Isopods are widely distributed in all environments, especially in aquatic systems; their distribution includes oceans as well as estuarine environments (^{Blake & Scott, 1994}). Neastacilla californica (^{Boone, 1918}); originally described as Astacilla californica, has a distribution range in Southern California (^{Boone, 1918}, ^{Menzies & Barnard, 1959}; ^{Stebbins, 2012}) and has also been reported in the Gulf of California, Mexico (^{Hendrickx et al., 1997}; ^{Espinosa-Pérez & Hendrickx, 2001}, ²⁰⁰⁶).

Information presented in this short communication comes from a baseline monitoring study of benthos conducted in Guerrero Negro Lagoon 28.3°N, 114.4°W (Fig. 1) that occurred monthly from July to December in 2012. Benthic samples were obtained at ten stations by scuba divers by using a 0.5×0.5 m quadrat (four replicates per station).

Figure 1 Black dots indicate the distribution reported of Neastacilla californica (^{Boone, 1918}) on the Pacific coast and in the Gulf of California. 1 Topolobampo Bay, 2 Roca Consag. Black star shows the collection point of the sample presented in this study. 

All samples were kept on ice, transferred to 10% formaldehyde solution, and preserved in 70% ethanol for later taxonomic identification. Schematic drawings and photographs were made using a dissecting stereoscope equipped with a camera lucida and a microscopic camera. A comparative table using diagnostic characteristics published for Neastacilla californica and the specimen collected in this study was assembled.

The sample was collected at 4.3 latitudinal degrees from the southernmost Pacific coast reports and corresponds to a female of Neastacilla californica (^{Boone, 1918}) with 7.4 mm total length (TL) (Figs. 2a-2c), collection date July 2012, locality Guerrero Negro Lagoon, BCS Mexico, 28°3′3.3”N, 114°4′57.03”W (Cat. No. CEFACIB-37).

Figure 2 Neastacilla californica (^{Boone, 1918}) female from Guerrero Negro Lagoon, Mexico [CEFACIB-37]. a) Female dorsal view, b) dorsal view, c) right lateral view, d) angular slit between the head and PI (detail), e) details of epibionts (presumably foraminiferous). P: I-III, pereonites I, II and III, PIV: pereonite IV, PV-PVII: pereonites V, VI, and VII, forming the pleon; *Conical protrusion on the head, **Conical protrusion bilobulate on PIV. 

N. californica is distinguished by the fusion of the first pereonite (PI) to the head, and it has a small and angular slit on its side that follows the eyes (Fig. 2d), in contrast to N. falclandica (type species), which has a partial groove without a real lateral slit (^{Lew-Ton & Poore, 1986}).

The subsequent pereonites, PII and PIII, gradually increase in length and width; the fourth pereonite (PIV) is considerably longer than the others (2.4 mm approximately the 33% of TL), similar to N. falclandica according to ^{Lew-Ton & Poore (1986)}, and its anterior part is wider than its posterior part (1.6 and 0.8 mm, respectively); the pleon is formed by the fusion of pereonites PV, PVI, and PVII. Pereopods PI-PIV are rigid and front-facing and have abundant setae for helping with filter feeding (dactyls PI-PIV are clawless); pereopods V-VII are sturdy and short, indicating ambulatory function (PV and PVI dactyls with claw).

Remarks: no corporal ornamentation is observed, except for a conic protrusion in the middle of the head and a conic bilobulate protrusion in the middle of the pereonite IV. A comparison of the main diagnostic features of Neastacilla californicacorporal sections between published information and the found specimen is shown in Table 1, attempting to be as descriptive and specific as possible because although some features are probably not indicative of the species, they are for this particular specimen.

Since the oral structures have not been described in detail, and because all of the structural features are key points that allow strengthening its taxonomy, continuing with the morphological description of Neastacilla californica is recommended.