Holotype. One adult female from a digestive caecum (Fig. 1A) of the hesionid polychaete Hesione picta Müller, 1858 (see Hartman, 1959) (UF 1594, KEYS-0778) collected in Monroe County, Florida Keys, Long Key, LONF1 tower dive site, W of Florida Keys Marine Laboratory (24.844°N, 80.864°W), at depth of 2 m, by Gustav Paulay. Body length of polychaete 28 mm long, 5 mm wide, 16 chaetigers (Fig. 1B, C).

Asterocherid with reduced fifth leg, represented by low protuberance armed with single distal seta. Genital double-somite with acute chitinous projection on posterolateral corners. Pediger 1 with posterolateral corners rounded, not produced, pediger 3 with posterior margin weakly curved. Anal somite with crenulated posterior margin. First segment of antennary endopod shorter than basis. Coxal sclerite with spinules. Coxal seta on leg 4 reduced, spiniform, third exopod of leg 4 with four spines, shorter than segmental width.

Total body length from anteriormost end of cephalosome to posterior margin of caudal rami: 998 µm. Body (Fig. 2A) robust, with broad, rounded prosome, body widest at first pedigerous somite, slightly flattened dorsoventrally. Length ratio of prosome/ urosome = 2.2. First pedigerous somite with leg 1 completely fused to cephalosome. Pedigerous somites gradually tapering posteriorly. Pedigerous somite 4 narrowest, partially covered by third pedigerous somite in dorsal view. Posterolateral corners of pedigerous somites 1–3 rounded, lacking processes. Fifth pedigerous somite wider than fourth. Urosome 254 µm long, with three somites, genital double-somite 170 µm long barrel-shaped, slightly longer than wide, representing 67% of urosome (Fig. 2B). Genital openings located dorsolaterally, at widest section of somite, with adjacent row of short setules and low, rounded integumental expansion. Postero-lateral corners of genital double-somite smooth. Preanal somite subrectangular, 45 µm long, slightly shorter than succeeding anal somite (51 µm), both ornamented with spinules on lateral margin. Caudal rami 43 µm long, slightly shorter than anal somite; armed with 6 setae. Innermost terminal seta 130 µm, outermost terminal seta 185 µm, inner dorsal seta 134 µm, outer dorsal seta 167 µm, two long, relatively thicker median terminal setae, outer 315 µm and inner seta 338 µm.

Antennule (Fig. 2C) 392 µm long, excluding setae; 19-segmented. Segmentation (between brackets), segmental homologies (Roman numerals), and setation (s=setae, ae= aesthetascs) as follows: (1)I-2s, (2)II-2s, (3)III-2s, (4)IV-2s, (5)V-2s, (6)VI-2s, (7)VII-2s, (8)VIII-2s, (9)IX-XIII-7s, (10)XIV-2s, (11)XV-2s, (12) XVI-2s, (13)XVII-2s, (14)XVIII-2s, (15)XIX-0, (16)XX-2s, (17)XXI-ls+ae, (18)XXII-XXIII-1s, (19)XXIV-XXVIII-8.

Antenna (Fig. 2D) with slender, elongate basis carrying short, 1-segmented exopod and long, well-developed endopod. EXP longer than wide, armed with one long seta. ENP1 slightly shorter than basis. ENP2 armed with 1 seta, ENP3 longer than second, armed with short seta and stout, slightly curved terminal claw.

Oral cone (Fig. 2I) with usual structure of asterocherids, produced into siphon-like distal portion, reaching insertion of leg 1.

Mandible (Fig. 2E) consisting of long, slender stylet and 2-segmented palp, bearing 2 unequal apical setae; palp segments ornamented with setules.

Maxillule (Fig. 2F) bilobed, consisting of short, narrow outer lobe, armed with 4 subequally long distal setae, and wider, medially inflate and longer inner lobe, ornamented with row of short setules, bearing 4 long and 1 short pinnate setae.

Maxilla (Fig. 2G) two-segmented, including short subrectangular proximal syncoxa and distal elongate basis, longer than proximal segment, with row of small spinules proximally. Distally curved basipodal claw ornamented with spinules.

Maxilliped (Fig. 2H) consisting of syncoxa, subrectangular basis and 4-segmented endopod; syncoxa unarmed, basis with minute inner seta and row of short spinules on distal outer margin. ENP segments armed with 1, 1, 0, and 1 setae, respectively; terminal claw thick, weakly curved.

Legs 1– 4 (Fig. 3A–D) biramous, all rami 3-segmented. Coxal sclerites subrectangular, with posterior margins smooth except for leg 1, with row of +10 spinules (arrowed in Fig. 3A). Coxae of legs 1–4 with inner coxal seta; in leg 4 seta reduced, represented by short spiniform element (Fig. 3D). Legs 1–4 with outer basipodal seta; leg 1 bearing short, stout inner basipodal seta. Leg 1 with row of small spinules along inner margin of basis. Outer spine on first exopodal segment of leg 1 strong, with curved tip, reaching insertion of proximalmost spine of third exopodal segment. Medial spine on leg 1 EXP3 being 1.5 times as long as other two spines on same segment (arrowed in Fig. 3A). ENP2 of legs 1–4 with bifurcate projections at outer distal corner; projection longest in leg 4. Leg 4 with reduced outer seta on third endopodal segment.

Spine and setal armature of legs 1–4 as follows:

Leg 5 (Figs 2A, 3E): strongly reduced, fused to somite; represented by rounded lateral expansion ornamented with row of 6–7 spinules, armed with slender, smooth distal seta, 70 µm long.

Male. Unknown.

Long Key, Florida Keys, Monroe County, Florida, USA (24.844°N, 80.864°W).

The new species is named after Dr. Sergio Salazar Vallejo, senior researcher at El Colegio de la Frontera Sur, for his valuable contributions to the taxonomy and diversity of tropical benthic polychaetes and for finding the copepod specimen herein described.

The benthic polychaete containing the copepod, H. picta, is a widespread species distributed in the western Atlantic Ocean from Florida to Brazil. Locally, it was found in rubble/ sand / seagrass bottom of the type locality. The original host of the copepod remains unknown.

The specimen examined was identified as a species of Monocheres by its possession of a reduced fifth leg, represented by a papilla-like process arising directly from the somite and armed with a single distal seta. All other characters resemble those known in members of Asterocheres (Stock 1966; Kim 2010). The new species can be distinguished from the two other species of the genus, M. mauritianus and M. cagarrensis, by several differences, as presented in Table 1. Some of the characters used by Johnsson and Bustamante (1997) to compare M. mauritianus and M. cagarrensis were not included in this analysis because they rely on the accuracy of the observation and even different drawing styles, like the serrate projection of the second exopodal segment of leg 1 or the presence/absence of denticles on the first and second endopodal segments of all swimming legs. Instead, other characters that were deemed stronger were added, like the lack of an inner basipodal spine in M. cagarrensis and the presence of spinules along the posterior margin of the coxal sclerite of leg 1. The main apomorphies include the presence of spines along the posterior margin of the first leg coxal sclerite, the shape of the cephalosome, and the reduced, spiniform coxal seta on leg 4. The differences presented in Table 1 serve to clearly distinguish the three species of this genus.

The new species was described based on a single specimen; this is not unusual among the asterocherid copepods; the type species of Monocheres, M. mauritianus, was also described on a single female specimen collected from the cauliflower coral Pocillophora damicornis (L.). This is the third species described in Monocheres after its description 50 years ago; there was a 31 year period between the description of the first one, M. mauritianus, and the finding of M. cagarrensis in Brazil; almost 20 years later a third species was unexpectedly found as described herein.

Because of the peculiar circumstances by which this specimen was recovered, it is difficult to determine the nature of its association with any of the local benthic invertebrate groups. The associations of asterocherid copepods take place with different invertebrate taxa and the host remains unknown for many species, but asterocherids have not been reported as symbionts of polychaetes (Boxshall and Halsey 2004; Bandera and Huys 2008). These copepods are all ectosymbionts except for Collocherides astroboae Stock, 1971, living as an endosymbiont in the stomach of ophiurids. Hence, it is assumed that the hesionid polychaete H. picta, usually living under rocks, consumed this copepod as a prey or among portions of its food, possibly from sclerobiotic sponges or coral. The copepod remained in the digestive chamber for some time before the fixation of the polychaete and thus, some structures or muscles were expected to be damaged but they were not; the specimen (not an exuvia) was in good condition for taxonomical analysis. It is likely that this individual remained in the caecum for a short time before the polychaete was collected and preserved.

It is interesting to note that H. picta has been recorded in association with ophiurids living under rocks (De Assis et al. 2012). There are more than 20 known species of asterocherid copepods which are ectosymbiotic in ophiurids, including species of Asterocheres, Collocheres Canu, 1863, Collocherides Stock, 1971, and Ophiurocheres (Humes, 1988) (Humes 1998; Doignon et al. 2004), which supports the alternative notion that this copepod was possibly consumed by the polychaete directly from an ophiurid during this hypothetical symbiosis. Hence, the original host of this copepod remains unknown but it is expected that this finding will motivate new observations on these associations involving ophiurids and copepods in the region. A similar situation was reported by Kolbasov et al. (2007); they described a new species of a facetotectan crustacean larva from specimens found together with other food items in the gut of a fish, but in this case the larva is deemed as free-living, with no indication of a symbiotic behaviour.

Other members of the genus Monocheres have been known from corals and sponges and only from islands (Stock 1966; Johnsson and Bustamante 1997); this is also the case in the new species, found in the Florida Keys. It is speculated that both isolation and habitat specialization could have a role in the divergence of this genus, with a striking reduction of the fifth leg that strongly diverges from the main asterocherid pattern.