BULLETIN OF MARINE SCIENCE. 60(2): 389-395, 1997 ONUPHIDAE OF THE GULF OF CALIFORNIA AND THE GULF OF TEHUANTEPEC, MEXICAN PACIFIC Laura Gonzalez-Ortiz, Pablo Hernandez-Alcantara and Vivianne Solis-Weiss ABSTRACT On the continental shelf of both the Gulf of California (GC) and the Gulf of Tehuantepec (GT), the Onuphidae is one of the best represented families, both in terms of abundancc and frequency. Sampling was done on board the R(V EL PUMA with a Smith McIntyre grab at 93 stations (41 in GC and 52 in GT) at depths between 20 and 220 m. A total of 1294 onuphids (765 in GT and 529 in GC) were collected at 56 stations (78% in GC and 46% in GT), We identified 25 species (6 in GT and 25 in GC). Diopatra ohliqlla was the most abundant spe- cies in the Gulf of Tehuantepec with 723 specimens present at 15 stations. In the Gulf of California, the most abundant was Killhergollllphis Pli/chra (134 in 6 stations), and Moorcol1l1- phis elsial' was the most frequent (46 in 16 stations). The division of this fauna in biogeo- graphic categories showed that it is predominantly amphiamerican. The family Onuphidae has been reported from all oceans and at all depths around the world (Glemarec, 1991). Although several genera have been found to be widely distributed worldwide and from shallow to abyssal waters, most of them are geographically and bathymetric ally restricted (Paxton, 1986). The family is best represented in the Southern Hemisphere, maybe due, according to Paxton (1986) to a dispersal center located there. About 220 species of onuphids worldwide have been recorded in 19 genera (Paxton, 1986; Orensanz, 1990). In the Mexican region of the Pacific Ocean, 11 genera and 56 species have been reported (Hernandez-Alcantara and Solfs-Weiss, 1991). In the Mexican Pacific, the Onuphidae is one of the best represented families in terms of species richness. However, their abundance and the percentage they represent in relation to the remainder of the polychaete fauna varies geographi- cally. For example, in the Mazathin zone (southwestern region of the Gulf of Cali- fornia), 11 species, which constituted 23% of the polychaete fauna, have been recorded (Arias-Gonzalez, 1984). In the continental shelf of Sinaloa (southern Gulf of California), 10 species representing 11 % of the fauna were collected; in the northern region of the Gulf of California, the five species found represent 7% of the polychaete fauna (Sarti-Martinez, 1984). In the Jalisco littoral, 11 species rep- resenting 7% of that collection have been recorded and on the Salina Cruz coasts (Gulf of Tehuantepec), five species represent 13% of the polychaetes collected (Mondragon, 1992). With the increasing current sampling effort, a wealth of information has been provided about groups of species of different zoogeographic origin coexisting in the same latitudes but under different environmental conditions. In this study, it is our aim to supplement the existing information about the biogeography of the onuphids from the Mexican Pacific, by analyzing the latitudinal and bathymetric distribution in two of the most important marine regions of Mexico: the Gulf of California and the Gulf of Tehuantepec. STUDY AREA The Gulf of California is considered to be the most important evaporation basin in the Pacific Ocean (Roden and Emilsson, 1979). It is located in the northern region of the Mexican Pacific (20°30'- 389