**1. Introduction**

Zoantharians (Anthozoa: Hexacorallia: Zoantharia) make up a considerable benthic component of tropical and sub-tropical shallow-water reefs, similar to Scleractinia (hard corals) and Actiniaria (sea anemones) [1–4]. Zoantharians are sessile colonial anemone-like organisms with two rings of tentacles surrounding circular to polygonal disks, and forming colonies of polyps [5]. They grow in dense mats or small patches throughout the shallow intertidal rocky zones, or coral reef ecosystems, and play an important ecological role in their tropical and sub-tropical habitats [6–12].

Zoantharian coverage on coral reef ecosystems can be extensive [9,10]. For instance, Karlson (1981) noted that two species, *Zoanthus sociatus* (Ellis, 1768) and *Zoanthus solanderi* (LeSueur, 1818) had extensive coverage at a northern intertidal area in Jamaica, which he subsequently named the '*Zoanthus* zone', as originally named by Tom Goreau in the 1950s [13]. Additionally, Lopez et al. (2018) reported a zoantharian zone located at Cabo Verde Islands, where molecular methods and morphological analysis confirmed the presence of two zoantharian species. Zoantharians, such as *Palythoa caribaeorum* (Duchassaing and Michelotti, 1860) and *Z. sociatus* cover large areas in subtidal and intertidal zones [7,9,10,12]. Additionally, many, but not all zoantharian species maintain symbiotic re-

**Citation:** Belford, S. Shallow-Water Species Diversity of Common Intertidal Zoantharians (Cnidaria: Hexacorallia: Zoantharia) along the Northeastern Coast of Trinidad, Southern Caribbean. *Oceans* **2021**, *2*, 477–488. https://doi.org/10.3390/ oceans2030027

Academic Editor: Rupert Ormond

Received: 30 October 2020 Accepted: 14 July 2021 Published: 20 July 2021

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lationships with Symbiodiniaceae (zooxanthellae), hence determining the identity of these holobionts adds more understanding of their ecology and physiological characteristics.

Molecular analyses of zoantharians assist in species identification, where morphological identification is difficult, or impossible. For example, phenotypic plasticity in zoantharians, specifically large variation in morphological characteristics, such as polyp shape, colony shape, size and oral disk color, may cause them to be overlooked in ecological surveys, even though their numbers may be abundant [5,8,14,15]. Given the difficulties in morphological and molecular examination of zoantharians, more priority should be given to determining their distribution and diversity in the Caribbean, especially since zooxanthellate zoantharians play key ecological roles in marine ecosystems [10]. Although success in identifying potential unidentified zoantharian species through morphological characteristics has been proven to be successful [16], the addition of molecular analyses has alleviated issues with morphological ambiguities [16,17]. Additionally, zoantharian species diversity including molecular analyses of zooxanthellate symbionts of family Symbiodiniaceae will continue to add more information about symbiont ecology in light of global climate change [11,18,19].

Although distributed world-wide in tropical and subtropical waters in the Atlantic and Indo-Pacific regions [8,20], zoantharians and Symbiodiniaceae genera identity at the southern-most part of the Caribbean, specifically Trinidad and Tobago, are limited. Two zoantharian genera, *Zoanthus* and *Palythoa* spp., are commonly observed occupying shallow waters, and in different intertidal zones along the northeastern coast of Trinidad [9,10,12,14], but relatively few studies have highlighted morphological and molecular details of these species in this part of the southern Caribbean region.

The purpose of this study is to quantitatively assess zoantharian benthic coverage, and use morphological and genetic analyses to identify zoantharians, and confirm Symbiodiniaceae genera identities. Molecular knowledge of these benthic organisms will be recorded here for the first time at this southernmost part of the Caribbean Sea. Consequently, all ecological aspects of identifying the extent of zoantharian benthic coverage provides more understanding of the dynamic coral reef ecosystem. In general, zoantharian distribution in the Caribbean needs to be continually monitored, and species identification confirmed, especially in the face of increased climate change and anthropogenic activities [9].
