3.3 Bleaching susceptibility

One of the most devastating consequences of global warming is coral bleaching. Bleaching occurs when scleractinian corals, hydrocorals and octocorals lose their photosynthetic symbiotic algae or pigments [21, 111, 123–125], which leads to the white calcium carbonate skeleton being visible through the transparent host tissue. The frequency and severity with which coral bleaching occurs have increased in recent years [126]. Numerous investigations have demonstrated that coral bleaching events are a serious threat to coral reefs worldwide, where they have caused a severe deterioration in reef health (e.g. increase in coral disease, decrease in reef calcification and loss of habitat for related reef organisms [25, 123, 127–129]. The severity of coral bleaching depends on several factors, including specific coral species impacted [130], symbiotic algae assemblages [131] and thermal history [132].

substratum [51, 91]. Millepora spp. are also found in many environments and waves, water movement, light intensity and habitat depth were identified as key factors influencing their distribution and growth forms [51, 82, 91–93]. On barrier reefs, the amount of wave energy is highest on the reef crest, where wave breaking first occurs and subsequently attenuates towards fore reef and lagoonal environments (Figure 5) [94, 95]. This gradient in wave energy, combined with Millepora's sensitivity to wave-induced breakage, were showed to strongly influence colony and size distributions of M. cf. platyphylla at Moorea (French Polynesia), with highest densities recorded on the fore reef and larger colonies on nearshore reefs [91]. M. cf. platyphylla colonies occurred in a contagious pattern of distribution (i.e. colonies close to one another), as described for other Caribbean species [96], and colony breakage and subsequent fragment re-attachment were suggested as explanations for such colony aggregations [58]. Three Millepora species were also identified on the reefs of Reunion Island [97], where each species is distributed according to their proximity with the shore and reef crest, mostly related to the wave energy dispersal. M. cf. exaesa is the first species encountered close to shore on the shallow reef flat (2 m depth), replaced by M. tenera when approaching the reef crest, and M. cf. platyphylla colonies live from the crest to 35 m depth on the outer slope. Millepores are important reef framework builders, second only after scleractinian hard corals [51, 82]. Their complex structure is a habitat for other species adapted to stinging cells, including scavenger crustaceans (e.g. crabs, shrimps and barnacles, [51, 98–100]), as well as fish [38, 101–103], serpulids [104, 105], spionid polychaetes [51] and scleractinian corals [106]. Interestingly, high fire coral cover on Caribbean reefs was associated with increased fish richness species [86]. Many studies have described hydrocorals as opportunistic species that show rapid growth rates with high fecundity [51] and the ability for clonal propagation through fragmentation [58]. Fire corals are capable of colonizing both natural and artificial substrates, including dead gorgonians, rocks and ships [107, 108], as well as living seagrass stems, hydrocorals, gorgonians, scleractinian corals and other reef invertebrates (e.g. giant clams) through pursuit, contact and overgrowth

Invertebrates - Ecophysiology and Management

Wave energy dispersal on a barrier reef (modified from [94, 95]). The fore reef experiences strong wave action from incoming waves that break on the reef crest, near the upper slope, with a significant decrease in swell exposure towards deeper waters. The reef crest dissipates 70% of the incident swell wave energy with gradual

wave attenuation from the back reef to nearshore fringing reefs.

Figure 5.

22

Zooxanthellate hydrocorals are thought to be extremely sensitive to bleaching [130, 133] and can be threatened by future climate change. Millepora spp. have been reported to be among the first cnidarians to lose their zooxanthellae symbionts during widespread bleaching events [134] and they have suffered local or regional extinctions from bleaching in the Pacific [78, 85, 135]. Numerous investigations of bleaching events on Caribbean and Florida Keys reefs have reported bleaching of Millepora colonies [133, 136–139], with M. alcicornis, a finely branched species, being the most severely affected reef corals. Such coral morphology has been described to be more susceptible to bleaching than encrusting and massive species [140]. Yet, bleached colonies of M. alcicornis remained alive during a bleaching event affecting a north-eastern Brazilian reef [133], which is in accordance with previous reports that Millepora species are also the first to recover from short-term bleaching [136, 137]. In the Maldives Archipelago (Indian Ocean), Millepora was reported to be the major reef-building coral in shallow reefs (7 m depth), producing some 'Millepora zones' [141]. Three species were well documented, the massive species M. cf. platyphylla [46, 142] and two branching ones, M. tenera [51, 143, 144] and M. latifolia [143]. However, many recent surveys of the Maldivian reefs have identified another pattern of distribution, where none to low abundances of Millepora species were recorded (1–2 depending on the species) [145–148]. Gravier-Bonnet and Bourmaud [148] suggested that milleporids were extirpated from several Maldives atolls, following the 1997–1998 El-Nino Southern Oscillation event (ENSO). ENSO has induced a strong bleaching and massive coral mortality (of up to 90%) in the tropical Indian Ocean, including the Maldives [145, 149]. On the Great Barrier Reef, Millepora spp. were also the most susceptible taxa to the mass bleaching event of 1998, with 85% of mortality [130], while they showed no evidence of bleaching at Moorea, although scleractinian corals were severely bleached at this location [150]. During 2014–2017, the worst documented bleaching event observed [26, 27], M. cf. platyphylla showed no sign of bleaching at Moorea, although about 60% of scleractinian corals were bleached on the fore reefs (Figure 6A). Since February 2019, Moorea's reefs are suffering from another mass bleaching event, with colonies of M. cf. platyphylla showing sign of bleaching and mortality (Figure 6B). Differential susceptibilities to this bleaching event were also observed between M. cf. platyphylla colonies (Figure 6C). Ongoing surveys will help quantifying bleaching susceptibility and mortality among coral taxa and locations, as well as between fire coral growth forms and genotypes (Dubé et al. in prep). Nevertheless, a previous study has shown that temperature is the primary factor related to bleaching in M. alcicornis, but that synergism with exposure to solar radiation may play a key role in hydrocoral bleaching [151]. Also, multifocal bleaching in hydrocorals, consisting of numerous scattered bleached spots, has been first described as a syndrome caused by an infectious disease affecting several colonies of M. dichotoma in the Red Sea [152]. 16S rRNA gene sequencing showed that affected tissues match sequences of bacteria belonging to Alphaproteobacteria and Bacteroidetes members previously associated with various diseases in scleractinian corals [153]. Yet the mechanisms of multifocal bleaching, its aetiology and mode of transmission remain unknown. Nevertheless, many studies have addressed the aetiology and effects of bleaching in Anthozoan species, wherein changes in the expression of genes and proteins were observed, and particularly heat shock proteins and transcription factors [154–159]. A recent study demonstrated that bleached specimens of M. alcicornis in Mexican Caribbean undergo a moderate decrease in symbiont's density and photosynthetic pigments, in addition to differential expression of 17 key proteins, such as calmodulin, actin and collagen

often coupled with calcium homeostasis, exocytosis and cytoskeleton organization

Bleaching susceptibility of M. cf. platyphylla during massive bleaching events occurring on the fore reefs at Moorea Island (French Polynesia). (A) View of the fore reef at Moorea during the bleaching event of 2016, showing healthy colonies of M. cf. platyphylla and bleached colonies of scleractinian corals, mostly of the Pocillopora genus. M. cf. platyphylla was sensitive to the recent bleaching event of 2019 at Moorea, where colonies bleached and died (B) from the rise in temperature, while other colonies showed sign of resistance to bleaching on the same reef (C). Photographs are courtesy of Yannick Chancerelle (A) and Yann Lacube

Ecology, Biology and Genetics of Millepora Hydrocorals on Coral Reefs

DOI: http://dx.doi.org/10.5772/intechopen.89103

Coral reefs are also threatened by ocean acidification associated with the increasing CO2 partial pressure, which depresses net calcification of corals and hydrocorals [160, 161]. Physiological responses of reef organisms to ocean acidification are relatively well known [162, 163]. Examples include changes in gene expression consistent with metabolic suppression, increased oxidative stress, antioxidant system, apoptosis and symbiont loss [164, 165]. Yet little information on the effects of ocean acidification on the physiology of fire corals is available in the current literature. Luz and colleagues [166] demonstrated that the antioxidant defense system of M. alcicornis is capable of coping with acidic conditions for a short period of time, while long-term exposure induces oxidative stress with consequent oxidative damage to lipids and proteins, which could compromise hydrocoral health and influence negatively the zooxanthellae-coral symbiosis and ultimately lead to bleaching [167].

in Anthozoan species [139].

Figure 6.

(B and C).

25

#### Figure 6.

Zooxanthellate hydrocorals are thought to be extremely sensitive to bleaching [130, 133] and can be threatened by future climate change. Millepora spp. have been reported to be among the first cnidarians to lose their zooxanthellae symbionts during widespread bleaching events [134] and they have suffered local or regional extinctions from bleaching in the Pacific [78, 85, 135]. Numerous investigations of bleaching events on Caribbean and Florida Keys reefs have reported bleaching of Millepora colonies [133, 136–139], with M. alcicornis, a finely branched species, being the most severely affected reef corals. Such coral morphology has been described to be more susceptible to bleaching than encrusting and massive species [140]. Yet, bleached colonies of M. alcicornis remained alive during a bleaching event affecting a north-eastern Brazilian reef [133], which is in accordance with previous reports that Millepora species are also the first to recover from short-term bleaching [136, 137]. In the Maldives Archipelago (Indian Ocean), Millepora was reported to be the major reef-building coral in shallow reefs (7 m depth), producing some 'Millepora zones' [141]. Three species were well documented, the massive species M. cf. platyphylla [46, 142] and two branching ones, M. tenera [51, 143, 144] and M. latifolia [143]. However, many recent surveys of the Maldivian reefs have identified another pattern of distribution, where none to low abundances of Millepora species were recorded (1–2 depending on the species) [145–148]. Gravier-Bonnet and Bourmaud [148] suggested that milleporids were extirpated from several Maldives atolls, following the 1997–1998 El-Nino Southern Oscillation event (ENSO). ENSO has induced a strong bleaching and massive coral mortality (of up to 90%) in the tropical Indian Ocean, including the Maldives [145, 149]. On the Great

Invertebrates - Ecophysiology and Management

Barrier Reef, Millepora spp. were also the most susceptible taxa to the mass bleaching event of 1998, with 85% of mortality [130], while they showed no evidence of bleaching at Moorea, although scleractinian corals were severely bleached at this location [150]. During 2014–2017, the worst documented bleaching event observed [26, 27], M. cf. platyphylla showed no sign of bleaching at Moorea, although about 60% of scleractinian corals were bleached on the fore reefs

and Bacteroidetes members previously associated with various diseases in

24

scleractinian corals [153]. Yet the mechanisms of multifocal bleaching, its aetiology and mode of transmission remain unknown. Nevertheless, many studies have addressed the aetiology and effects of bleaching in Anthozoan species, wherein changes in the expression of genes and proteins were observed, and particularly heat shock proteins and transcription factors [154–159]. A recent study demonstrated that bleached specimens of M. alcicornis in Mexican Caribbean undergo a moderate decrease in symbiont's density and photosynthetic pigments, in addition to differential expression of 17 key proteins, such as calmodulin, actin and collagen

(Figure 6A). Since February 2019, Moorea's reefs are suffering from another mass bleaching event, with colonies of M. cf. platyphylla showing sign of bleaching and mortality (Figure 6B). Differential susceptibilities to this bleaching event were also observed between M. cf. platyphylla colonies (Figure 6C). Ongoing surveys will help quantifying bleaching susceptibility and mortality among coral taxa and locations, as well as between fire coral growth forms and genotypes (Dubé et al. in prep). Nevertheless, a previous study has shown that temperature is the primary factor related to bleaching in M. alcicornis, but that synergism with exposure to solar radiation may play a key role in hydrocoral bleaching [151]. Also, multifocal bleaching in hydrocorals, consisting of numerous scattered bleached spots, has been first described as a syndrome caused by an infectious disease affecting several colonies of M. dichotoma in the Red Sea [152]. 16S rRNA gene sequencing showed that affected tissues match sequences of bacteria belonging to Alphaproteobacteria

Bleaching susceptibility of M. cf. platyphylla during massive bleaching events occurring on the fore reefs at Moorea Island (French Polynesia). (A) View of the fore reef at Moorea during the bleaching event of 2016, showing healthy colonies of M. cf. platyphylla and bleached colonies of scleractinian corals, mostly of the Pocillopora genus. M. cf. platyphylla was sensitive to the recent bleaching event of 2019 at Moorea, where colonies bleached and died (B) from the rise in temperature, while other colonies showed sign of resistance to bleaching on the same reef (C). Photographs are courtesy of Yannick Chancerelle (A) and Yann Lacube (B and C).

often coupled with calcium homeostasis, exocytosis and cytoskeleton organization in Anthozoan species [139].

Coral reefs are also threatened by ocean acidification associated with the increasing CO2 partial pressure, which depresses net calcification of corals and hydrocorals [160, 161]. Physiological responses of reef organisms to ocean acidification are relatively well known [162, 163]. Examples include changes in gene expression consistent with metabolic suppression, increased oxidative stress, antioxidant system, apoptosis and symbiont loss [164, 165]. Yet little information on the effects of ocean acidification on the physiology of fire corals is available in the current literature. Luz and colleagues [166] demonstrated that the antioxidant defense system of M. alcicornis is capable of coping with acidic conditions for a short period of time, while long-term exposure induces oxidative stress with consequent oxidative damage to lipids and proteins, which could compromise hydrocoral health and influence negatively the zooxanthellae-coral symbiosis and ultimately lead to bleaching [167].
