Garcia-Hernandez, J.E.

Abstract

The Caribbean scleractinian reef coral Agaricia undata (Agariciidae) is recorded for the first time as a host of the coral-gall crab Opecarcinus hypostegus (Cryptochiridae). The identity of the crab was confirmed with the help of DNA barcoding. The association has been documented with photographs taken in situ at 25 m depth and in the laboratory. The predominantly mesophotic depth range of the host species suggests this association to be present also at greater depths. With this record, all seven Agaricia species are now listed as gall-crab hosts, together with the agariciid Helioseris cucullata. Within the phylogeny of Agariciidae, Helioseris is not closely related to Agaricia. Therefore, the association between Caribbean agariciids and their gall- crab symbionts may either have originated early in their shared evolutionary history or later as a result of host range expansion. New information on coral-associated fauna, such as what is presented here, leads to a better insight on the diversity, evolution, and ecology of coral reef biota, particularly in the Caribbean, where cryptochirids have rarely been studied.

Keywords Associated fauna . Brachyura . Coral reefs . Cryptochiridae . Marine biodiversity . Symbiosis

A team of researchers from Naturalis Biodiversity Center, ANEMOON Foundation, and colleagues from various intitutes, performed several bodiversity surveys in the Dutch Caribbean (Bonaire, Curaçao, Sint Eustatius). These expeditions were organized by professor Bert Hoeksema, who is the coral expert at Naturalis and honorary professor of tropical marine biodiversity at the University of Groningen. The team discovered, amongst others, some new associations for Christmas tree worms (Spirobranchus spp.) as symbionts within different coral species. These worms live partly inside the coral and appeared to be able survive when the host coral becomes covered by encrusting animals, who then serves as a secondary host. Surprisingly, most discoveries were made in very shallow water (less than 2 m depth), where scuba divers may spend less time when compared to greater depths.

Caribbean Christmas tree worms (Annelida: Polychaeta: Serpulidae: Spirobranchus) are considered host generalists in their associations with anthozoan (Scleractinia) and hydrozoan (Millepora) stony corals. As planktonic larvae, they settle on coral surfaces and start secreting a calcareous tube to be used as a dwelling. This tube usually becomes overgrown by the host coral (except for its opening) and may get encapsulated deep inside the coral skeleton. In this manner, the well-protected worms grow and survive predation and other hazards, allowing them to live for over four decades. When the host corals are overgrown by other organisms, such as octocorals and sponges, these may act as secondary hosts.

The honeycomb cowfish, Acanthostracion polygonius Poey, 1876 (Ostraciidae) is easily distinguishable from other Caribbean coral reef fish. Both females and males possess a heavy external bony box with uniformly hexagonal scale plates as armor, as well as a pair of spines projecting from the carapace above the eyes and anterior to the £ fin (Moyer 1984). They are known to feed on a variety of invertebrates, including tunicates, alcyonaceans, shrimps, gastropods, and at least 15 species of sponges (Randall 1967; Wulff 1994).

During a coral reef biodiversity survey at Curaçao (June 2017), two A. polygonius individuals were engaged in what appeared to be either a failed mating ritual dance or a male–male territorial dispute (Fig. 1). It is likely that the encounter is that of a male–male confrontation since the distinctive humming sound of the male prior to gamete release was not heard, nor any spawning was observed (Moyer 1984).

The fighting behavior resembled that of Lactoria diaphana from the Indo-Pacific; upon seeing each other, the pair proceeded to flash and display their bright neon-blue coloration (Moyer 1984). Aggressively charging each other, they took turns sucking/biting their respective underside belly as they rose in the water column (from 6 m depth) in a circular motion (Fig. 1). Upon reaching the proximity of the surface, with the larger (more dominant) trunkfish attached to the smaller fish, the pair broke off and swam toward the reef in opposite directions. Within the scientific literature, little is known regarding cowfish social structure, reproduction, and territorial competition. This record shines light into a previously known ritual, yet still misunderstood behavior of trunkfishes in the Caribbean.

This article was published in the following report:

MARINE BIODIVERSITY SURVEY OF ST. EUSTATIUS, DUTCH CARIBBEAN 2015 by Naturalis Biodiversity Center and the Netherlands ANEMOON Foundation

This section of the report focuses on the diversity of marine sponges from various coral reef environments around the coastal waters of St. Eustatius. A total of 36 sites were surveyed during the course of the marine expedition in order to catalogue the marine sponge fauna. Sponge surveys were performed using the roving diver technique, and presence of each sponge species was recorded by digital photography and underwater paper. 

Crabs of the genus Platypodiella (Brachyura: Xanthidae) are known for their association with zoantharians (Hexacorallia: Zoantharia), predom- inantly of the genus Palythoa (Den Hartog and Holthuis 1984; Den Hartog and Tu ̈rkay 1991). They make cavities inside their zoantharian hosts, which they use as shelter.

During the Statia Marine Biodiversity Expedition to St. Eustatius (Lesser Antilles, Dutch Caribbean) in June 2015, small crabs (~5–10 mm wide) of Platypodiella spectabilis (Herbst, 1794) were most commonly observed either inside or in close proximity to encrusting Palythoa caribaeorum (Duchassaing and Michelotti, 1860) (Fig. 1a, b). A larger crab of the same species (~30 mm wide) was found hiding in between sponges and coral (M.A. Faasse and G.W.N.M. van Moorsel, pers. comm.), whereas crevices in dead coral may also be a common habitat (Martin and Zimmerman 2007). All crabs were characteristically colored in variable patterns of orange, yellow, black and white (Fig. 1; Martin and Zimmer- man 2007).
Unexpectedly, two small P. spectabilis individuals were found in cavities inside Niphates digitalis (Lamarck, 1814) sponges associated with the parazoanthid Umimayanthus parasiticus (Duchassaing and Michelotti, 1860). These crab dwellings were similar to burrows ob- served in Palythoa (Fig. 1c). Although N. digitalis was one of the most commonly observed sponges of St. Eustatius, no P. spectabilis was found in sponges without a zoantharian associate.