Littoral caprellids (Crustacea: Amphipoda) from the Mexican Central Pacific coast, with the description of four new species

The caprellids of shallow-water localities from the Mexican Central Pacific coast are investigated. The Mexican Pacific coast is poorly known, unlike more northern sites such as the California coast where c. 40 species have been reported. Hence, this is the first study dealing with the caprellidean fauna of this area. Seven species in three genera were found (four of which are new to science): Aciconula acanthosoma Chess, 1989; Caprella equilibra Say, 1818; Caprella mendax Mayer, 1903; Caprella pitu sp. nov.; Liropus isabelensis sp. nov.; Paracaprella carballoi sp. nov.; and Paracaprella isabelae sp. nov. All the species are fully illustrated. http://zoobank.org/urn:lsid:zoobank.org:pub:B04D3837-E7E1-4DA5-A8ED-28CA7BF1E1AF


Introduction
Caprellids (Crustacea: Amphipoda) are small marine crustaceans that inhabit algae, hydroids, ascidians, anthozoans, bryozoans, sponges, sea grasses and sediments (McCain 1968;Guerra-García 2001). They feed mainly on detritus (Guerra-García and Tierno de Figueroa 2009) although some caprellid species can prey on other organisms (such as copepods or other amphipods) or graze on epibiotic fauna and flora. Many fish feed on caprellids (Caine 1991) and, therefore, caprellids are being considered as a potential resource in aquaculture (Woods 2009). Furthermore, caprellids are useful as bioindicators of marine pollution and environmental stress (e.g. Takeuchi et al. 2004). In spite of interest in them, knowledge of caprellids in some geographical areas, especially in central and South America, is still scarce (McCain and Steinberg 1970).
In contrast to the North American Pacific coast, the caprellidean amphipods of the Eastern Tropical Pacific have been poorly studied (García-Madrigal 2007). In this area, only six species have been cited of which two are deep species (Abyssicaprella galatheae McCain, 1966 andCaprella ungulina Mayer, 1903 from Costa Rica and Peru and the Galapagos Archipelago) and four are littoral species (Caprella californica Stimpson, 1857, from British Columbia to Coquimbo, Chile; Caprella equilibra Say, 1818 and Caprella scaura Templeton, 1836, cosmopolitan species;Paracaprella barnardi McCain, 1967, from the west coast of Panama). However, c. 40 species have been found from California to Alaska (Dougherty and Steinberg 1953;Laubitz 1970;Laubitz and Lewbel 1974;Martin 1977;Chess 1989;Watling and Carlton 2007;Guerra-García and Hendrycks 2013).  Journal of Natural History 81

Remarks
The specimens are in agreement with the original description by Chess (1989) with material from Santa Catalina island (California). Typical features of the species are present such as all pereonites dorsally spinose or basis of gnathopod 2 with acute distolateral projection. However, the studied specimens have some intraspecific variation in the head projection pattern compared with the four prominent curved projections from the material type: two anterior prominent and two posterior reduced ones (about one-third of the length of anterior ones). In some specimens from Mazatlán (St1 and St2), one or two posterior projections can be absent or very reduced. The length is slightly smaller in Mexican A. acanthosoma (male to 5.5 mm, female to 4.5 mm) than in the type material (male and female to 7.3 mm and 6.3 mm, respectively). Additionally, pereonites 3 and 4 have fewer lateral spinose projections.
Other generic characters are present such as the six-articulate pereopod 5, elongate, with long setae (although slightly less setose than in the original description) and the distal article reduced to a small cone, instead of the typical dactylus.

Habitat
Aciconula acanthosoma was found attached to different species of Thecata and Athecata hydroids, gorgonians (Leptogorgia rigida, Leptogorgia peruviana, Pacifigorgia sp., Pacifigorgia cf. agassizii, Muricea sp., Muricea cf. californica), a bryozoan Bugula sp., and the seaweed Zonaria cf. farlowii with epiphytic hydroids. Chess (1989) collected specimens from different substrata but they were more abundant on seaweeds (e.g. Cystoseira neglecta, Sargassum palmeri or Zonaria farlowii). In the present study, however, they were more abundant on sessile animals, being absent in most of the sampled seaweeds. Although in temperate ecosystems the highest densities of caprellids can be found in seaweeds (Guerra-García 2001), in the tropical region caprellids are mainly associated with hydroids and secondarily with gorgonians and other corals (Guerra-García 2006; Scinto et al. 2008). The unique available seaweed as substrate, Zonaria cf. farlowii, had a high coverage of hydroids which could indicate a strong relationship between A. acanthosoma and cnidarians. According to Chess (1989), A. acanthosoma feeds on sponges and ascidians. Recently in a study on feeding habits of Pacific Mexican caprellids, Alarcón-Ortega et al. (2012) reported that this species feeds mainly on detritus (75% of the gut content), crustaceans (17.5%) and hydroids (6.7%). In agreement with Guerra-García and Tierno de Figueroa (2009), the caprellid species with molars in mandibles (such as genus Aciconula) are characterized by a diet mainly based on detritus, although in general caprellids are rather opportunistic because they can change their feeding habits depending on the substrates to which they cling.

Remarks
The morphology of the specimens observed is in agreement with the description and figures included in Krapp-Schickel (1993) for Mediterranean specimens. This cosmopolitan species has a distinctive ventral projection between gnathopods 2, head with rectangular rostrum, gnathopod 2 basis robust and short with high and denticulate anterior carina. The largest male length found during the present study was 10 mm in length. Taking into account that Krapp-Schickel (1993) found specimens up to 20 mm, presumably our specimens are subadults. However, this variation could also be attributed to environmental variation between different geographical regions. Three species with similar characteristics have been described from the North American Pacific coast: Caprella equilibra, Caprella mendax Mayer, 1903 andCaprella pilidigita Laubitz, 1970. All of them show the ventral projection between gnathopods 2. According to Laubitz (1970), C. pilidigita is clearly different from the others, especially based on the structure of gnathopod 2 (dactylus with hairs along inner margin, propodus three times as long as broad, basis one-third of total appendage) and the setation of antenna 1. Caprella mendax was described by Mayer (1903) but its distinction from C. equilibra is very tenuous (some authors such as Dougherty and Steinberg (1953) and Stoddart and Lowry (2003) consider C. mendax to be a synonym of C. equilibra). In fact, our specimens showed characteristics of both species as summarized in Table 2.
Widespread species such as C. equilibra can show natural morphological variation in their range or may encompass cryptic species. Disentangling species boundaries and/or cryptic species is a common issue for a great number of marine groups and especially for crustaceans (Cabezas et al. 2013, and references therein). In this sense, Cabezas et al. (2013) provided the first evidence for cryptic species in Caprellidae with a study of the cosmopolitan species Caprella penantis Leach, 1814, and supporting evidence that this species is a complex of at least four species. However, another study on the exotic Paracaprella pusilla Mayer, 1890 has demonstrated that this species exhibits great genetic uniformity throughout its distribution range (Ros et al. 2013). In the case of C. equilibra, further studies are necessary to clarify this issue. Due to this and the presumable absence of superadult males, the material examined has been assigned to C. equilibra despite a certain similarity with C. mendax, such as the length of pereonite 5 larger than pereonite 4 or antenna 1 length.   Habitat Caprella equilibra has been found on a great variety of substrates such as seaweeds, hydroids, bryozoans, sponges or ascidians from the intertidal to 3000 m deep (Krapp-Schickel 1993). In the present study, C. equilibra was more abundant on hydroids and bryozoans, scarce on gorgonians (Pacifigorgia cf. agassizii) and absent on seaweeds. Despite its worldwide distribution and ubiquity of substrates, it has been only found in Isla Isabel within our study area. Guerra-García and Tierno de Figueroa (2009) included C. equilibra as detritivorous predators based on a high percentage of detritus in the gut content (> 80%) besides hydroids (> 7%) and copepods (> 6%). More recently, Alarcón-Ortega et al. (2012) showed with material from the present study that the gut content of C. equilibra was approximately 40% of both detritus and hydroids, so that the substrate is not only used as habitat but can also be a source of food.

Remarks
Typical features of the species are present such as body smooth, a small projection at the base of each gnathopod 2 or length of pereonite 5 larger than pereonite 4. Mouthparts typical of genus. Antenna 1 longer than pereonites 1, 2 and 3 with 16 articles in flagellum. Gnathopod 2 inserted posteriorly on pereonite 2; basis shorter than half pereonite 2 and one-quarter of total appendage length, with an anterior denticulate carina; propodus twice as long as broad, palm with one proximal grasping spine, and poison tooth and triangular projection distally. Pereopods 5, 6 and 7 increasing in length, with propodus with proximal grasping spines and concave palm. Abdomen typical of genus. However, our specimens showed characteristics of both Caprella mendax and C. equilibra (Table 2), although, according to Mayer (1903) and Laubitz (1970), because the main differences are based on the structure of the second gnathopod and the length of pereonite 5 being greater than pereonite 4 in the first species, we have tentatively assigned them to C. mendax. Laubitz (1970) found specimens up to 16.4 and 11.2 mm in length for males and females, respectively, but the largest male and female lengths in our study were 9.3 and 7 mm.
One of the more distinctive features in both C. mendax and C. equilibra is the presence of a ventral acute projection between gnathopods 2, which is absent in the material studied. McCain (1968) reported a variant of C. equilibra, associated with the gorgonian Leptogorgia, along the coast of Virginia, North Carolina and South Carolina in which the spine was reduced or absent and the body not quite as stout   as in the typical form. Our material was also found on gorgonians (Leptogorgia rigida, Leptogorgia peruviana, Muricea sp. and Pacifigorgia sp.). In agreement with McCain (1968), this substrate may have some relation to the reduction of the spine and stoutness of the body. Further studies would be necessary to clarify this question and to elucidate the potential synonymy between C. equilibra and C. mendax.

Habitat
Caprella mendax occurs predominately in the intertidal zone and shallow waters (Watling and Carlton 2007), although it has also been found up to 80 m deep (Martin 1977). It lives on a great variety of substrates such as seaweeds (Hammer and Zimmerman 1979) or hydroids (Dougherty and Steinberg 1953). In our study, C. mendax was more abundant in shallow waters (up to 5 m) on seaweeds (Zonaria cf. farlowii) with epiphytic hydroids and on gorgonians (Leptogorgia rigida, Leptogorgia peruviana, Muricea sp. and Pacifigorgia sp.).

Distribution
Type locality. California. Other records: Dillon Beach, Moss Beach and Pacific Grove-Monterey Bay, California (Dougherty and Steinberg 1953); Vancouver Island and Hecate Strait (British Columbia), San Juan Islands (Washington) (Laubitz 1970); off Humboldt Bay (Martin 1977); Mazatlán is the most southerly record of C. mendax if validity of species is confirmed.

Etymology
The species is dedicated to Emilio Sánchez 'Pitu', son of the first and second authors; used as a noun in apposition.

Diagnosis
Head with rostrum short and triangular. Body stout and wide with tiny tubercles. Peduncle of antenna 1 scarcely setose. Gnathopod 2 basis short with an anterior carina; palm of propodus with a acute projection medially and a rounded distal one.

Description
Male holotype. Body length: 8.7 mm. Lateral and dorsal view ( Figure 9): Head rostrum short and triangular. Body stout and slightly flattened, with numerous tiny tubercles. Pereonites 2-7 decreasing in length respectively. In dorsal view, body wide, with lateral and flat expansions especially in pereonites 3 and 4 (about as wide as long). Strong pleural development.
Gills ( Figure 9): Present on pereonites 3-4, rounded. Mouthparts ( Figure 10): Upper lip symmetrically bilobed with small setulae apically. Mandibles without palp, mandibular molar process strong, incisor and lacinia mobilis five-toothed, left and right mandible with three and two pectinated setae, respectively. Lower lip with inner lobes well-demarcated, inner and outer lobes setose apically. Maxilla 1 outer lobe with seven robust setae, distal article of the palp with six apical robust setae and 12 lateral setae. Maxilla 2 inner lobe oval and outer lobe rectangular, about 1.5 times as long as the inner lobe. Maxilliped inner plate oval with two robust and short setae and eight plumose setae; outer plate with four robust setae and eight long setae; palp four-articulate, with numerous long setae, article 4 with row of setulae on its grasping margin. Antennae (Figure 9 and 11): Antenna 1 about half of body length; peduncle scarcely setose; article 1 and 2 enlarged; flagellum nine-articulate. Antenna 2 flagellum two-articulate and setose, carrying robust setae in the distal article; swimming setae present.
Gnathopods ( Figure 11): Gnathopod 1 basis as long as ischium, merus and carpus combined, with a anterior denticulate carina and tiny tubercles; propodus palm with two proximal grasping spines and setae along the palm; dactylus elongate with rows of setulae. Gnathopod 2 inserted in the middle of pereonite 2 and with numerous tiny tubercles along all surface; basis short, about one-sixth as long as total length of gnathopod, with an anterior carina; propodus oval, length about 1.5 times width, palm of propodus without grasping spines, with an acute projection medially and a rounded distal one; dactylus short and wide with tiny tubercles.
Abdomen ( Figure 12): With a pair of two-articulated appendages, a pair of lateral lobes and a single dorsal lobe.
Allotype female. Body length: 5.2 mm. Similar to male (Figures 9 and 11). Body wide, especially in pereonites 3 and 4 (width about twice length). Oostegites present, being slightly setose on pereonite 3. Antenna 1 peduncle not enlarged. Gnathopod 2 similar to male but propodus palm with a minor excavation between medial and distal projections. Abdomen with a pair of lateral lobes and a single dorsal lobe ( Figure 12).

Intraspecific variation
Most of the morphological characters from C. pitu were rather constant. However, there are slightly differences between populations from the type locality and other sites based especially on body length and the morphology of the pereopods 5-7 (Figures 9 and 13). In the type locality (Mazatlán), the individuals were more robust and longer than in other localities further south such as Isla Isabel and Bahía Banderas (length varied from 8.7 to 7.8 mm in males, and 5.2 to 4.3 mm in females). The length of the pereopods was apparently larger in specimens from Isla Isabel and these specimens were referred to as a 'long-leg' form. These differences are based mainly on the ratio between width and length of the merus and propodus (independently for males or females), which gives a relative appearance of greater length. For example, the propodus of pereopod 7 was one-third as wide as long in Mazatlán versus one-quarter in Isla Isabel; and the carpus was one-half and one-third, respectively (Figures 12 and 14).

Remarks
The new species is close to Caprella penantis Leach, 1814 in the general shape of the body. Caprella penantis is considered a cosmopolitan species with remarkable intraspecific morphological variation (Mayer 1903;McCain 1968;Laubitz 1972;Cabezas et al. 2010) and the taxonomic status of the different forms has been controversial for years (Cabezas et al. 2013).
Although C. penantis has been cited through the Pacific Ocean, the nearest record to the Mexican Pacific coast was cited by Laubitz (1972) from Monterey Bay but without location, date of collection or drawings. According to Watling and Carlton (2007), C. penantis could be an introduction to the California coast. Laubitz (1972) wrote that specimens of C. penantis from California were less setose and smaller than specimens from the Atlantic, but they showed the typical stout body and strong pleural development. In fact, our specimens are clearly smaller than C. penantis (8.7 mm as opposed to 14 mm). Recently, a genetic and morphological study on the C. penantis group has refuted the cosmopolitan distribution of this species and has highlighted the existence of at least four species (Cabezas et al. 2013). Our specimens were included in this study as 'C. penantis from Mexico' and the results demonstrated that it represents a different species from the C. penantis group. Additionally, Cabezas et al. (2013) pointed out the reciprocal monophyly of the two forms of C. pitu and suggested that the absence of gene flow between populations from Mazatlán and Isla Isabel could be an indication of the existence of two distinct species. However, we have considered these as a single species due to the scarcity of morphological differences between both forms and until more detailed studies can be carried out.
Caprella pitu and C. penantis can be distinguished mainly on the basis of the following characteristics: male gnathopod 2 propodus less setose and with medial projection in C. pitu and with proximal projection in C. penantis (or without projection in some varieties); female gnathopod 2 propodus similar to male in C. pitu, with medial projection, small excavation and distal rounded projection, while in C. penantis female gnathopod 2 propodus is different to male with palm slightly convex with a pair of proximal grasping spines; propodus periopod 5-7 without proximal grasping spines in C. pitu, versus with grasping spines in C. penantis; carpus of periopod 5-7 elongate in C. pitu (half as wide as long in pereopod 7) and subquadrate in C. penantis.
McCain (1968) reported the absence or reduction of grasping spines on the propodus of pereopods 5-7 in specimens of C. penantis taken on the gorgonian Leptogorgia from Florida. Caprella pitu has been found exclusively on gorgonians and, in agreement with Aoki and Kikuchi (1995) for Caprella andreae, the adaptation to an ecologically isolated habitat such as gorgonians could have led to its speciation. Potentially, the material of McCain (1968) lacking grasping spines could also belong to a different species.
Another morphologically similar species from the California coast is Caprella natalensis Mayer, 1903(= C. angusta in Dougherty and Steinberg 1953, and Laubitz 1970. It is considered to be the Pacific coast equivalent of C. penantis (Laubitz 1972) and it is very abundant along the California coast (Dougherty and Steinberg 1953;Martin 1977;Watling and Carlton 2007). The main differences between C. pitu and C. natalensis are basically similar to the differences with C. penantis. Additonally, the three species can be differentiated because pereonite 5 is usually longer than pereonites 6 and 7 in C. natalensis whereas in C. pitu and C. penantis it is shorter (three pereonites are subequal). Pleural development is not present in adults of C. natalensis.

Habitat
Caprella pitu has been found exclusively clinging to several species of gorgonians (Leptogorgia sp., Leptogorgia rigida, Leptogorgia peruviana, Pacifigorgia sp and Pacifigorgia cf. agassizii) from 2 to 25 m deep. It was more abundant on L. rigida and P.cf. agassizii (some samples with more than 500 individuals). In some samples from Mazatlán, it shared the substrates with other species such as Aciconula acanthosoma and Caprella mendax; however it was the only species in the samples from Isla Isabel and Bahía Banderas.

Etymology
Named isabelensis alluding to the National Park of Isla Isabel (Nayarit), México.

Description
Male holotype. Body length: 3.25 mm. Lateral and dorsal view ( Figure 15): Body dorsally smooth. Head rounded, eyes present. Pereonite 1 fused with head, suture present. Pereonite 2 with a pair of anterolateral acute and directed downwards projections. Pereonite 3 with a pair of mediolateral projections. Pereonite 5 the longest. Pereonite 7 the shortest. Gills (Figure 15): Present on pereonites 3-4, oval. Gills on pereonite 3 about 2.5 times longer than those on pereonite 4. Mouthparts ( Figure 16): Upper lip symmetrically bilobed with small setulae apically. Mandibles with molar process and three-articulate palp; distal article of palp the longest, with one seta apically (lost in the dissection); second article of palp with one distal plumose seta; incisor and lacinia mobilis five-toothed; left and right mandibles with three and two pectinated setae respectively. Lower lip with inner lobes well-demarcated, inner and outer lobes setose apically. Maxilla 1 outer lobe carrying five robust setae; distal article of the palp with five apical setae. Maxilla 2 inner lobe oval, carrying five distal setae, and outer lobe rectangular, with six apical setae. Maxilliped inner plate rectangular carrying from inner to outer margin a nodular seta, two plumose setae and one seta; outer plate with four setae and one plumose seta apically; palp four-articulate, scarcely setose, third article provided with a projection.
Gnathopods ( Figure 17): Gnathopod 1 basis longer than ischium, merus and carpus combined (length about 1.5 times); propodus palm with proximal grasping spine and setae along the palm present. Gnathopod 2 inserted on the anterior half of pereonite 2; basis slightly longer than pereonite 2, with tiny tubercles on the proximal half; ischium elongated, half as long as basis; merus rounded; carpus short and triangular; propodus elongated, longer than basis, and four times as long as wide; palm with proximal projection with one grasping spine, followed by another proximal acute projection, margin setose; dactylus with setulae and widened medially.
Pereopods ( Figure 18): Pereopod 3 and 4 small, one-articulate, with three or four setae apically, respectively. Pereopod 5 one-articulate, with five setae and one plumose seta apically. Pereopod 6-7 increasing in length, six-articulate and attached to the posterior end of the pereonite; propodus palm carrying a row of robust setae.
Allotype female. Body length: 2.6 mm. Similar to male except for the following characteristics (Figures 15 and 17): oostegites present, being slightly setose on pereonite 3 and 4; pereonite 2 and 3 lacking anterolateral and lateral projections, respectively; gnathopod 2 ischium less elongated than in male (one-quarter as long as basis), propodus oval, as long as basis, with only one proximal projection, dactylus not setose. Abdomen with a pair of lateral lobes and single dorsal lobe ( Figure 18).

Remarks
Liropus isabelensis represents the 10th species from the genus Liropus Mayer, 1890. The other species included in the genus are as follows: L. africanus Mayer, 1920, L. azorensis Guerra-García, 2004, L. cachuchoensis Guerra-García, 2008, L. elongatus Mayer, 1890, L. gracilis Chevreux, 1927, L. japonicus Mori, 1995, L. minimus Mayer, 1890, L. minusculus Guerra-García and Hendrycks, 2013and L. nelsonae Guerra-García, 2003. Most of the species have an Atlantic or Mediterranean distribution except L. japonicus, from Japanese waters, and L. minusculus, from California, USA. Therefore, the new species is the second record of the genus Liropus from the East Pacific coast. A morphological comparison among Liropus species is given in Table 3. Liropus isabelensis can be distinguished from all other species mainly by the following characteristics: anterolateral acute and downwards-directed projections on pereonite 2, presence of mediolateral projections on pereonites 3 and gnathopod 2 ischium elongated. Liropus minusculus is the geographically nearer species and shares some characteristics with L. isabelensis such as pereopod 5 one-articulate and the anterolateral projections in pereonite 2 although this is more acute and downwards directed in the latter. Both species have the smallest size, although L. africanus and L. japonicus are also characterized by a very small size.

Habitat
This species was found attached to species of Thecata and Athecata hydroids, gorgonian (Muricea cf. californica), a bryozoan (Bugula sp.), the seaweed Zonaria cf. farlowii with epiphytic hydroids and several red seaweeds. They occur from 3 to 25 m deep, although always in low densities. Other caprellids, such as Paracaprella spp. and Aciconula acanthosoma, live on these substrates together with L. isabelensis.

Etymology
The species is dedicated to Dr José Luis Carballo for his important contribution to the knowledge of the benthic fauna from the Pacific coast of México.

Diagnosis
Head rounded. Pereonite 2 with a rounded and narrow anteroventral projection in adult males, absent in females. Mandibles with molar process and palp reduced to a long seta. Flagellum of antenna 1 nine-articulate. Propodus of gnathopod 2 palm with rectangular projection proximally, carrying two proximal grasping spines and a distal robust tooth. Pereopod 3 and 4 two-articulate. Pereopod 5-7 without grasping spines. Abdomen with a pair of setose uni-articulate appendages.
Gills (Figure 19): Present on pereonites 3-4, oval. Gills on pereonite 3 about twice as long as those on pereonite 4. Mouthparts ( Figure 20): Mandibles with molar process and palp reduced to a long seta; incisor and lacinia mobilis five-toothed; left and right mandibles with three and two pectinated setae, respectively. Maxilla 1 outer lobe carrying six robust-stout setae; distal article of the palp with three robust-stout setae and three marginal setae. Maxilla 2 inner lobe oval, carrying four distal setae, and outer lobe elongated, with five apical setae. Maxilliped inner plate rectangular carrying three setae apically; outer plate with six submarginal and two apical setae; palp four-articulate, setose; third article provided with a large distal process and setose apically; terminal article with a row of setulae on grasping margin, and one seta subdistally. Upper and lower lip lost in dissection.
Antennae (Figures 19 and 21): Antenna 1 shorter than the combined lengths of head and pereonite 2-3. Peduncle scarcely setose; flagellum nine-articulate. Antenna 2 a little shorter than peduncle of antenna 1; proximal peduncular article with a small acute projection distally; swimming setae absent; flagellum two-articulate, proximal article c. 2.5 times the length of the distal one.
Gnathopods ( Figure 21): Gnathopod 1 basis as long as ischium, merus and carpus combined; propodus palm with a single proximal grasping spine; grasping margin of propodus serrated with setae; distal ventral margin of dactylus with one tooth. Gnathopod 2 inserted on the anterior half of pereonite 2; basis elongated slightly shorter than pereonite 2, with two distal short processes on lateral margin; propodus length about twice width; propodus palm with rectangular projection proximally, carrying two proximal grasping spines and a distal robust tooth; grasping margin setose; dactylus thickened medially and setose.
Pereopods (Figures 19 and 22): Pereopod 3 and 4 two-articulate, both with four setae apically on distal article and two setae subdistally on basal article. Pereopod 5-7 increasing in length, six-articulate and attached to the posterior end of the pereonite, with several plumose setae. Palm of propodus of pereopod 5 with a proximal short knob bearing a small spine; palm of pereopod 6-7 with several proximal short knobs, each bearing a small spine. Grasping spines absent.
Allotype female. Body length: 4.4 mm. Similar to male except for the following morphological characteristics (Figures 19 and 21): oostegites present, being slightly setose on pereonite 3 and 4; anterolateral projection on pereonite 2 absent; flagellum of antenna 1 eight-articulate; propodus of gnathopod 2 oval, with a proximal knob bearing a grasping spine, dactylus not setose. Abdomen with a pair of lateral lobes and single dorsal lobe carrying two setae ( Figure 22).

Habitat
Paracaprella carballoi was mainly found attached to the seaweed Zonaria cf. farlowii with abundant small hydroids on underside of thallus, between 3-6 m depth. On these substrates it was relatively abundant. Also it has been found on the gorgonian Leptogorgia rigida, although in low densities. Other caprellids on these substrates were Aciconula acanthosoma, Caprella mendax, C. pitu and Liropus isabelensis.

Distribution
Paracaprella carballoi has been found so far from the type locality, Isla de los Pájaros (Mazatlán), México.

Etymology
The species is dedicated to Isabel Sánchez, daughter of the first and second authors. She was born just after the sample survey of this study.

Diagnosis
Head rounded and dorsally humped. Large bifid sharp-pointed anterolateral projection on anterior margin of pereonite 2 in males, simple and rounded in females. Pereonite 3 with a rounded anterolateral projection in males. Short ventral forwarddirected projection with 'raspberry'-like surface between the gnathopods 2. Mandibles with molar process and without palp. Flagellum of antenna 1 10-articulate. Coxa of gnathopod 2 with a tubercle with 'raspberry'-like surface in males. Propodus palm of gnathopod 2 with rectangular projection proximally, bearing one proximal grasping spine and a distal long robust tooth in males. Pereopod 3 and 4 two-articulate. Pereopod 5-7 without grasping spines. Abdomen with a pair of setose uni-articulate appendages in males. Mouthparts ( Figure 24): Upper lip symmetrically bilobed without setulae apically. Mandibles with molar process and without palp; incisor and lacinia mobilis five-toothed; left and right mandibles with three and two pectinated setae, respectively. Lower lip with inner lobes well-marked and with a medial suture, outer lobes setose apically. Maxilla 1 outer lobe carrying six robust-stout setae; distal article of the palp with three robust and two long apical setae. Maxilla 2 inner lobe oval, carrying four distal setae, and outer lobe elongated, with five apical setae. Maxilliped inner plate rectangular carrying three setae apically; outer plate with six submarginal setae; palp four-articulate, scarcely setose; third article provided with a large distal process and setose apically; terminal article with a row of setulae on grasping margin, and two setae subdistally.

Description
Antennae (Figures 23 and 25): Antenna 1 shorter than the combined lengths of head and pereonite 2-3. Peduncle setose; flagellum 10-articulate. Antenna 2 a little shorter than peduncle of antenna 1; proximal peduncular article with a short acute projection distally; swimming setae absent; flagellum two-articulate, proximal article 2 times the length of the distal one.
Gnathopods ( Figure 25): Gnathopod 1 basis as long as ischium, merus and carpus combined; propodus palm with a single proximal grasping spine; grasping margin of propodus with setae; ventral margin of dactylus with several teeth. Gnathopod 2 inserted on the anterior half of pereonite 2; coxa bearing anteriorly a tubercle with 'raspberry'-like surface; basis elongated slightly shorter than pereonite 2, without proximal serrated knob on ventral margin; propodus length about twice width; propodus palm with rectangular projection proximally, carrying one proximal grasping spine and a distal long robust tooth; grasping margin setose; dactylus thickened medially and setose.
Pereopods (Figures 23 and 26): Pereopod 3 and 4 two-articulate, with five and four setae apically, respectively, and one seta distally on basal article. Pereopod 5-7 increasing in length, six-articulate and attached to the posterior end of the pereonite, with several plumose setae. Palm of propodus of pereopod 5 with a proximal knob bearing a small spine; palm of pereopod 6-7 with several proximal knobs, each bearing a small spine. Grasping spines absent.
Allotype female. Body length: 4.2 mm. Similar to male except for the following characteristics (Figures 23 and 25): oostegites present, being slightly setose on pereonite 3; small and rounded anterolateral projection on pereonite 2, lacking anterolateral projection on pereonite 3; flagellum of antenna 1 six-articulate; propodus of gnathopod 2 oval, with a proximal knob bearing a grasping spine and a plumose seta, dactylus not setose. Abdomen with a pair of lateral lobes and single dorsal lobe carrying two plumose setae ( Figure 26).

Habitat
This species has been found on several substrates along the coast of Isla Isabel from 1 to 25 m depth. It was more abundant on hydroids and bryozoan (Bugula sp.) at 6 m depth, although it was also recorded on gorgonians (Pacifigorgia cf. agassizi), and several red seaweeds. Other caprellids such as Aciconula acanthosoma, Caprella pitu, C. equilibra or Liropus isabelensis live on these substrates together with P. isabelae.
Paracaprella carballoi is morphologically close to P. barnardi and P. pusilla, but can be distinguished by the following characters: both P. carballoi and P. pusilla have a smooth body dorsally with an anterodorsal blunt protuberance on pereonite 2, whereas P. barnardi has a large tubercle in this position; the anterolateral projection of pereonite 2 is rounded and narrow as opposed to the sharp-pointed projection in the others; pereonite 4 and 5 are subequal in length whereas pereonite 3 is the longest; Table 4. Comparison of selected characters in males among the species of Paracaprella based on data from Mayer (1903), McCain (1967, 1968), Quitete (1971, Laubitz (1972), Arimoto (1976), Sivaprakasam (1977), Diaz et al. (2005), Guerra-García et al. (2006) and Winfield and Ortiz (2013   basis of gnathopod 2 elongated without proximal serrated knob but with two distal short processes on lateral margin; gnathopod 2 propodus palm with rectangular proximal projection bearing two grasping spines, but only one in all other species of the genus (except P. crassa, without grasping spines and carrying three teeth); and mandibular palp as a simple seta (similar to P. barnardi, P. pusilla and P. guerragarciai). Paracaprella isabelae can be distinguished from all other species mainly by the following characteristics: head rounded and dorsally humped (similar to P. crassa, with which it also shares a blunt posterior protuberance on pereonite 2); large and bifid sharp-pointed anterolateral projection on pereonite 2 (in other species can be sharp-pointed but always single); rounded anterolateral projections on pereonite 3 (similar to P. alata although triangular on pereonite 3 and small and rounded on pereonite 4 in this case; triangular in P. digitimanus; and absent in the other species of the genus); short ventral forward directed projection with 'raspberry'-like surface between gnathopods 2; pereonite 4 and 5 subequal in length and pereonite 3 the longest, similar to P. carballoi; and mandibular palp absent as is the case of P. alata and P. digitimanus.
According to Winfield and Ortiz (2013), the genus Paracaprella has a geographic distribution encompassing temperate, subtropical and tropical seas. However, only P. barnardi and P. pusilla have been recorded so far from the American Pacific coast. While P. barnardi has a more local distribution (it has only been recorded from the type locality in Culebra Island, Panama), P. pusilla is the species of the genus with a more worldwide distribution and it is the only species found in European waters, presumably introduced by ship fouling (Ros and Guerra-García 2012;Ros et al. 2013), even though it has only been cited from the American coast in Chile (Guerra-García and Thiel 2001). Hence, both P. carballoi and P. isabelae represent the northernmost records of the genus on the East Pacific coast.