75% of total cases), along with hawksbill (15%), and green (10%) turtles.
  • Anthropogenic factors were the primary cause of injury or stranding in 75% of cases with entanglement in ghost fishing gear being the most common (66% of all cases). Other causes of morbidity, such as from turtles being kept as pets (6%), boat strikes (<1%), bycatch (<1%), and poaching (<1%) were recorded less frequently.
  • Olive ridley turtles were more likely to have injuries associated with entanglement than other species and showed a peak in admissions during the northeast monsoon, in the period following the known arribada nesting season in nearby India.
  • Turtles admitted to rehabilitation following entanglement were released a mean of 70 days sooner and had 27% lower mortality rates than for other causes of admission.
  • This study highlights the high prevalence of ghost net entanglement of sea turtles within the Maldives. The topic of ghost fishing is of global importance and international cooperation is critical in tackling this growing issue.
Turtle rescue research paper map (journal pone)
Turtle rescue research paper graph (journal pone)

Identifying Key Factors for Coral Survival in Reef Restoration Projects Using Deep Learning

Author(s): Gaétan Morand, Simon Dixon, Thomas Le Berre.
Journal of Aquatic Conservation: Marine and Freshwater Ecosystems
Publication date: 07 September 2022 > https://doi.org/10.1002/aqc.3878
The raw data files (SQL & CSV) are made available by SEANOE Sea scientific open data edition

Abstract

  • Coral restoration has emerged globally as a form of life support for coral reefs, awaiting urgent mitigation of anthropogenic pressures. Yet its efficiency is difficult to assess, as sizeable transplantation programmes handle hundreds of thousands of fragments, with survival rates inherently time intensive to monitor. Owing to limited available data, the influence of most environmental and methodological factors is still unknown.
  • To address this issue, machine learning and computer vision were used to track individual colonies’ survival, in a world first. Fragments from several species of Acropora and Pocillopora were transplanted over 12 sites across two Maldivian atolls. These colonies grew on coral frames, placed between 1 and 30 m deep. Analysis of monitoring pictures provided health and growth data on 77,574 individual coral colonies to inform the influence of genus, depth, initial fragment size, and substrate on their survival.
  • Among 77,574 fragments, individual survival rate was 31% after 2 years (21% after 4 years), which is much lower than most reported results. Deeper placement was an important success factor for Acropora transplants, but not for Pocillopora. In both genera, smaller initial fragment size was key to increased survival rates. Pocillopora fragments survived better than Acropora fragments at shallow depths (≤7 m), regardless of initial fragment size. Deeper, both genera had similar survival rates, which were influenced by initial fragment size and depth with comparable importance. During the mid-2019 heat wave, previously transplanted Acropora fragments were 38% more likely to die than Pocillopora fragments.
  • Overall, the total volume of live coral steadily increased over time, by more than 3.7 × 106 cm3 per year, as the volume increase in surviving fragments more than compensated for the volume loss due to mortality. This finding supports the use of targeted coral restoration to accelerate reef recovery after mass bleaching events.

Coral Restoration Effectiveness: Multiregional Snapshots of the Long-Term Responses of Coral Assemblages to Restoration

Author(s): Margaux Y. Hein, Roger Beeden, Alastair Birtles, Naomi M. Gardiner, Thomas Le Berre, Jessica Levy, Nadine Marshall, Chad M. Scott, Lisa Terry, Bette L. Willis.
Publication date: 17 April 2020, Diversity 2020, 12(4), 153; https://doi.org/10.3390/d12040153

Abstract

Coral restoration is rapidly becoming a mainstream strategic reef management response to address dramatic declines in coral cover worldwide. Restoration success can be defined as enhanced reef functions leading to improved ecosystem services, with multiple benefits at socio-ecological scales. However, there is often a mismatch between the objectives of coral restoration programs and the metrics used to assess their effectiveness. In particular, the scales of ecological benefits currently assessed are typically limited in both time and space, often being limited to short-term monitoring of the growth and survival of transplanted corals.

In this paper, we explore reef-scale responses of coral assemblages to restoration practices applied in four well-established coral restoration programs. We found that hard coral cover and structural complexity were consistently greater at restored compared to unrestored (degraded) sites. However, patterns in coral diversity, coral recruitment, and coral health among restored, unrestored, and reference sites varied across locations, highlighting differences in methodologies among restoration programs. Altogether, differences in program objectives, methodologies, and the state of nearby coral communities were key drivers of variability in the responses of coral assemblages to restoration. The framework presented here provides guidance to improve qualitative and quantitative assessments of coral restoration efforts and can be applied to further understanding of the role of restoration within resilience-based reef management.

ONGOING RESEARCH

Reefscapers at Sheraton Maldives coral reef propagation

Reefscapers Diaries

During the global coral bleaching event of 2016, we started recording detailed observations of the natural reefs and our coral frames.
These monthly diaries document our efforts to mitigate the effects of coral bleaching, and increase biodiversity of marine species.

Reefscapers at Sheraton Maldives coral reef propagation

Coral Spawning in the Maldives

Read about our pioneering research, studying coral spawning events both on the wild reef and on our propagated coral frames.
We have also transferred coral gametes to our Lab, and successfully settled coral larvae to produce coral polyps (a first in the Maldives).

Reefscapers at Sheraton Maldives coral reef propagation

Maldivian Corals Database

What are the most commonly observed species of corals in the Maldives?
Which species of corals are recommended for coral propagation and reef regeneration?
How can we differentiate between the different species of corals in the Maldives?

Reefscapers at Sheraton Maldives coral reef propagation

Propagating Coral Microfragments

Scleractinian “massive” stony corals are essential to reef ecosystems, but are neglected by global restoration research (fewer than 5% of studies). Propagation tends to focus on a small group of faster-growing coral species for ease of proliferation and rapid results.

Reefscapers at Sheraton Maldives coral reef propagation

National Turtle ID Project

We invite citizen scientists from around the Maldives to submit turtle photos to us for inclusion in our national database.
The turtle facial scutes (scales) form a unique pattern (similar to fingerprints in humans) enabling identification of individuals.

Reefscapers at Sheraton Maldives coral reef propagation

Dolphin ID Database

A dolphin can be individually identified based on the notches, scars and markings present on their dorsal fin.
We take quality photos of the dolphins we encounter, then crop/optimise the image before analysing with matching software.

Reefscapers at Sheraton Maldives coral reef propagation

Sea Turtles & Environmental Enrichment

We provide stimulating environments to encourage curiosity in our sea turtle patients, to aid with rehabilitation and recovery.
We study turtle behaviours by introducing novel ‘pool toys’ and filming individual interactions over time, for analysis.

Reefscapers at Sheraton Maldives coral reef propagation

Zooplankton Monitoring

Zooplankton feeds the world’s largest animal, the blue whale, as well as Baa Atoll’s world-famous aggregations of manta rays.
Our continuous study collects baseline data on zooplankton communities around Baa atoll, with the hope to expand nationwide.

Reefscapers at Sheraton Maldives coral reef propagation

Marine Aquaculture in our Fish Lab

At our Marine Discovery Centre at Landaa Giraavaru, we have a unique fish and aquaculture science lab.
We study breeding techniques of various marine species, including anemonefish, shrimp, and jellyfish.