**Contents**


## **About the Editor**

**Simone Montano** is a researcher at the University of Milano-Bicocca, Department of Earth and Environmental Sciences (DISAT) in Italy, and Vice-Director of the Marine Research and High Education Center, Magoodhoo Island, Maldives. He is a marine biologist, mainly interested in the ecology and biology of the coral reef ecosystem. His current research activities focus on the assessment of coral health and diseases, with particular attention on new and emerging coral symbioses. All his activities are aimed to understand the dynamics that will drive this ecosystem under a climate change scenario, in order to develop and propose environmental management plans. To date, he has published more than 60 peer-reviewed papers in international scientific journals. He is a PADI diving instructor with an Advanced European Scientific Diver license, with >800 scientific dives and a total of >1000. His field work experiences include the Caribbean (St. Eustatius, Bonaire, and Curacao), the Indo-Pacific area (Maldives, Mauritius, Yemen, India, and Thailand), and the Red Sea (Egypt, Saudi Arabia).

## **Preface to "Diversity of Coral-Associated Fauna"**

Mutualistic, commensalistic, and parasitic associations are extremely abundant in coral reef ecosystems. Reef-building corals are usually considered the most likely to provide numerous different habitats and to bear a huge number of symbiotic relationships. However, many other invertebrate groups such as sponges, bryozoans, and other cnidarians are known to establish strict symbiotic relationships with other marine organisms, even though their inter-specific interactions are poorly investigated. To date, symbiotic associations have mainly been studied by considering pairwise relationships, but in the vast majority of cases one host is typically inhabited by several other organisms (e.g., epibionts, commensals, and parasites) that may interact with each other and with the two partners. Unfortunately, even though these symbioses have been found to be more common than previously known, information regarding the nature, origin, and existence of any correlation with environmental factors is far from being fully elucidated.

In line with this, we believe that it is necessary to understand how these co-occurring organisms influence the symbiotic association considered, and how their combined effects influence the two partners. This information could be used to understand the mechanisms by which ecological interactions can mediate species' responses to disturbances and used to predict the ability of single organisms to persist in a rapidly changing environment.

For this reason, this book aims to explore the hidden diversity of coral reefs, focusing on some neglected components of the biodiversity of this extraordinary marine ecosystem, significantly improving our knowledge of the diversity, ecology, and role of the coral-associated fauna.

A special thanks goes to all authors of the papers published in this Special Issue. Their contributions regarding the ecological interactions in tropical coral reef ecosystems made by the combination of multidisciplinary approaches, taxonomic expertise, and dedicated biodiversity surveys revealed the existence of many previously unknown associations.

I strongly believe that similar in-depth studies addressed to identify and describe other hidden symbioses will be increasingly necessary in the future.

> **Simone Montano** *Editor*

## *Editorial* **The Extraordinary Importance of Coral-Associated Fauna**

#### **Simone Montano 1,2**


Received: 14 September 2020; Accepted: 15 September 2020; Published: 16 September 2020

**Abstract:** Coral reefs are one of the most diverse marine ecosystems on Earth and one of the richest in terms of species interactions. Scleractinian corals are usually the most likely to provide numerous different habitats and to support many symbiotic relationships. However, many other invertebrate groups, such as sponges, bryozoans, and other cnidarians, establish strict symbiotic relationships with other marine organisms. Despite the nature of these relationships—as well as the factors that drive their establishment—being unclear in most cases, a few studies have already shown that some associations may increase the resistance of their hosts to external disturbances. Thus, the potential ability of each member of these diverse symbiotic assemblages to influence the fitness and long-term survival of their hosts bring the coral-associated fauna to the top of the list of coral reef studies. Unfortunately, the widespread degradation of coral reef ecosystems may threaten the existence of the intimate relationships that may go unrecognized complicating our understanding of the intricate networks connecting the fates of reef species. Therefore, this unprecedented loss of biodiversity calls for synergic conservation and monitoring actions aimed at significantly increasing our efforts to search for and describe as much of the diversity of coral-associated organisms as possible, shedding new light on the complex, elusive mechanisms controlling coral reef functioning.

**Keywords:** biodiversity; scleractinian; coral reefs; symbiosis; global change; impacts

Coral reefs encompass the highest biodiversity of any marine ecosystem of the planet [1]. This abundance is primarily due to the topographic complexity created by many benthic organisms, such as reef-building corals, sponges, bryozoans and other cnidarians that play a key role in creating the complex three-dimensional architecture of coral reef and providing a plethora of habitats to support an extraordinary diversity of organisms from all kingdoms of life [2].

The highly diverse fauna associated with these sessile reef organisms is dominated by invertebrates, belonging to numerous phyla—such as Arthropoda, Mollusca, Echinodermata, Anellida, Porifera and Cnidaria—depending on their hosts for food, refuges and habitats, and usually establishing strict symbiotic relationships in form of mutualistic, commensalistic and parasitic associations [3,4]. The coral-associated fauna assumes a considerable and unique importance considering that each member of these diverse symbiotic assemblages has the potential to influence the fitness and long-term survival of their host [2].

Reef-building corals, for example, are known to form associations with about a thousand of microand macro-organisms that, in many cases, appear to be strictly host specific. Despite the fact that the large number of them may contribute to the reduced health and mortality of corals through feeding or boring activities, many other species can be considered fundamental to the persistence and resilience of their host corals [4]. Indeed, more than 50% of coral-associated invertebrates are obligate coral dwellers, with some of them known to actively participate in nutrient recycling [5], to alleviate detrimental effects

of sedimentation and actively defend colonies from coral-feeding organisms [6,7], or to slow down the progression of diseases as shown by the crabs of the genus *Cymo* [8]. More recently, coral symbiotic hydrozoans of the genus *Zanclea* has been proved to both reduce coral susceptibility to diseases and protect their hosts from predation [9], highlighting how far we are from the understanding of the mechanisms by which ecological interactions can mediate species' responses to disturbances.

Unfortunately, how many species are living on the coral reefs as well as the species of microand macroinvertebrates living in association with other reef organisms is still not clear. Most of the unknown reef communities consist of cryptofauna [10] that may be difficult to recognize in the field due to their tiny size [11,12], camouflage behavior [13,14], and because they live in habitats that are often overlooked, such us as caves, sediment or coral rubble [15], or because they are located in deep environments as the mesophotic zones [16]. This gap in knowledge can be exacerbated both in shallow and deep coral reefs if the parasites diversity is included since most species in most major parasite groups are still undiscovered or unnamed [17].

Bearing in mind the likely high degree of specialization and co-dependence of these symbiotic relationships, this lack of information appears dramatic in the light of the increasing number of threats contributing to the global decline of coral reefs [18]. Indeed, habitat degradation could have serious negative effects on the diversity of reefs and may disrupt these symbiotic relationships [19], intensifying the loss of biodiversity [20]. Thus, if preserving biodiversity is now considered a priority for any natural ecosystem, it is increasingly vital for the future of coral reefs in which thousands of coral-associated organisms could be negatively impacted by global change, on scales ranging from local declines to global extinction; these losses could have major downstream consequences for coral reef ecosystem function and stability [17].

The fundamental value of the papers published in this Special Issue is twofold. On one hand, it highlights the still-scarce knowledge of the ecological interactions in tropical coral reef ecosystems and the possible existence of many other so-far-unknown similar associations that deserve our attention. On the other hand, it highlights how the combination of multidisciplinary approaches, taxonomic expertise and dedicated biodiversity surveys can significantly improve our knowledge about the diversity, ecology and role of coral-associated fauna. Therefore, we hope that these studies can stimulate the exploration of neglected areas in reef ecology, increase significantly our effort in searching and describing as much the diversity of coral-associated organisms and systematically investigate the coral-associated biodiversity by adding coral-associated fauna surveys to largescale biodiversity monitoring programs.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Article*
