Acanthistius (Anthiinae) is distributed in the Indo West Pacific, Southeast Pacific and South Atlantic Ocean. The genus is constituted by 13 species, of which only Acanthistius pictus (Tschudi 1846) is recorded in Chile. A. pictus is an endemic serranid of the Eastern Pacific coast distributed from southern Perú (16°S) to north of Chile (26°S) (^{Pequeño et al. 2011}). A. pictus is a rocky reef fish of shallow waters (5-30 m), that lives associated to caves and Lessonia trabeculata kelp forests (^{Cisternas & Sielfeld 2008}). It is a carnivorous species that feeds mainly on crustaceans (^{Medina et al. 2004}). It is also an economically important species which supports small-scale artisanal fishing activities in the north of Chile (^{Godoy 2013}).

The first record of A. pictus for Chile was conducted by ^{Tschudi (1845}), who determines that its distribution extends between 20°S and 26°S. The 17 specimens that are preserved in London (MNHN) and Hamburg (ZMH) museums were collected exclusively in northern Chile (^{Fish base 2015}), consistent with the first record. In this note, we report the southern expansion range of A. pictus from 26°S to 34°S, in Chilean waters.

In March of 2012, a recreational spearfisher caught one specimen of A. pictus at 5 m depth in the coastal area of Pichilemu (34ºS 72ºW). The specimen was frozen whole and transported to laboratory of Pontificia Universidad Católica de Chile. The diagnosis features used for specimen identification was according to ^{Pequeño et al. (2011}). The main features considered were: Head: Premaxilla, dentary, vomer and palatine with a thin band of villiformes teeth (always less than 3 rows of teeth); not abruptly expandable jaw; more than 18 pseudobranchs; less than 90 sawing in preopercle; one antrorsa thorn in the ventral edge of preopercle. Fins: caudal fin truncated or forked; less than 21 pectoral rays; third, fourth and fifth pelvic radios unfused; last dorsal radius unstretched; caudal fin with unfused radios; more than 11 dorsal spines. Morfometrics: preopercular distance greater than 5% of standard length, head length; more than 28% of the standard longitude; orbital diameter more than 10% head length; pelvic fin length of less than 35% of standard length; length of anal fin less than 20% of standard length. Scales: more than 90 scales in rows on the sideline; interorbital distance more than 6 scales; suborbital distance with more than five scales. Total length (LT) and total weight (TW) were recorded. The specimen was preserved in alcohol and deposited in the "Colección de Flora y Fauna, Profesor Patricio Sánchez Reyes, Pontificia Universidad Católica de Chile (SSUC-Pi 0087)". The determination analysis showed that the sample examined corresponds to an adult specimen of A. pictus with a total length of 38.2 cm and total weight of 1061 g. This result extends the distribution range of A. pictus from 26°S to 34°S (Fig. 2).

Field and anecdotal observations suggest that the southern distribution of A. pictus would extend to Valparaíso (33°S) (^{Mann 1954}; ^{Chirichigno 1974}; ^{Moreno & Castilla 1982}; ^{Ojeda et al. 2000}), but these studies do not detail the sites of capture and collection registered. We consider that Pichilemu (34°S) is the southernmost documented record for A. pictus. A. pictus is a component of rocky reef fish assemblage which showed links to more septentrional regions (^{Ojeda et al. 2000}; ^{Rojas & Pequeño 2000}). This finding adds to recent records of other three rocky reef fish species Cheilodactylus variegatus, Chromis crusma and Graus nigra which have been reported to extend their distributions to Valdivia (39°S) (^{Pequeño 2001}, ^{Vargas and Pequeño 2001}). Different hypotheses have been proposed to explain this trend: a) the limited exploration of the coastal Chile, referring to the geographical distribution of coastal rocky reef fish and; b) the septentrional invading fish species (^{Mann 1954}) due to climate change or during warm El Niño Southern Oscillation events (^{Vargas & Pequeño, 2004}). Unfortunately, subtidal ecological information in Chile is scarce, which for the moment makes it impossible to assess these different hypotheses about the determinants of distribution change, but it is possible to infer an interesting pattern on the southern extension of the distribution of rocky reef fish species with northern affinities. Such evidence is relevant when ichthyogeographical considerations are proposed.

FIGURE 1: Acanthistius pictus SSUC-Pi 0087. FIGURA 1: Acanthistius pictus SSUC-Pi 0087 

FIGURE 2: Acanthistius pictus (1) distributional range by ^{Tschudi (1845}) and 2: extended range by present work. FIGURA 2: Acanthistius pictus (1) rango de distribución por ^{Tschudi (1845)} y 2: rango extendido en el presente trabajo.