A remarkable infestation of epibionts and endobionts of an edible chiton (Polyplacophora: Chitonidae) from the Mexican tropical Pacific Laura Regina Alvarez-Cerrillo 1 Facultad de Ciencias Universidad Nacional Auto ´ noma de Me ´ xico Ciudad de Me ´ xico, MEXICO Paul Valentich-Scott Santa Barbara Museum of Natural History Santa Barbara, CA 93105 USA William A. Newman Scripps Institution of Oceanography La Jolla, CA 92093 USA ABSTRACT Although epibiosis is common in polyplacophorans, we describe an unusual presence of epibionts and endobionts in a single adult specimen of Chiton articulatus collected in Guerrero, Mexico, from an eroded habitat of crevices with high wave activity. The epibiont and endobiont specimens covered nearly 90% of the central and lateral areas of the chiton valves while the border of mantle girdle showed no epibiosis. Crustose and filamentous algae, and crustacean arthropods from two common barnacle families, Chthamalidae and Balanidae, rep- resent the observed epibionts. Polychaete (Annelida), bivalve mollusks from two families: Pteriidae (Pinctada mazatlanica) and Mytilidae (Leiosolenus aristatus), and crustacean arthro- pods from the burrowing barnacle family Cryptophialidae (Cryptophialus wainwrighti) represent the observed endobionts. In addition, finding of Cryptophialus wainwrighti represents a new geographic range extension from the type locality in Sinaloa to Guerrero. Epibiosis studies of invertebrates in the intertidal rocky shore, such as the dominant C. articulatus, can assist in understanding ecological relationships and patterns of diversity in coastal communities. Additional Keywords: epibiosis, endobiosis, basibiont, Cirripedia, Chthamalus spp., Balanidae, Polychaeta, Bivalvia, Leiosolenus aristatus, Pinctada mazatlanica, Acrothoracica, Cryptophialus wainwrighti INTRODUCTION Common in aquatic habitats, epibiosis is the association between a living substrate organism (basibiont) and a sessile organism (epibiont) attached to the basibiont’s outer surface without trophically depending on it (Wahl, 2010). In endobiosis, an organism (endobiont) lives under the external surface of its basibiont (Wahl, 1989, 1997; Wahl and Mark, 1999; Trigui El-Menif et al., 2008; Wahl, 2010; see Taylor and Wilson, 2002 for a more complex terminology). In some studies epibiosis is included gen- erally as fouling (e.g., Mendez et al., 2014), biofouling (e.g., El Ayari et al., 2015), or without specific terminol- ogy (e.g., Buschbaum et al. 2007). Epibiosis is found worldwide, especially in marine environments, where any exposed solid surface is likely to be colonized by organisms (Wahl, 1989). Sessile organisms are the major constituents of these communi- ties (Canning-Clode and Wahl, 2010; Mendez et al., 2014). The basibionts more frequently studied are mol- lusks (Wahl and Mark, 1999; Wahl, 2010), especially those with economic importance such as gastropods and bivalves (e.g., see Table 19.2 in Du ¨rr and Watson, 2010). Epibiosis has been poorly documented for the class Polyplacophora, where epibionts and endobionts occur in/on the chiton valves. Arey and Crozier (1919) reported adventitious organisms on the dorsal surface of Chiton tuberculatus Linnaeus, 1758, including epizoic barnacles and algae, with other organisms living between the algae. Reports of chiton epibiosis have also been represented by pictures, such as in MacGinitie and MacGinitie (1968: 388, fig. 243) where Mopalia hindsii is pictured with its valves covered by algae and invertebrates. Bullock and Boss (1971) documented epibiotic calcareous algae, bryozoans, polychaete tubes, and the detrimental endobiont Leiosolenus aristatus (Dillwyn, 1817) boring into the valves of Chiton stokesii Broderip, 1832, in the southernmost part of the Panamic Province, and C. tuberculatus, from the Caribbean. Watters (1981) reported another eastern Pacific mytilid, Leiosolenus spatiosa Carpenter, 1857, in the valves of the chiton, Acanthochitona hirudiniformis (Sowerby I, 1832). Other epibionts reported on the valves of Chiton tuberculatus include species of the sessile barnacle genus Tetraclita Schumacher, 1817, calcareous tube-dwelling polychaetes, Spirorbis Daudin, 1800 and Serpula Linnaeus, 1758, and green algae including Ulva Linnaeus, 1753. The algae provide protection for juvenile mollusks, nematodes, archiannelids, and protozoans. Bullock and Boss (1971) did not consider any of the reviewed epibionts to be 1 Author for correspondence: letgopvd@gmail.com; Present address: Facultad de Ciencias del Mar, Universidad Auto ´ noma de Sinaloa, Mazatla ´ n, Sinaloa, Mexico. THE NAUTILUS 131(1):87–96, 2017 Page 87