How will coral reefs look like in the future?

Adaptive mechanisms and sublethal effects in corals under climate change


How will coral reefs look like in the future?

Adaptive mechanisms and sublethal effects in corals under climate change

First experiment: Thermal stress effects on the survival and settlement of Mediterranean octocoral larvae

The current climate change causes a dramatic increase in the frequency and intensity of warmer sea surface temperature (SST). This increasing SST is the main driver behind coral mass mortality events worldwide. Warming can also have sublethal impacts during the vulnerable early stages of development in coral species. Coral sexual reproduction usually occurs during warmer months and under optimal conditions. Increased SST during the spawning period may approach the thermal threshold for corals and has been seen to impact larval settlement or survival in adult colonies, which may compromise the most vulnerable coral species. While some coral species will be able to persist as SST rises, others will begin to disappear, causing a phase-shift in species composition in tropical coral reefs or temperate coralligenous communities as climate change progresses.

To evaluate the effects of thermal stress on the survival and recruitment of coral larvae, the CoralChange project started preliminary experimental studies in late-June 2020. For this study, we selected two of the most emblematic species of the Mediterranean:

The red coral: Corallium rubrum (Linnaeus, 1758). This Mediterranean endemic species presents male and female colonies, and sexually reproduces in mid-July. C. rubrum is an internal brooder, meaning that eggs are fertilized within the polyps of the female colonies, where the larvae develop until being released.

The white gorgonian: Eunicella singularis (Esper, 1791). This is the only Mediterranean octocoral which houses symbiotic algae within its tissue. It presents male and female colonies, and sexually reproduces in early-July. E. singularis is an internal brooder, similar to C. rubrum.

What is the difference between octocorals and hexacorals?

Both belong to the class Anthozoa (ánthos: “flower”; zóa: “animals”) but:

  • Octocorals have polyps with 8 pinnate tentacles
  • Hexacorals have polyps with 6 (or multiples of 6) simple tentacles

As with hexacorals and sponges, octocorals are ecosystem engineers, forming three-dimensional frameworks which provide structural complexity and shelter for many species. These species are often called Marine Animal Forests because they mimic the complex infrastructure and biodiversity comparable to a terrestrial forest.

To obtain larvae from our two study species, different colonies were sampled by SCUBA diving on 25th June in the Natural Park of Cap de Creus (NW Mediterranean Sea) under permits issued by the Generalitat de Catalunya, Department of Territory and Sustainability. Sampled colonies were transported to the Experimental Field Service of the University of Barcelona (UB) and maintained under natural conditions until larval release.

Larval release of E. singularis started on July 3rd. The larvae appear bright pink and their size ranges between 2 – 2.5 mm. The larvae are lecithotrophic (non-feeding), so consequently their development until settlement is based solely on the maternal provisions transferred to the egg.

In C. rubrum, the larval release started on July 22nd. The C. rubrum larvae appear bright white and larval size ranges between 1 – 1.5 mm. Similar to E. singularis, the larvae of C. rubrum are lecithotrophic and they are extremely active swimmers.

Experimental set-up

In all research studies, experimental design is crucial to conducting a scientifically sound study. A control treatment and replication is always necessary when designing an experiment. To experimentally assess the effects of ocean warming on octocoral larval survival and recruitment, the collected larvae from both study species were maintained for 20 days under one of three temperature treatments:

  1. 20ºC (ambient temperature conditions, CONTROL)
  2. 24ºC (recent summer high temperatures, T-MEDNet)
  3. 26ºC (temperature predicts by 2100)

Temperature was maintained using one water bath for each experimental temperature. Each water bath contained a heater connected to an electric controller and submersible pumps to maintain circulation. HOBO dataloggers were placed in each water bath to record precise temperature every 15 minutes. This allowed for consistent and accurate temperatures during the experimental duration.

For each species (C. rubrum and E. singularis), 10 replicated glasses (250 ml) with 10 larvae in each, were maintained under each thermal treatment (n = 100 larvae for each treatment). Each glass also contained coralline algae (Litophyllum stictaeforme) with size ranging between 2.65 – 4.30 cm2 as suitable substrate for larval settlement.

To assess survival, larvae were counted daily and mortality was recorded. To assess recruitment, larval settlement was noted each day. To explore possible sublethal effects of thermal stress on larvae, ten different subset of larvae (10 larvae for each subset) from each temperature were collected after 5 days of exposure. These samples have been analysed to assess (1) larval size according to carbon organic content, and (2) their energetic content according to calorimetric analyses.

Preliminary results show different effects of thermal stress on survival and recruitment rates of larvae in both species. However, for a solid ecological interpretation, we need first apply various statistical models to assess the degree of impact caused by thermal stress on the larvae of these two important species of octocorals.


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