Subphylum Cephalochordata includes marine and estuarine organisms known as lancelets (Branchiostomidae Bonaparte 1846), which inhabit sandy-muddy bottoms sometimes mixed with shell particles, as well as coarser sediments (^{Vargas & Dean, 2010}). Currently there are about 29 species of cephalochordates that live in temperate and tropical marine environments in shallow and deep areas all over the world and usually experience a planktonic early stage and later becomes part of the benthic community (^{Subirana et al., 2020}). These species are important for studying the evolution of chordates and coastal human communities harvest them as food in shallow soft bottoms in China and other Asian countries (^{Karleskint et al., 2013}).

The presence of Branchiostoma californiense Andrews 1893 in the eastern Pacific has been documented in different marine areas of Mexico (^{Campos-Dávila et al., 2019}; ^{Galván-Villa et al., 2017}; ^{Velázquez-Velázquez, 2021}). Also, its occurrence was documented in Nicaragua (^{Poss & Boschung, 1996}) and Costa Rica (^{Vargas & Dean, 2010}). Moreover, the genus Branchiostoma was registered at La Unión Bay sediments (^{Hartmann, 1959}) and as planktonic larva at Los Ranchos area, Jiquilisco Bay (^{Galdámez, 2014}). Probably it was B. californiense in both cases. The present study documents and confirms the existence of this species within two estuaries in El Salvador.

La Unión Bay is an important estuarine body located within the Gulf of Fonseca on the eastern part of El Salvador. It receives important freshwater discharges during rainy season and in the dry period salinities are higher than the adjacent ocean due to oceanographic processes (^{Valle- Levinson & Bosley, 2003}). These salt concentrations causes that mangrove trees reach low heights (^{Barraza, 2017}). Jiquilisco bay is located on the southeastern part of El Salvador and was declared Ramsar site in 2005. Salinity varies from 0 to 34 parts per thousand approximately, according to season and area. It is the largest estuary within the country and species richness is high. There are some touristic activities in the area.

Both bays present important biodiversity that sustain fisheries and shellfish extraction as well as habitat for permanent and migrating species including endangered ones, such as sea horses (Hippocampus ingens), hawksbill turtle (Eretmochelys imbricata), crocodile (Crocodylus accutus) and others.

Specimens were collected during different benthic surveys at Jiquilisco and La Union Bays (Fig. 1.) in April 2018 and November 2019, respectively. These organisms were preserved in ethanol 90% in labeled jars. Size parameters were determined using a caliper (+ 0,01mm accuracy). Photographs were taken using a stereo microscope (LEICA EZ4E). Morphological characteristics were compared to the description of ^{del Moral-Flores et al. (2016)} allowed species confirmation. Two specimens were deposited at the Natural History Museum of El Salvador with codes 40 -1095 MUHNES and 40 - 1096 MUHNES.

Fig. 1 Sampling áreas in El Salvador: Puerto El Triunfo, Jiquilisco Bay (J) and Perico island at La Unión Bay (U). 

The taxonomic data of this species is detailed below:

Material examined. Seven specimens were collected with a Petite Ponar Grab at Puerto El Triunfo, Jiquilisco Bay (13º 15’ 58.3” N, -88º 33’ 8.2” W) from two samples of sediments at a depth of 7 meters in April 2018. Length oscillated from 14,9 to 34,2mm (average (= 24,8; SD = 6,8). Also, three specimens were taken from intertidal mixed sediments at Perico island, La Union Bay (13º 15’ 59.1” N, -88º 33’ 8.2” W) during November 26th, 2019. Length varied from 24,8 to 28,6mmm ( = 27,0; SD = 2,1mm). In both cases sediments were sandy-muddy. Figures 2 and 3 present respectively a fresh specimen collected at Perico island, and the sand flat of the same island.

Fig. 2 Specimen of B. californiense collected at a sand flat at Perico island, La Unión Bay. Length: 12,5mm (each square: 3mm). 19-October-2020 

Fig. 3 Sampling place (sand flat) at Perico island 

Description. Body white, elongate and thin. Presence of notochord. Rostral process evident on the anterior zone. Oral area with segmented cirri (30-39). Rostral, dorsal, and ventral fins present. Gonad pouches in the ventral region (26-31). Caudal fin as extension of the latter two. Whitish fins (Figures 4, 5 and 6, Specimen 40 -1095 MUHNES).

Fig. 4 Anterior part of Branchiostoma californiense 

Fig. 5 Central part of the body of B. californiense showing gonad pouches. 

Fig. 6 Posterior area of B. californiense 

Distribution. From California (USA) to Panama in sandy or mixed sediments (^{Poss & Boschung, 1996}).

Previous documentations about lancelets for El Salvador included specimens collected from soft bottom at La Unión Bay and a planktonic larva at Jiquilisco Bay (^{Galdámez, 2014}; ^{Hartmann, 1959}; Galdámez, 2014) respectively, therefore this third record allowed to confirm the presence of B. californiense in the same places in intertidal and subtidal muddy sand. ^{Galván-Villa et al. (2017)} mentioned that this species presents a wide distribution in the eastern tropical Pacific and suggested more sampling efforts in northern Central America.

Our data proved that this species present a continued occurrence from Mexico to El Salvador, and south to other central American countries (^{Meek & Hildebrand, 1925}; ^{Poss & Boschung, 1996}; ^{Vargas & Dean, 2010}). Although its occurrence in Guatemala requires corroboration.

This species is not used as food for humans or domestic animals in El Salvador, as in other world regions (^{Karleskint et al., 2013}). Probably it is an important energetic source for other marine fauna, such as benthic fishes (^{Ruppert et al., 2000}) in this tropical area.

Other fauna present in Jiquilisco Bay sediment samples were mostly polychaetes. Similar trend occurred in the sandy sediments mixed with mud and fragmented shell from Perico island, where also small crustaceans and gastropods were abundant.