3.1 Distribution, abundance and ecological roles

Fire corals occur worldwide in tropical seas and are limited in distribution from the intertidal zone to depths of approximately 50 m [51, 82, 83]. Although fire corals can be abundant locally [84–86] and dominate shallow water communities in some coral reef ecosystems [87–90], they usually cover less than 10% of the overall reef

The advent of DNA barcoding greatly helped delimiting species of many marine invertebrates [71–74]. Consequently, the more recent works on Millepora spp. used a combination of morpho and molecular characterization. Mitochondrial sequence data were successfully used to delineate milleporid species from the Caribbean, revealing two genetic entities: M. squarrosa Lamarck 1816 and a species complex composed of M. alcicornis Linnaeus 1758–M. complanata Lamarck 1816 [67]. Similarly, the four Millepora species from the Brazilian province were discriminated using the 16S mitochondrial gene coupled with morphological characters [68]. Recently, a study on milleporids from the Red Sea successfully distinguished three species M. platyphylla, M. dichotoma and M. exaesa Forskal 1775, using both morphological and molecular characterization [57]. Similarly, Boissin and colleagues (submitted) successfully used 16S sequences to delineate the three Millepora species

Fire corals are found in tropical/subtropical regions around the globe, nearly ubiquitous on reefs in the Atlantic, Indian and Pacific Oceans (Figure 4). Currently, 10 species are considered valid in the Indo-Pacific and 6 in the Atlantic Ocean [57, 61, 65, 68, 75]. The species status of two other Indo-Pacific species, M. nodulosa Nemenzo 1984 and M. latifolia Boschma 1948, are still unclear [65]. Several Indo-Pacific species show an extensive geographic distribution from west of the Indian Ocean to west (M. dichotoma, M. tenera), centre (M. platyphylla) or east of the Pacific Ocean (M. exaesa, M. intricata), while M. foveolata Crossland 1952 and M. boschmai de Weerdt and Glynn 1991 have restricted distributions (Philippines and Indonesia, respectively, Figure 4). In the Atlantic, two species are endemic to the Caribbean province (M. complanata, M. squarrosa) and three species are endemic to the Brazilian province (M. braziliensis Verrill 1868, M. nitida Verrill 1868, M. laboreli Amaral 2008), while M. alcicornis is present in both provinces as well as in the Canary Islands, Cape Verde and Ascension Island (Figure 4) [46, 76]. However, with recent morpho-molecular re-evaluations of species boundaries in this group, our understanding of the biogeographic patterns is still evolving. The recent highlight of cryptic species between the Red Sea and the rest of the Indo-Pacific provinces [57] pointed out that M. platyphylla, M. dichotoma and M. exaesa in the Indo-Pacific need taxonomic re-description. The number of Indo-Pacific species was thus raised from 7 to 10 in the last few months. This number is likely to grow in future years, as M. cf. exaesa for instance includes several lineages over its Indo-Pacific range and likely represents another case of species complex (Boissin

Additionally, the range of M. platyphylla (now M. cf. platyphylla) was recently extended back to the eastern Pacific [77] from where it was documented as extirpated decades ago [78]. In the Atlantic, M. alcicornis has recently established in the Canary Islands (Macaronesia), far north of its tropical distribution [79], possibly by means of drifting material from the Caribbean Sea or transportation through ballast waters of large vessels and fouling of hulls [79–81]. Long-distance dispersals in milleporids have also been demonstrated in the Pacific, with Millepora colonies recorded on drifting pumice [80]. This alternative mode of dispersal can explain such a wide geographic distribution for a species with a short pelagic stage (see Section 5.3). However, as noticed by Lewis [51], it is surely remarkable that a family of worldwide distribution, with a long geological history and apparent ecological

from Reunion Island.

Invertebrates - Ecophysiology and Management

2.3 Biogeography

et al., unpublished).

20

success, is represented by less than 20 species.

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.

(Figure 3) [59, 60]. This ability to inhabit different substrates and its rapid colonization rates [79] provide a competitive advantage for potential habitat expansions. Although fire corals compete with other corals, they also contribute to coral survival during Acanthaster outbreaks [106], highlighting their key ecological role in reef resilience. In fact, the corallivorous predator Acanthaster planci tends to avoid Millepora species [109], thus providing predator-free sanctuaries to nearby

Ecology, Biology and Genetics of Millepora Hydrocorals on Coral Reefs

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

3.2 Endosymbiosis with photosynthetic dinoflagellates (Symbiodiniaceae)

Many members of the phylum Cnidaria, including corals, octocorals, sea anemones and hydrocorals, host unicellular dinoflagellate endosymbionts (i.e. zooxanthellae) belonging to the family Symbiodiniaceae [110]. These associations are often obligatory and of fundamental importance to coral reef ecosystems as they enhance the growth of calcifying corals that form the reef. For instance, the zooxanthellae contribute to host nutrition (up to 95% of the energy requirements in scleractinian corals [111]) and skeletogenesis by providing photosynthetically fixed carbon, while the cnidarian host provides inorganic nutrients and refuge from herbivory to its symbionts [112–114]. Previous studies have demonstrated that the association of Cnidaria–Symbiodiniaceae is not stochastic, but mostly determined by host phylogeny and geography [115, 116]. Like scleractinian corals, hydrocorals feed heterotrophically on a variety of resources (mostly planktonic feeders [51, 117]) and rely on a mutualistic symbiosis with Symbiodiniaceae algae for autotrophic nutrition and calcification [118, 119]. While coral-Symbiodiniaceae associations have been extensively studied over the last decades (reviewed in [120]), only two studies have recently investigated hydrocoral-Symbiodiniaceae associations on Caribbean reefs [121, 122]. Rodriguez and colleagues [122] showed that Symbiodiniaceae species that associate with M. alcicornis vary as a function of its geography, with

Symbiodinium sp. (formerly clade A) found in samples from Mexico and Breviolum sp. (formerly clade B) in the eastern Atlantic, with the exception of samples from the Canary Island and Cape Verde Islands that comprised Cladocopium sp. (formerly clade C). Unpublished data collected across M. cf. platyphylla Indo-Pacific range showed that this species can associate with the genera Symbiodinium (dominant symbiont), Cladocopium and more rarely with Brevolium in French Polynesia, Papua New Guinea and the south-western Indian Ocean (Dubé et al. in prep.; Boissin et al. in prep.). The other Indo-Pacific species (M. cf. dichotoma and M. cf. exaesa) investigated so far show the same Symbiodiniaceae associations (Boissin

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

scleractinian corals.

et al. in prep.).

3.3 Bleaching susceptibility

thermal history [132].

23

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

#### Figure 5.

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 3) [59, 60]. This ability to inhabit different substrates and its rapid colonization rates [79] provide a competitive advantage for potential habitat expansions. Although fire corals compete with other corals, they also contribute to coral survival during Acanthaster outbreaks [106], highlighting their key ecological role in reef resilience. In fact, the corallivorous predator Acanthaster planci tends to avoid Millepora species [109], thus providing predator-free sanctuaries to nearby scleractinian corals.
