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We studied the association of 11 species of fishes with 5 species of echinoids at Roatán, Honduras, from 27 August to 1 September 2017. Fishes associated most frequently with the echinoid Diadema antillarum (34.3% of echinoids, six fish species, n = 146 echinoids), followed by Echinometra viridis (25.0%, three fish species, n = 12), Echinometra lucunter (7.5%, ten fish species, n = 1,834), Eucidaris tribuloides (3.4%, four fish species, n = 116), and Tripneustes ventricosus (7.1%, one fish species, n = 28). Of 196 fishes seeking shelter beside echinoids, Malacoctenus aurolineatus was the most common (41.8% of fishes, three echinoid species), followed by Stegastes adustus (38.8%, three echinoid species), Stegastes diencaeus (6.6%, three echinoid species), Sargocentron coruscum (6.1%, five echinoid species), Chaetodon capistratus (1.5%, one echinoid species), Gobioclinus filamentosus (1.5%, one echinoid species), Pomacanthus paru (1.0%, two echinoid species), Labrisomus nuchipinnis (1.0%, two echinoid species), Equetus punctatus (0.5%, one echinoid species), Microspathodon chryurus (0.5%, one echinoid species), and Thalassoma bifasciatum (0.5%, one echinoid species). None of the fishes associated exclusively with echinoids or was specialized for associating with echinoids, indicating the association was facultative. All fishes were small (< 12 cm). Fishes associated most frequently with the longest-spined echinoid, D. antillarum, supporting the hypothesis that fishes seek shelter among the spines of echinoids to benefit from increased protection from predation.

Introduction: Echinoids (sea urchins) provide shelter for a variety of facultative or obligatory ectosymbionts. Objective: To evaluate the hypothesis that decapods and fishes prefer to associate with echinoid individuals and species that have longer spines. Methods: We visually studied the frequency of decapod crustaceans and fishes associated with echinoids in shallow water (< 4 m) and deeper water (5-20 m) at Los Cabos, Baja California Sur, Mexico, during 1-6 January 2019. Results: We inspected 1 058 echinoids of six species. Five decapod species associated with three species of echinoids. When compared with other echinoid species, in shallow water, decapods associated 5.1 times more often with the longest-spined echinoid Diadema mexicanum (7.0 times more decapods per individual D. mexicanum); in deeper water, association frequency was similar for all echinoid species. Fourteen fish species associated with four echinoid species. In shallow water, fishes associated 2.6 times more with D. mexicanum (4.5 times more fishes per individual). There was no preferred echinoid species in deeper water. Longer-spined D. mexicanum had more decapods and fishes. Associations were more frequent in shallow water. Multiple individuals and species of decapods and fish often associated together with a single D. mexicanum. The decapod that presumably is Tuleariocaris holthuisi showed a possible obligatory association with one of the equinoids (D. mexicanum); the other decapods and all fish species are facultative associates. Conclusion: Our results support the hypothesis that decapods and fishes associate most frequently with …