Comments on Microhabitat Specialization and a Depth Range Extension for a Chaenopsid Tube Blenny in the Gulf of California, Mexico

Acanthemblemaria balanorum Brock (Clubhead Blenny; Fig. 1) is a chaenopsid tube blenny endemic to the Tropical Eastern Pacific. Adults of this species, as all members of the Chaenopsidae, inhabit vacated invertebrate tubes or tests (Stephens 1963; Lindquist 1985). In the case of A. balanorum, the shelter of choice is the vacated test of Megabalanus Hoek barnacles, a genus characterized in the Tropical Eastern Pacific by a complex of species (Henry and McClaughlin 1986) that typically live in the upper 10 m on shallow rocky reefs (Brusca and Hendrickx 2008). In the Gulf of California (GOC), Mexico, A. balanorum overlaps in distribution with two congeners, A. crockeri Beebe and Tee-Van and A. hastingsi Lin and Galland, and these species are known to exhibit depth partitioning, with A. balanorum inhabiting relatively shallower depths, A. crockeri inhabiting relatively deeper depths, and A. hastingsi overlapping near the edges of the depth ranges of the other two species at intermediate depths (Lindquist 1985). A detailed study of the relationships among these three congeners in the southern GOC (Lindquist 1985) reported that A. balanorum inhabits shelters (5barnacles) down to approximately 7 m depth. Guides to the fishes of the region (e.g., Allen and Robertson 1994; Humann and DeLoach 2004) report a similar depth range. In November 2010, I observed and collected several individuals of A. balanorum at a depth of 21 m at the base of a pinnacle off the south end of Maria Cleofas, the southernmost point in the Islas Marias archipelago, southern GOC. These individuals, like all individuals of this species that I have observed, inhabited vacant barnacles (Megabalanus). This observation represents a significant depth range extension for this normally shallow subtidal fish (and may also represent an extension for the barnacle; Brusca and Hendrickx 2008). This ability of a microhabitat specialist to colonize abnormal macrohabitats (in this case much deeper than normal waters) when its microhabitat (5barnacles) is available supports a hypothesis that these specialists are resource (5shelter) limited. Similar shelter limitation has already been experimentally demonstrated in the GOC congener, A. crockeri, which increases in average density with shelter addition (Hastings and Galland 2010). I observed additional evidence that A. balanorum is a shelter-limited microhabitat specialist in July 2009 at Las Animas, a small island and a series of small pinnacles in the central GOC. That site proved to be ideal A. balanorum habitat, with several large boulders completely covered by broad, very dense Megabalanus fields down to 5 m depth. Within these barnacle fields, I observed large numbers of A. balanorum, more densely distributed than any other chaenopsid population reported to date (e.g. in Lindquist 1985; Clarke 1996; Thomson and Gilligan 2002; P.A. Hastings, pers. comm.). During an opportunistic survey of the area, I placed a 0.25 m quadrat on five randomly selected areas on the top of a large, flat boulder at 5 m depth in order to survey chaenopsids and Bull. Southern California Acad. Sci. 110(2), 2011, pp. 52–55 E Southern California Academy of Sciences, 2011