Abstract

Coral reefs are one of the most productive and diverse ecosystems on Earth. However, climate warming is occurring at an unprecedented rate and has negatively affected coral reefs worldwide. Evaluating the life history of reef-building species carries important implications for coral reef conservation. This chapter examines the taxonomy, biogeography, ecology, symbiosis, morphology and reproductive biology of Millepora hydrocorals, an important but relatively unstudied component of coral reefs. An emphasis is also placed on the influence of variable reef environments on Millepora life history traits, providing a fascinating opportunity to study the interplay between ecology and evolution. Special attention is given to ecological and evolutionary benefits of asexual versus sexual reproduction in the maintenance of genetic and phenotypic diversity. Lastly, this chapter discusses whether life-history strategies of Millepora hydrocorals and tolerance to different stressors can influence their ability to adapt and survive to future climate change, and other natural and anthropogenic disturbances.

Keywords: Millepora, coral reefs, taxonomy, biogeography, symbiosis, morphology, reproduction, population genetics

## 1. Introduction

#### 1.1 Coral reefs: biodiversity and threats

Coral reefs were formed only 230 million years ago and are largely limited to warm shallow waters [1], yet they are among the most biologically diverse and economically important marine ecosystems. Coral reefs do not only shelter thousands of species; they also provide critical services to humans, including fisheries, coastal protection, medicines and tourism activities [2–4]. The economic value of coral reefs worldwide has been estimated to be around 30 billion US\$ of net benefit per year [5]. Often called the rainforest of the sea due to their outstanding biodiversity, coral reefs only cover less than 0.1% of the ocean seafloor [6, 7] or approximately 5% that of rainforest areas [8]. Coral reefs thrive under nutrient-poor and oligotrophic waters [9–11], but yet harbour more than 25% of all known marine species [12, 13]. This ecosystem is sustained through efficient nutrient recycling strategies developed by corals [14] and algae [15], the primary reef producers, and

other key organisms, i.e., microbes [16] and sponges [17]. In coral reef ecosystems, many calcifying benthic organisms contribute to reef accretion and build the complex and massive three-dimensional structure of reefs, including scleractinian corals, the major habitat architects, and hydrocorals [18]. These reef-builders are key components of coral reef health and biodiversity as they offer food, shelter and nurseries for thousands of reef-dwelling organisms and fishes [19].

Reefs are dynamic systems that are frequently punctuated by perturbations [20]. For instance, human activities can alter both global (climate change associated with CO2 emissions) and local reef health (e.g. coastal habitat loss, pollution, sewage, overfishing and invasive species) [21, 22]. As a consequence, several reports of coral reef declines have been recorded, averaging 30–50% reductions in reef cover globally [4, 23, 24], including recent losses of coral cover following the multiple global bleaching events that occurred between 2014 and 2017 [25–27]. Since coral reefs are integrated ecosystems, declines in reef-building corals are often accompanied with declines of other species, such as many coral reef fishes [28], further hampering their capacity to deliver important ecosystem services to more than 500 million people [23, 29]. Yet, reef corals and the ecosystem they create can recover and a key factor underpinning such recovery is the ability of coral species to grow back, to maintain or renew their populations. Such an ability to respond to acute and chronic stressors in coral species is often linked to morphological traits, reproductive strategies and symbiont partners (among others). As we progress further into the Anthropocene, understanding and predicting these stress responses require prior knowledge on the life history traits of keystone reef corals, and some assessment of the influence that environmental changes may have on those traits.

2. Biodiversity and biogeography

only 29 were using genetic approaches.

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

The literature search in web of science identified 326 publications referring to Millepora hydrocorals, wherein

Ecology, Biology and Genetics of Millepora Hydrocorals on Coral Reefs

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

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],

from the last interglacial sea-level high-stand, 129,000–116,000 ya [56].

2.1 Fossil records

Figure 1.

2.2 Species delimitation

17

#### 1.2 Millepora hydrocorals

To date, the vast majority of studies on species' life history traits in coral reefs have mainly focused on scleractinian corals due to their key role in providing much of the habitat framework and structural complexity of reefs [30–34]. The extent to which other non-scleractinian reef-building organisms might rescue reef populations in response to environmental change is largely unknown. More information on such organisms is therefore needed. Millepora hydrocorals, known as fire corals because of their painful sting via toxic nematocysts [35, 36], are important components of reef communities where they, similar to scleractinian corals, contribute to reef accretion and community dynamics [37, 38]. Millepora species are Hydrozoans (Medusozoans), and together with hermatypic corals (Anthozoans), belong to the phylum Cnidaria [39]. Millepora spp. are members of the monogeneric family Milleporidae, the sub-order Capitata and the order Anthomedusae [40–42]. Milleporidae and Stylasteridae are the only two Hydrozoan families producing skeletons of calcium carbonate. The first scientific report of Millepora spp. was from Linnaeus in 1758 [43], with subsequent species reports and descriptions by several authors (e.g. [44, 45]), and the seminal work of Boschma [46, 47]. There has been a surge of interest in fire corals over the last two decades (Figure 1), and especially in the last 7 years. These recent studies focused on genetics and coincided with the development of new molecular markers [48–50]. While there is much known now about fire corals (reviewed in [51]), the literature is scattered, particularly that of biogeography and population genetic research, and needs to be summarized. In this chapter, we will document what is known about taxonomy, biogeography, ecology, symbiosis, morphology and reproductive biology of Millepora hydrocorals, using both published and unpublished information, and will highlight areas where knowledge is especially lacking.

Figure 1.

other key organisms, i.e., microbes [16] and sponges [17]. In coral reef ecosystems, many calcifying benthic organisms contribute to reef accretion and build the complex and massive three-dimensional structure of reefs, including scleractinian corals, the major habitat architects, and hydrocorals [18]. These reef-builders are key components of coral reef health and biodiversity as they offer food, shelter and

Reefs are dynamic systems that are frequently punctuated by perturbations [20]. For instance, human activities can alter both global (climate change associated with CO2 emissions) and local reef health (e.g. coastal habitat loss, pollution, sewage, overfishing and invasive species) [21, 22]. As a consequence, several reports of coral reef declines have been recorded, averaging 30–50% reductions in reef cover globally [4, 23, 24], including recent losses of coral cover following the multiple global bleaching events that occurred between 2014 and 2017 [25–27]. Since coral reefs are integrated ecosystems, declines in reef-building corals are often accompanied with declines of other species, such as many coral reef fishes [28], further hampering their capacity to deliver important ecosystem services to more than 500 million people [23, 29]. Yet, reef corals and the ecosystem they create can recover and a key factor underpinning such recovery is the ability of coral species to grow back, to maintain or renew their populations. Such an ability to respond to acute and chronic stressors in coral species is often linked to morphological traits, reproductive strategies and symbiont partners (among others). As we progress further into the Anthropocene, understanding and predicting these stress responses require prior knowledge on the life history traits of keystone reef corals, and some assessment of the influence that environmental changes may have on those traits.

To date, the vast majority of studies on species' life history traits in coral reefs have mainly focused on scleractinian corals due to their key role in providing much of the habitat framework and structural complexity of reefs [30–34]. The extent to

populations in response to environmental change is largely unknown. More information on such organisms is therefore needed. Millepora hydrocorals, known as fire corals because of their painful sting via toxic nematocysts [35, 36], are important components of reef communities where they, similar to scleractinian corals, contribute to reef accretion and community dynamics [37, 38]. Millepora species are Hydrozoans (Medusozoans), and together with hermatypic corals (Anthozoans),

Anthomedusae [40–42]. Milleporidae and Stylasteridae are the only two Hydrozoan families producing skeletons of calcium carbonate. The first scientific report of Millepora spp. was from Linnaeus in 1758 [43], with subsequent species reports and descriptions by several authors (e.g. [44, 45]), and the seminal work of Boschma [46, 47]. There has been a surge of interest in fire corals over the last two decades (Figure 1), and especially in the last 7 years. These recent studies focused on genetics and coincided with the development of new molecular markers [48–50]. While there is much known now about fire corals (reviewed in [51]), the literature is scattered, particularly that of biogeography and population genetic research, and needs to be summarized. In this chapter, we will document what is known about taxonomy, biogeography, ecology, symbiosis, morphology and reproductive biology of Millepora hydrocorals, using both published and unpublished information,

which other non-scleractinian reef-building organisms might rescue reef

belong to the phylum Cnidaria [39]. Millepora spp. are members of the monogeneric family Milleporidae, the sub-order Capitata and the order

and will highlight areas where knowledge is especially lacking.

nurseries for thousands of reef-dwelling organisms and fishes [19].

Invertebrates - Ecophysiology and Management

1.2 Millepora hydrocorals

16

The literature search in web of science identified 326 publications referring to Millepora hydrocorals, wherein only 29 were using genetic approaches.
