The lancelets or amphioxi are small chordates with a cosmopolitan distribution, inhabiting shallow marine and brackish waters, mainly associated with coarse sediments such as sand, gravel, or shell material (^{Da Silva et al., 2008}, ^{Richardson and McKenzie, 1994}). Currently, lancelets are important for phylogenetic studies of vertebrates due to its phylogenetic proximity with the vertebrate clade (^{García-Fernández and Benito-Gutiérrez, 2009}, ^{Gee, 2006}, ^{Vargas and Dean, 2010}). The chordate family Branchiostomidae is composed of 2 valid genera: Branchiostoma and Epigonichthys. The genus Branchiostoma is the most diverse, with 23 valid species, 2 of them (Branchiostoma californiense Andrews, 1893 and Branchiostoma elongatum Sundevall, 1853) distributed throughout the Tropical Eastern Pacific (^{Del Moral-Flores et al., 2016}, ^{Poss and Boschung, 1996}). According to ^{Gill (1895)}, the species in this genus are characterized by bearing bilateral gonads, a rayed sympodium, low dorsal fin, and sagittiform expansion of the caudal fin membranes.

B. californiense has been previously recorded in the California coast, USA, the Gulf of California, Jalisco, and Oaxaca, Mexico (^{Beebe and Tee-Van, 1941}, ^{Del Moral-Flores et al., 2016}, ^{Hubbs, 1922}, ^{Poss and Boschung, 1996}), Corinto harbor, Nicaragua (^{Poss & Boschung, 1996}), the Gulf of Nicoya, Costa Rica (^{Vargas & Dean, 2010}), and Chame Point in Panama (^{Meek & Hildebrand, 1923}); the other species B. elongatum has been reported from Chile, Peru, and the Galapagos islands (^{Poss and Boschung, 1996}, ^{Snodgrass and Heller, 1905}, ^{Vergara et al., 2012}). Studies of lancelet communities in the Mexican Pacific region are limited. As part of a study on the diversity of the marine fauna of the Sanctuary of Bahía Chamela and the coast of Sinaloa, some specimens identified as B. californiense were collected and these represent records from the southeast area of the Gulf of California and the central Mexican Pacific.

Four specimens of B. californiense were collected from the sandy bottom of Bahía Chamela, Jalisco, in the central Mexican Pacific and 1 more from Isla Melendres, Sinaloa, in the southeast Gulf of California. The specimens from Bahía Chamela were collected in October 2013 from the shallow subtidal zone (5–8 m deep) in 3 different sites and the specimen from Isla Melendres was collected in August 2015. Sediment samples were taken with a rectangular biological dredge (26 × 44 cm, mesh size = 2.5 cm, and cod-end mesh size = 1.3 cm) conducted on lineal trawling for 5 min. All specimens were fixed in 5% formalin, later preserved in 70% ethanol and subsequently deposited in the invertebrate collection of the Laboratory of Marine Ecosystems and Aquaculture (LEMA, for its acronym in Spanish), Centro Universitario de Ciencias Biológicas y Agropecuarias, Universidad de Guadalajara (Zapopan, Mexico). Identification of specimens was based on the descriptions given by ^{Hubbs (1922)} and ^{Kirkaldy (1895)}.

Phylum Chordata Haeckel, 1874

Subphylum Cephalochordata Owen, 1846

Class Leptocardii Müller, 1845

Order Amphioxiformes Anonymous

Family Branchiostomatidae Bonaparte, 1846

Genus Branchiostoma Costa, 1834

B. californiense Andrews, 1893 (Fig. 1)

Figure 1 Lateral view of the external features of Branchiostoma californiense (LEMA-CCC2) from Bahía Chamela. (A) Head region; (B) trunk region; (C) tail region. 

Material examined. LEMA-CCC1 (1, 25.2 mm TL (total length)), 19°32′04″ N 105°05′05″ W, 7 m, Bahía Chamela, Jalisco, Mexico, 10 October 2013. LEMA-CCC2 (1, 28.1 mm TL), 19°33′20″ N 105°06′22″ W, 8 m, Bahía Chamela, Jalisco, Mexico, 10 October 2013. LEMA-CCC3 (2, 24.1–26.1 mm TL), 19°33′23″ N 105°06′20″ W, 5 m, Bahía Chamela, Jalisco, Mexico, 12 October 2013. LEMA-CCC4 (1, 40.1 mm TL), 24°57′16″ N 108°03′5″ W, 1 m, Isla Melendres, Sinaloa, Mexico, 6 August 2015.

Description. Body elongate. Ratio of body length to body depth 11.7 (10.4–13.2). Ratio of preatriopore length to postatriopore length 2.42 (2.2–2.6). Notochord extends forward of the oral hood and forms a well-developed rostral process. Buccal region with numerous fine cirri (up to 40), some of them longer than the tip of the rostrum. Rostral fin thickened and round termination. Myotome formula: preatriopore 43 (39–46) + atriopore-anal 17 (16–19) + postanal 9 (9–10) = total 69 (66–72). Dorsal fin of moderate height. Dorsal fin-chambers 364 (343–395). Preanal fin-chambers 58 (56–61). Ratio of body length to body depth 12.06 (10.4–13.36). Caudal fin with expansions of dorsal and ventral fins long and shallow. Anus located far behind middle of lower lobe of caudal fin. Color of body white and fins translucent.

Distribution. Coast of California, USA to Chame Point, Panama (^{Poss & Boschung, 1996}).

Although B. californiense is widely distributed and abundant along the tropical eastern Pacific (Fig. 2). The lack of information is due to the difficulty of collecting them because they have small size and live buried in sandy bottoms. This is the second record of lancelets for the coast of Jalisco in the central Mexican Pacific, previous record was collected in 1935 in Bahía Tenacatita (LACM-22314); since then it had not been reported again. A previous record in Sinaloa corresponds to 1 specimen collected in Bahía Mazatlán, near to the Isla Venados (^{Poss & Boschung, 1996}), approximately 250 km south of Isla Melendres (where the specimen of this work was found). Records of B. californiense are concentrated in 3 regions: southern California (USA), Gulf of California (Mexico), and Central America (from Nicaragua to Panama) with a great gap without records between the last 2 regions. Possibly the absence of records between the Gulf of California and Nicaragua is due to the lack of sampling effort on this group in this large area of over 2,800 km of coastline (^{Bastida-Zavala et al., 2013}, ^{Del Moral-Flores et al., 2016}, ^{Fuentes-Farías et al., 2005}).

Figure 2 Map displaying records of Branchiostoma californiense based on previously known material (black circles) and the present study material (black triangles). 

^{Poss and Boschung (1996)} recognized great morphometric variation among specimens of B. californiense but without a well-defined geographic pattern. As expected, specimens from Bahía Chamela and Isla Melendres are similar to specimens of the Gulf of California (Mexico) and Central America (Nicaragua to Panama) but show some differences from the records from California (USA). The most remarkable difference between specimens from the Gulf of California and those recorded in Bahía Chamela is in the number of preatriopore myotomes (42–47 and 39–46, respectively) (Table 1). There is also a tendency to increase the number of dorsal and preanal fin-chambers from the northern to the southern specimens recorded in Mexico. ^{Hubbs (1922)} measured less preanal fin-chambers (50 vs. 58) and presented lengths of mature specimens from 5.8 to 8.35 cm and immature specimens from 3.75 to 5.6 cm, whereas we found a mature individual of 2.61 cm. According to Hubbs, specimens from San Luis Gonzaga Bay, Gulf of California (collected by the Albatross), are not fully typical in the form of tail and fins, but differences may be due to the poor preservation of the gulf specimens. On the other hand, ^{Kirkaldy (1895)} reported 7.4 cm as the maximum size but did not report the minimum of specimens collected in California, USA. Specimens from Bahía Chamela are comparatively smaller than previous records from the California coast (^{Hubbs, 1922}) but are of similar size (5–37 mm) to those collected in Costa Rica (^{Vargas & Dean, 2010}). In this case, ^{Vargas and Dean (2010)} reported that smaller individuals probably represent recruits for this species. However, specimens of Bahía Chamela are mature individuals and the lower number of gonad pouches compared to previous reports is likely due to their smaller size. More sampling effort should be performed in the southern Pacific coast of Mexico (between Colima and Chiapas) and in Central America (from Guatemala to El Salvador) to confirm if there is a continuous distribution of the Californian lancelet and to evaluate morphometric differences among populations. Besides, a phylogenetic study of these populations is also necessary to determine if they belong to only 1 species or are a complex of species such as was recently recorded for Asymmetron lucayanum (^{Kon et al., 2006}, ^{Kon et al., 2007}).

Thanks to Cristina Zuñiga, Valentina Fernández, Thalía González, Diego Moreno, Abel Trejo, and Cande Hernández for their help during sampling work. To Jorge Vega and all the personnel in the Biological Station of Chamela for access to the facilities. We are grateful to Adriana Reyes for taking pictures and to Stefanie Kaiser for comments on the final version of the manuscript. This work was supported by the Comisión Nacional para el Conocimiento y Uso de la Biodiversidad (Conabio) (grant number JF023) and the University of Guadalajara (grant number P3E2014).