2. Biodiversity and biogeography

## 2.1 Fossil records

Hydrocorals have a relatively long evolutionary history since many fossils from Tertiary deposits have been assigned to Millepora. However, Boschma [52] recognized only two species, Millepora tornquisti from Eocene rocks (56–33 mA) of Madagascar and M. alcicornis in Pleistocene deposits (2.58 mA–11,700 ya) from the Panama Canal zone [53]. Other branching milleporids were also reported from the Upper Cretaceous (100–66 mA) in northern Spain [54]. Recently, fossils of M. alcicornis have been recorded in deposits from the Early Miocene (23 mA) [55]. M. exaesa fossils were also recorded in more recent deposits in the Seychelles, dating from the last interglacial sea-level high-stand, 129,000–116,000 ya [56].

## 2.2 Species delimitation

As in many corals, the morphological species concept was traditionally applied to the species delimitation of Millepora, which is based on colony growth forms. Millepora species have a great diversity of growth forms and can be encrusting, branching, plate-like, massive or even columnar (Figure 2). Interestingly, the typical growth forms of Millepora species are broadly the same in the Red Sea and the Indo-Pacific. Arrigoni and colleagues [57] hypothesized a morphological convergence for these species. Similarly, in the Atlantic, there is also one plate-like and one branching species, as well as other massive/encrusting forms. As these growth forms do not form monophyletic groups on the phylogenetic reconstructions [57],

Figure 2.

Growth forms, pores and polyps of three Millepora species. (A–C) M. cf. exaesa encrusting growth form, pores and polyps, respectively; (D–F) for the massive M. cf. platyphylla and (G–I) for the branching M. cf. tenera. D photograph is courtesy of Gilles Siu.

they seem to have appeared independently and likely evolved in relation to the hydrodynamic conditions of their environment.

This group is also known for its great phenotypic plasticity [46], and environmental factors are known to greatly influence the morphology of Millepora colonies. Recently, Dubé and colleagues [58] demonstrated phenotypic plasticity among clonal colonies distributed in habitats with different hydrodynamic characteristics (see Section 4). To further complicate the matter, fire corals have been shown to overgrow stony corals, hydrocorals and gorgonians, which gives them additional peculiar growth forms (Figure 3) [59, 60]. Consequently, about 100 nominal species were described [61]. While Duchassaing and Michelotti [44, 62] identified 24 Millepora species based on trivial morphological differences, Hickson [45, 63] reckoned that there was only one Millepora species, M. alcicornis, and that all other morphological growth forms were only ecological variations. There is a true 'species boundary problem' within Millepora and it has been subject to much debate for over 150 years [45–47, 64, 65].

characters. Using a more elaborate quantitative approach on pore characters, Moschenko [66] considered 11 traits (e.g. numbers and diameters of gastropores and dactylopores, distances between dactylopores and gastropores, number of dactylopores per gastropore) in one plate-like (M. platyphylla Hemprich and Ehrenberg 1834) and five branching Millepora species (M. cruzi Nemenzo 1975, M. dichotoma Forskal 1775, M. intricata Milne Edwards 1860, M. murrayi Quelch 1884 and M. tenera Boschma 1949). His results distinguished only M. platyphylla, while all branching species shared important overlap in trait values with gradual transition from one species to another [66]. However, M. cruzi and M. murrayi have been subsequently synonymized (with M. tenera and M. intricata, respectively) and this could explain some of the trait overlaps between species. More recently, Razak and Hoeksema [65], based on colony growth forms and pore characters, revised the Indonesian Millepora species and synonymized 6 of the 13 recognized Indo-Pacific species. In particular, the gastropore and dactylopore diameters were shown to be discriminant among many Millepora species [57, 67, 68]. Boissin and colleagues (submitted) analyzed 13 pore characters and could distinguish the three species present in Reunion Island. This latest study showed that gastropore and dactylopore numbers, as well as diameters, were informative and should be used as standard traits in future Millepora studies. This study also showed that polyp features were discriminant, such as the presence or absence of capitate tentacles or capitations, and the presence, absence or abundance of Symbiodiniaceae. Additional biological traits seem to be helpful to delineate milleporid species, such as reproductive periods, medusoid features and nematocyst morphology [36, 57, 69, 70].

Ecology, Biology and Genetics of Millepora Hydrocorals on Coral Reefs

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

Millepora hydrocorals overgrowing living reef corals at Europa Island (Indian Ocean), including massive Porites (A, C and D), Distichopora sp. (B) and Astrea sp. (D). M. cf. platyphylla can overgrow giant clam shells (C).

Figure 3.

19

Apart from colony growth forms, pore traits are the most widely used characters in Millepora species delimitation. The pores in Millepora are like the corallites for the scleractinian corals, accommodating the polyps. There are two types of polyps in Millepora species: feeding polyps (gastrozoids) which are provided with a gastrovascular cavity opening by a mouth, and defensive polyps (dactylozoids) without a mouth. The gastropores and the dactylopores, from which the gastrozoids and the dactylozoids are able to extend outside to catch food, are organized in cyclosystems formed by a circle of dactylopores surrounding a single gastropore (Figure 2). While Boschma [46] concluded that the colony growth form was the most important character for the distinction of species, and that the other characters were not sufficient delimiting criteria, subsequent studies have used pore

#### Figure 3.

they seem to have appeared independently and likely evolved in relation to the

This group is also known for its great phenotypic plasticity [46], and environmental factors are known to greatly influence the morphology of Millepora colonies. Recently, Dubé and colleagues [58] demonstrated phenotypic plasticity among clonal colonies distributed in habitats with different hydrodynamic characteristics (see Section 4). To further complicate the matter, fire corals have been shown to overgrow stony corals, hydrocorals and gorgonians, which gives them additional peculiar growth forms (Figure 3) [59, 60]. Consequently, about 100 nominal species were described [61]. While Duchassaing and Michelotti [44, 62] identified 24 Millepora species based on trivial morphological differences, Hickson [45, 63] reckoned that there was only one Millepora species, M. alcicornis, and that all other morphological growth forms were only ecological variations. There is a true 'species boundary problem' within Millepora and it has been subject to much debate for over

Growth forms, pores and polyps of three Millepora species. (A–C) M. cf. exaesa encrusting growth form, pores and polyps, respectively; (D–F) for the massive M. cf. platyphylla and (G–I) for the branching M. cf. tenera. D

Apart from colony growth forms, pore traits are the most widely used characters in Millepora species delimitation. The pores in Millepora are like the corallites for the scleractinian corals, accommodating the polyps. There are two types of polyps in Millepora species: feeding polyps (gastrozoids) which are provided with a gastrovascular cavity opening by a mouth, and defensive polyps (dactylozoids) without a mouth. The gastropores and the dactylopores, from which the gastrozoids and

the dactylozoids are able to extend outside to catch food, are organized in cyclosystems formed by a circle of dactylopores surrounding a single gastropore (Figure 2). While Boschma [46] concluded that the colony growth form was the most important character for the distinction of species, and that the other characters were not sufficient delimiting criteria, subsequent studies have used pore

hydrodynamic conditions of their environment.

Invertebrates - Ecophysiology and Management

150 years [45–47, 64, 65].

18

photograph is courtesy of Gilles Siu.

Figure 2.

Millepora hydrocorals overgrowing living reef corals at Europa Island (Indian Ocean), including massive Porites (A, C and D), Distichopora sp. (B) and Astrea sp. (D). M. cf. platyphylla can overgrow giant clam shells (C).

characters. Using a more elaborate quantitative approach on pore characters, Moschenko [66] considered 11 traits (e.g. numbers and diameters of gastropores and dactylopores, distances between dactylopores and gastropores, number of dactylopores per gastropore) in one plate-like (M. platyphylla Hemprich and Ehrenberg 1834) and five branching Millepora species (M. cruzi Nemenzo 1975, M. dichotoma Forskal 1775, M. intricata Milne Edwards 1860, M. murrayi Quelch 1884 and M. tenera Boschma 1949). His results distinguished only M. platyphylla, while all branching species shared important overlap in trait values with gradual transition from one species to another [66]. However, M. cruzi and M. murrayi have been subsequently synonymized (with M. tenera and M. intricata, respectively) and this could explain some of the trait overlaps between species. More recently, Razak and Hoeksema [65], based on colony growth forms and pore characters, revised the Indonesian Millepora species and synonymized 6 of the 13 recognized Indo-Pacific species. In particular, the gastropore and dactylopore diameters were shown to be discriminant among many Millepora species [57, 67, 68]. Boissin and colleagues (submitted) analyzed 13 pore characters and could distinguish the three species present in Reunion Island. This latest study showed that gastropore and dactylopore numbers, as well as diameters, were informative and should be used as standard traits in future Millepora studies. This study also showed that polyp features were discriminant, such as the presence or absence of capitate tentacles or capitations, and the presence, absence or abundance of Symbiodiniaceae. Additional biological traits seem to be helpful to delineate milleporid species, such as reproductive periods, medusoid features and nematocyst morphology [36, 57, 69, 70].

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 from Reunion Island.

#### 2.3 Biogeography

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 et al., unpublished).

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 success, is represented by less than 20 species.

3. Ecology and symbiosis

Figure 4.

21

3.1 Distribution, abundance and ecological roles

Ecology, Biology and Genetics of Millepora Hydrocorals on Coral Reefs

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

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

Geographic distribution of the 16 recognized species of Millepora in the Atlantic and Indo-Pacific Oceans.

## Ecology, Biology and Genetics of Millepora Hydrocorals on Coral Reefs DOI: http://dx.doi.org/10.5772/intechopen.89103

Figure 4. Geographic distribution of the 16 recognized species of Millepora in the Atlantic and Indo-Pacific Oceans.
