Additional distribution and ecology records of the deep-water isopod Rocinela murilloi Brusca & Iverson , 1985 ( Isopoda , Aegidae ) in western Mexico

A series of specimens of the fish external parasite Rocinella murilloi Brusca and Iverson, 1985, is reported from 12 localities off western Mexico. These records are the first for southwestern Mexico (four) and add six new localities off the west coast of the Baja California Peninsula where the species was known from a single record. The specimens were recovered from bottomoperated nets, between 700 and 1235 m, slightly shallower than previous records. Environmental conditions confirm the presence of R. murilloi below the core of the Oxygen Minimum Zone, and this species appears to be highly tolerant to moderate to severe hypoxic conditions. General distribution of the species in the eastern Pacific is presented.


Deep-water isopod Rocinela murilloi off western Mexico
Nauplius, 26: e2018036 France, 1992), and there are large latitudinal intervals with single or no records, e.g., the SW coast of Mexico, south of the Gulf of California, off Central America except Costa Rica, and off Pacific Colombia and Ecuador.
During exploratory cruises off the west coast of the Baja California Peninsula and off SW Mexico, more material of R. murilloi was collected in 12 localities, thus extending our knowledge on the distribution and occurrence of this large species of Aegidae off western Mexico.Additional information on its ecology was also obtained.Two additional records from the Gulf of California are also included in this report.

Deep-water isopod Rocinela murilloi off western Mexico
Nauplius, 26: e2018036 Taxonomic remarks.The material examined fits well with the original description of the body of Rocinela murilloi by Brusca and Iverson (1985) and Brusca and France (1992): cephalon about twice as wide as

Deep-water isopod Rocinela murilloi off western Mexico
Nauplius, 26: e2018036 long; eyes large, dark, widely separated; antenna 1 short, antenna 2 extending to pereonite III; pereonite I the longest; coxae of pereonites V-VII extending posteriorly; pleotelson with a pair of submedian pits (Fig. 1).The original description (Brusca and Iverson, 1985) also included illustrations of the pereopods I, IV and VII, the maxilliped, the pleopods 1-5, and a dorsal view of the female holotype.Brusca and France (1992) completed the illustration series by adding drawings of the frontal margin of the carapace (ventral view), the mandible, the maxillule, the maxilla, the pereopod III, and the uropod (all from the holotype).SEM photographs of selected appendages and of the frontal portion of the carapace of R. murilloi (Figs.2-4) are provided here for the first time.The shape and ornamentation of pereopods I-III of the material examined (Fig. 2A, D, G) perfectly match the illustrations and descriptions provided by Brusca and Iverson (1985) and Brusca and France (1992), with the inferior margin of the propodus featuring a
The mandible of the examined material (Fig. 3A-F) also closely matches the description and illustrations provided by Brusca and France (1992), with a 3-articulated palp (Fig. 3A, B), the second article about twice as long as first, and with a distal row of about 10 serrate spines and 2 distal, single setae (Fig. 3B); the strongly projecting linguiform molar process (Fig. 3C) also matches the description, and is covered by regularly spaced rows of minute spines (Fig. 3D); the claw-like incisor is spoon-like, ending in a rather sharp tip (Fig. 3E), and the lobe between the molar process and the incisor (the lamina mobilis of Brusca and France, 1992) seems to be an isolated process surrounded by a field of numerous, sharp spinules (Fig. 3F).Brusca and France (1992) also provided an illustration and a SEM photograph of the ventral view of the frontal part of R. murilloi carapace, with a narrow, arrowhead shape frontal lamina that perfectly matches the material examined (Fig. 4A).
The surface of the pereopods I-III articles is covered by minute scales (Fig. 4B, C), not illustrated previously.

disCussion
Rocinela murilloi is a widely distributed deep-water aegid, ranging from California to Chile (Brusca and France, 1992;Hendrickx, 2008).It was reported for this geographical range by Espinosa-Pérez and Hendrickx (2001) but inadvertently omitted by the same authors for polar and temperate waters of the East Pacific (Espinosa-Pérez and Hendrickx, 2006).Hendrickx (2008) reported a total of 11 specimens from seven localities in the southern Gulf of California.The material examined in the present study amounts to two additional specimens from the southern Gulf of California (2 localities), 56 specimens from SW Mexico (4 localities), and 12 specimens from off the west coast of the Baja California Peninsula (6 localities) (Fig. 5).With a total of 81 specimens found in 19 localities, R. murilloi appears to be a widespread and relatively common species of fish parasite in deep-water off western Mexico.When considering new and previous records for this species (Tab.1), there is a clear lack of data for very wide areas in the eastern Pacific (Fig. 5), particularly between SW Mexico (17°45'16"N) and Costa Rica (ca.09°36.7'N), between Costa Rica and northern Peru (7°07'S), and between northern Peru and Chile (only two records in ca.18°40'S and 33°39'S).
The material reported by Hendrickx (2008) for the southern Gulf of California was collected at depths of 890 to 1274 m, with an epibenthic dissolved oxygen range of 0.20 to 0.76 ml/l, and a bottom temperature range of 3.6 to 5.3 ºC.Overall depth range of the additional material examined herein is 700 to 1235 m, thus slightly shallower than previously known (768 to 1866 m; Brusca and France 1992).The new material reported herein was collected in a lower dissolved oxygen concentrations range (i.e., 0.11-0.65 ml/l) and a slightly different temperature range (i.e.,

Deep-water isopod Rocinela murilloi off western Mexico
Nauplius, 26: e2018036  Hendrickx (2008).Sediment type at sampling localities, however, might not be directly related to the presence of the isopods, which are temporary external parasites of fishes.Present information and previous records (Hendrickx, 2008) indicate that R. murilloi is highly tolerant to oxygen depletion and is a member of the deep-water fauna inhabiting below the lower boundary of the Oxygen Minimum Zone (OMZ) occurring off western Mexico (see Hendrickx and Serrano, 2010).This OMZ is particularly wide, deep, and with anoxic core-values off the west coast of Baja California (Papiol et al., 2016) and off SW Mexico (Serrano, 2012).Although R. murilloi is presumably attached in a permanent way to their hosts and might therefore move over long distance over the sea floor, environmental conditions prevailing close to the bottom below the OMZ core, in a similar depth range, are considered stable, particularly in what concerns dissolved oxygen concentration and water temperature (Hendrickx and Serrano, 2010;Serrano, 2012).
Species of Rocinela are blood-sucking temporary parasites of marine fishes.They can easily detach from their host, swim and reattach to a new host.However, very little is known about the fish-parasite relationship (Brusca and France, 1992).Rocinela murilloi and other species of the genus may abandoned their host once they are captured in bottom trawls or slegdes (see Brusca and France, 1992;Wing and Moles, 1995), thus making the identification of hosts very difficult.Cruz-Acevedo et al. (2018) reported 50 species of bathypelagic and bathydemersal deep-water fishes between 300 and 2100 m off western Baja California, the area where most specimens of R. murilloi were Rocinella murilloiBrusca and Iverson, 1985  (Figs. 1-4) Figure 1.Rocinela murilloi Brusca and Iverson, 1985, female, 33.6 mm (ICML-EMU-9973), dorsal view.

Figure 5 .
Figure 5. Sampling localities in the eastern Pacific where Rocinela murilloi Brusca and Iverson, 1985 has been collected.

Table 1 .
Brusca and Iverson, 1985ords of Rocinela murilloiBrusca and Iverson, 1985in the eastern Pacific.N, number of specimens; LACM-AHF, Los Angeles County Museum of Natural History, ex-Allan Hancock Foundation, California; AMS, The Australian Museum, Sydney; SIO, Scripps Institution of Oceanography, California; USNM, United States National Museum, Washington D.C.Some closely set records overlap.

Table 2 .
Environmental data associated with the capture of Rocinela murilloiBrusca and Iverson, 1985 (present study).