“LOOKING FOR A HOME”: THE ECOLOGICAL ASSOCIATION BETWEEN THE CHITONS Ischnochiton striolatus (GRAY, 1828) AND OTHER MOLLUSCS SPECIES

The present note reports, for the first time, species of Polyplacophora found as inquilines in the gastropod Tubinella laevigata as well as documenting the same relationship with other species of mollusks in the Brazilian coast.

Polyplacophora are molluscs that occupy a wide variety of habitats such as sandstone reefs, rocky shores, seaweeds (Otaíza & Santelices, 1985;Moreira;Chapman & Underwood, 2007), marine angiosperms (Barros et al., 2013), from intertidal to the supratidal zones, places that provide protection against predators and desiccation (Schill et al., 2002). Nevertheless, the relationships these animals as inquilines is not well documented. Even though Boyle (1970) reports the adherence of Sypharochiton pelliserpentis (Quoy & Gaimard, 1835) and Acanthochiton zelandicus (Quoy & Gaimard, 1835) within and between the edges of oyster shells, data on the association between Polyplacophora and other molluscs is still scarce in the literature.
The present note reports the Ischinochiton striolatus (Gray, 1828) species of polyplacophoran found as an inquiline in the gastropod Tubinella laevigata (Anton, 1839) for the first time (Fig. 1A) through a year-long monitoring at Baixa Grande beach in the Northeast of Brazil. This note also reports punctual observations of the aforementioned Polyplacophora associated with the following molluscs:   The survey was carried out on Baixa Grande beach, in the Northeast of Brazil (4º55'45.37" S 37º4'45.12" W) between January 2015 and June 2016. The methodology employed was that of visual survey, where three researchers carried out an intense visual search for the gastropod T. laevigata on a sandstone reef (Area = 28.500 m²; Perimeter = 885 m). During low spring tides some sandstone reefs become fully exposed, which facilitates sampling.
The species T. laevigata was chosen because it is the most common gastropod (Beatriz C. Lopes, personal observation) and it is easy to visualise due to its contrasting colour (between cream and white) against that of the sandstone reef. After being located, T. laevigata specimens were examined in search of chitons inhabiting their shells. After this procedure, all animals were replaced exactly where they had been found and the number of chitons was recorded.
In total, 179 specimens of T. laevigata were found, of which, 45 presented at least one specimen of I. striolatus on their shell. The number of I. striolatus ranged from 1 to 4 per gastropod with a mean of 1.31 ± (0.69). On average, 25.14% of the T. laevigata found during the survey had chitons occupying their shells (Fig. 2). Although consolidated sediment is reported by several authors as the preferred habitat by these animals (Joger et al., 2008;Smith & Otway, 1997), chitons had not been found in malacofauna surveys associated with consolidated sediment in the region (Fig.  1E), this is possibly explained by I. striolatus preference for medium sized pebbles (Rodrigues & Absalão, 2005). Another possibility is related to the three hypothesis for polymorphic distribution mechanisms and the use of habitat by chitons elaborated by Mendonça et al. (2014): presence of algae that are part of the chiton's, use of rock substrate as shelter, and shape and colour of the rock substrate. We have made suggestions as to what kind of relationship was taking place between these molluscs. The following observations lead us to the conclusion that the association in question is inquilinism: 1) Although this relationship can be classified as commensalism, because there is a species that benefits (I. striolatus) from it, without causing damage to its host. The term commensalism, described by Van Beneden (1869) means "table companion". Therefore, such relationship is motivated by, or associated to the search for food. In this study, however, the host gastropods are carnivorous and the I. striolatus species feed on algae and diatoms, scraping the epibenthic film formed on the substrate (Piercy, 1987;Langer, 1983). Thus, this association is not motivated by the search for food, being a non-commensal relationship. 2) The frequent association of Polyplacophora with other molluscs may be motivated by the necessity of a suitable substrate for adhesion. On the beach where the survey took place there is no smooth surface, which would be ideal for a better adhesion. Chitons of the genus Ischnochiton are habitat-specialist, having specific requirements, such as pebble surfaces, in their choice of habitat (Grayson & Chapman, 2004;Rodrigues & Absalão, 2005).
3) The hosts' shell may serve as protection against predation and light. The host species have a habit of burying themselves in the substrate, and Polyplacophora have negative phototaxis (Kaas, Van Belle & Strack, 2006).
Inquilinism, a term coined by Gudger (1932), is defined as the regular association between different species that live together with no detriment to either of them, and without the implication of interdependence or parasitism (Fraaye & Jäger, 1995;Landman et al., 2014). The same type of association was recently found among native Brazilian gastropods (Lima et al., 2016): Stramonita brasiliensis (Claremont & D. Reid, 2011) associated with Pugilina tupiniquim (Abbate & Simone, 2015).
Polyplacophora are specialised in using pebbles as a microhabitat, but can adhere to other types of substrate in their absence. Compared to other intertidal or subtidal Beatriz C. Lopes, Ellano J. da Silva, Inês X. Martins environments, these molluscs have been found to be more frequently associated with pebbles (Grayson & Chapman, 2004). However, Barros et al. (2013) have observed chitons adhered to the marine angiosperm Halodule wrightii, rather than to the rocky substrate, and linked this to the fact that H. wrightii banks work as a feeding and nursery area. Mollusc shell is likely to work as an adequate substrate for Polyplacophora when their preferred substrate is not found in the environment.
Although these animals play an important role in the intertidal environment, studies on Polyplacophora are still scarce, especially in Northeast Brazil. This note sheds light on a relationship that is commonly observed, but only now documented. It is still necessary to investigate, in depth, which factors actually motivate I. striolatus to become an inquiline of other molluscs, using, for example, tests of preference for different substrates and micro biofouling associated with the host molluscs and their role in food intake by the Polyplacophora.