Adult female holotype (ECO-CHZ-08979) partially dissected, mounted on glycerine sealed with Entellan®, Playa Careyeros, Nayarit, Mexico, coll. Jani Jarquín-González, Patricia Salazar and Ramiro Gallardo, March 23, 2013, depth=2–3 m, algal patch from coral rock. Paratypes: three adult females, partially dissected, slides, mounted in glycerine sealed with EntellanÒ, same site, date, and collector (ECO-CHZ-08980), seven undissected adult female specimens preserved in ethanol, vial (ECO-CHZ-08981), same site, date and collector. Two undissected adult females (USNM-1221050), same sampling data.

Playa Careyeros (20°46'59.46"N, 105°30'35.48"W), state of Nayarit, central part of the Pacific coast of Mexico.

The species is named after the Mexican state of Nayarit, where this species was originally collected. The name of the species is a noun used in apposition.

Female: Body (Fig. 2A) broad, dorsoventrally flattened, arched along longitudinal axis. Cephalosome accounting for about half the body length, with its greatest width at first pedigerous somite, behind which the body gradually tapers posteriorly. Epimera of genital somite and preceding somite pointed and backwardly directed. Length of holotype: 1.03 mm measured from tip of rostrum to posterior margin of anal somite. Length range of type females from 0.93 mm to 1.05 mm, average length 0.96 mm, n=16. Rostrum fused to cephalosome, broad, downwardly directed.

Cephalosome with a few small, sparsely distributed sensilla on lateral margins (Fig. 2A), posterior margin smooth. Succeeding prosomites, bearing legs 2-4, with flat, laterally expanded subtriangular margins ornamented with a mixed pattern of long and minute spinules (detail in Fig. 2A). First urosomite, bearing leg 5, slightly longer than succeeding genital double-somite; posterior margin of genital double-somite smooth. Anal somite with rounded posterior margin; somite naked in dorsal view, but with a row of long setae along ventral margin (Fig. 3B). Anal area moderately deep, with inner rows of short setules along margin of anal operculum. Caudal rami cylindrical, about twice as long as wide, with 7 setae. Middle caudal apical seta (V) longest. Dorsal seta (VII) about half as long as ramus (Fig. 3C).

Antennule (Fig. 2B) 7-segmented; first segment slightly longer than second, ornamented with distal row of spinules. Armature of antennulary segments (s=setae, ae=aesthetasc) as: 1(1), 2(10s), 3(6s), 4(3s+2ae), 5(1s), 6(2s), 7(9s+1ae). Aesthetasc on fourth segment long, about 70% of antennule length.

Antenna (Fig. 2C). Coxa small, Allobasis with short abexopodal seta and longitudinal patch of spinules on outer margin. Exopod two-segmented, elongated, first segment with short slender seta, second segment bearing three setae distally, distal margin with row of short spinules. One exopodal seta modified, with regular pectinate ornamentation along both margins (see detail in Fig. 2C). Free endopodal segment with outer row of long spinules, armed with one spine and two lateral setae plus seven distal setal elements, four of them being articulated stout setae.

Mandible (Fig. 2D) short, tapering distally. Gnathobasis with narrow diastema, armed with 5-6 monocuspidate teeth, plus stout dorsal seta fused to gnathobasis, ornamented with row of short setules. Mandibular palp small, represented by a subrectangular coxa-basis segment and a single-segmented endopod armed with an inner row of spinules, six terminal setae plus a single outer seta.

Maxillule (Fig. 2E). Praecoxal arthrite armed with eight teeth and one distal short seta. Coxa and basis fused, indistinguishable, with three terminal setae. Endopodite represented by one seta. Exopodite one-segmented, with two setae.

Maxilla (Fig. 2F). Syncoxa robust, short, with two endites, the proximal small and bearing one broad seta, distal endite cylindrical, armed with two subequal, pinnate setae; allobasis forming a strong terminal claw and with one inner basal seta, one outer basal seta and two subequal outer endopodal setae.

Maxilliped (Fig. 3A) subchelate. Coxa and basis elongate. Basis with smooth surface, bearing a well-developed seta, as depicted. First endopodal segment robust, ornamented with single row of spinules and large patch of spinules, as shown. Endopodal claw slender, slightly curved, about 1.3 times as long as basis, with single accompanying seta.

Leg 1 (Fig. 3D). Coxa elongate, ornamented with single row of small spinules on inner and outer margins, plus single short seta on inner middle part of segment. Basis wide, inner margin and part of outer margin ornamented with spinules. Inner distal basipodal seta reaching distal margin of first endopodal segment. Outer basipodal seta reaching distal end of basis. Exopod three-segmented, second exopodal segment longest, about 1.5 times as long as first segment, with patch of minute spinules on outer distal margin. Two exopodal claws on distal position of third exopodal segment; outer claw half as long as inner claw. Endopod two-segmented, shorter than exopod. Endopodal segments wide, globose (sensu Wells 2007), ornamented with rows of short setules along the inner and outer margins. Terminal elements include a spine ornamented with distal row of minute spinules and two equally long slender setae.

Leg 2 (Fig. 4A). Coxa small, basis transversely elongated. Basis with outer seta. Endopod three-segmented, longer than exopod, exopod reaching midlength of third endopodal segment. Exopod three-segmented, with spinules on outer margins of third segment and spinules at insertion of spines on first and second segments.

Leg 3 (Fig. 4B). Coxa, basis, and relative length of endopodal and exopodal rami as in leg 2. Endopod and exopod three-segmented.

Leg 4 (Fig. 4C). Coxa and basis as in leg 3. Exopod three-segmented, third segment with 8 setal elements. Insertion points of exopodal spines and outer margins of second and third exopodal segments with rows of spinules. Endopod three-segmented, slightly longer than exopod, outer margins of segments ornamented with rows of short spinules.

Armature of swimming legs 1-4 as in Table 1.

Leg 5 (Fig. 3E) exopod and baseoendopod separated. Baseoendopod bearing single inner seta. External seta long, borne on elongate cylindrical lobe of baseoendopod reaching half the length of exopodal lobe. Exopodite slender, with 5 setal elements (I-V) (sensu Wells 2007), two inserted on inner margin (I, II), three (III-V) distal. Elements I-III represented by stout, distally pinnate setae, elements IV and V represented by equally long slender setae.

Male: Unknown.

The available keys to the species of Peltidium include those by Nicholls (1941), Lang (1948), and Wells (2007); when following the latter work, our specimens from Nayarit key down to a couplet leading to either Peltidium nichollsi Geddes, 1968 or Peltidium lerneri Geddes, 1968, both from the Bahamas. These species share the following characters with Peltidium nayarit sp. n.: female leg 5 with separate exopod and baseoendopod with 5 setae borne on distal and inner edges only; leg 1 with two-segmented endopod, distal endopodal segment with three setal elements; the number of setae on the distal endopodal segment of legs 2-4 is 3:5:4. The new species differs from these two Bahamian species in several characters, as follows. It diverges from Peltidium nichollsi in having a sharper frontal protuberance of the cephalosome and a longer cephalosome (length/width ratio= 1.26 vs. 1.43 in Peltidium nayarit sp. n.). In the new species the outer basipodal seta of leg 1 is slender, inserted on the proximal half of segment, whereas this seta is stouter, shorter, and borne on the distal half of the basipod in Peltidium nichollsi. The setal elements of both the exopod and endopod of leg 1 differ in these two species. The third exopodal segment Peltidium nichollsi bears two strong, stout subdistal elements in addition to the pair of terminal claws (Geddes 1968, fig. 6B) vs. a single slender seta in Peltidium nayarit sp. n. (Fig. 3D) and the terminal claws are equally long in Peltidium nichollsi, whereas the inner claw is about twice as long as the outer one in the new species. In Peltidium nichollsi the three distal elements of the second endopodal segment have different lengths, whereas these elements are equally long in Peltidium nayarit sp. n. Both species share the presence of one modified seta on the antennary exopod, but in Peltidium nichollsi only the tip of the seta is irregularly pectinate (Geddes 1968, fig. 7B), whereas in the new species this seta is regularly pectinate almost its entire length (see detail in Fig. 2C). Another species with a distally pectinate exopodal seta is Peltidium maldivianum Sewell, 1940, but it differs in characters of leg 1 and leg 5 from both Peltidium nichollsi (see Wells 2007) and Peltidium nayarit sp. n. The armature of leg 5 is also different between the new species and Peltidium nichollsi. In the latter, setae I and II are heavily pectinate (Geddes 1968, fig. 7G), they are also clearly shorter than setal elements III and V, and seta IV is shorter than elements III and V. In the new species elements I-III are not heavily pectinate but lightly spinulose (Fig. 3E), element II is longest and seta IV is almost as long as seta V.

The new species can be distinguished from Peltidium lerneri by the armature of leg 1. In Peltidium lerneri the exopodal claws are subequally long, whereas the inner is twice as long as the outer one in Peltidium nayarit sp. n. In addition, the inner spiniform seta of the second endopodal segment is clearly shorter than the other two setae vs. an equal length of the three elements in the new species. Also, as indicated by Wells (2007), in Peltidium lerneri the shape of the first endopodal segment is quadrate (Geddes 1968, fig. 8C), with straight outer margin and inner margin only slightly convex; this character allows distinction from the globose (i.e. both margins of segment convex) condition present in both Peltidium nichollsi and Peltidium nayarit sp. n. The middle of the three antennary exopodal setae is the longest in Peltidium lerneri (Geddes 1968, fig. 9B), but the corresponding seta is the shortest in the new species (Fig. 2C). Most importantly, the female leg 5 is different in these two species. In Peltidium lerneri setal element IV is very reduced, about 1/3 as long as adjacent seta V (Geddes 1968, fig. 8G), whereas the same element is as long as seta V in the new species (Fig. 3E). A reduced seta IV is also present in Peltidium proximum Nicholls, 1941 (Nicholls 1941). Also, the exopodal lobe is clearly shorter in Peltidium lerneri (length/width ratio=1.8) (Geddes 1968, fig. 8G) than it is in Peltidium nayarit sp. n., which is an elongate, slender structure (L/W ratio=4.14).

The new species shares also some important characters with Peltidium speciosum Thompson & Scott, 1903 (see Nicholls 1941), including a leg 5 with a very similar armature and structure except for a relatively robust exopodal segment (length/width ratio= 3.7 vs. 4.2 in Peltidium nayarit sp. n.) and a shorter outer baseoendopodal seta, reaching to about half the length of exopodal seta V (Nicholls 1941, fig. 8), thus differing from the new species, in which the same seta almost reaches the distal end of seta V (Fig. 3E). In the new species one of the antennary exopodal setae is modified, as described (Fig. 2C), whereas Peltidium speciosum lacks modified setae (Nicholls 1941 fig. 8). Several characters of leg 1 differ in these two species; in Peltidium speciosum, the inner basipodal seta is relatively longer than in the new species, it reaches to about half the length of the second endopodal segment (Nicholls 1941, fig. 8), whereas it is clearly shorter in Peltidium nayarit sp. n., barely reaching halfway along the first endopodal segment. The shape of the first and second endopodal segments differs in these two species, the first one is subquadrate as in Peltidium lerneri (Nicholls 1941, Geddes 1968) and the second is subrectangular, with straight margins, thus diverging from the subtriangular shape present in Peltidium nayarit sp. n. (Fig. 3D). Most importantly, in Peltidium speciosum there are four setal elements on the second endopodal segment, thus diverging from the new species (and also from Peltidium lerneri and Peltidium nichollsi), with only three such elements. The terminal exopodal claws of leg 1 are subequal in Peltidium speciosum and diverge from the pattern described herein for Peltidium nayarit sp. n. When Nicholls’ (1941) work is followed to identify our specimens from Nayarit, we reach Peltidium purpureum, which can be easily separated from the new species by its having 4 setal elements (2 claws, 2 setae) on the third exopodal segment of leg 1 vs. 3 elements (2 claws, 1 seta) in the new species, a different shape of the endopodal segments of leg 1, with narrower segments, and by its having 6 setal elements on the exopodal lobe of leg 5 instead of five found in Peltidium nayarit sp. n. Also, Peltidium purpureum has 3 maxillar endites (1 proximal, 2 distal) while Peltidium nayarit and all other Peltidium species mentioned in this paper (except maybe Peltidium maldivianum, whose maxilla has not been described) have lost the proximal endite.