A new species of Monocheres Stock (Copepoda, Siphonostomatoida, Asterocheridae) from shallow waters off Florida, USA: an unexpected discovery

Abstract The rare asterocherid copepod genus Monocheres, ectosymbionts of corals and sponges, contains only two species, one from Mauritius (Indian Ocean) and the other one from Brazil (western Atlantic). From the analysis of the digestive caecum contents of the benthic hesionid polychaete Hesione picta Müller, 1858, an adult female of an undescribed species of Monocheres was unexpectedly recovered; it is the third species of this rare asterocherid genus. The new species, Monocheres sergioi sp. n., has the distinctive reduction of the fifth leg as a process with a single seta. It differs from its two other congeners by several characters including the presence of an inner basipodal spine, the armature details of the third exopodal segment of leg 1, the shape of the cephalosome and pedigerous somites 3 and 4, and the ornamentation of the postero-lateral corners of the genital double-somite. The main synapomorphies include the presence of spinules along the posterior margin of the first leg coxal sclerite and the reduced, spiniform coxal seta on leg 4. The biology and feeding habits of the polychaete containing this specimen suggests that the copepod was ingested as an ectosymbiont from sponges or coral but it is also possible that it was consumed from an ophiurid echinoderm. This finding allows an expansion of the genus geographical distribution in the northwestern Atlantic. A key to the species of Monocheres is also provided.


Introduction
The copepod family Asterocheridae Giesbrecht, 1899 includes a highly diverse group of associated forms that have been recorded as ectosymbionts from a wide array of benthic invertebrates, including sponges, corals, ascidians, echinoderms, bryozoans, and mollusks (Boxshall and Halsey 2004). The type genus, Asterocheres Boeck, 1859 is clearly the most speciose in the family, and has more than 100 species (Kim 2010;Walter and Boxshall 2016). Several of the smallest asterocherid genera include only one or a few species that are rarely found. One of these groups is Monocheres Stock, 1966; it contains only two species. The first one, M. mauritianus Stock, 1966 was described from specimens obtained from corals collected in Mauritius, Indian Ocean (Stock 1966). More than 30 years later, a second species (M. cagarrensis Johnsson & Bustamante, 1997) was described from sponges in Brazilian islands off Rio de Janeiro (Johnsson and Bustamante 1997). This is a very unusual asterocherid genus, whose distinctive character is the strongly reduced fifth leg, represented by a papilla-like process fused to the pediger lateral margin and armed with a single distal seta (Stock 1966).
During a biological survey of the benthic invertebrates in the Florida Keys, benthic polychaetes of the genus Hesione were obtained. The taxonomical analysis of the genus includes the morphology of the enteric caeca, elongate internal sac-like structures. During the examination of dissected caeca of an individual of H. picta Müller, 1858, both a nereid polychaete and surprisingly, a copepod were found. The specimen was given to me for taxonomic analysis resulting in the identification of an undescribed species of Monocheres. In this report this specimen is described in full and compared with the other two known species of the genus; a key for the identification of the species of Monocheres is also provided.

Methods
The polychaete from which the copepod was extracted was obtained by hand during sampling dives in shallow littoral areas off Long Key, Florida Keys, Florida. The copepod specimen was transferred to glycerol and lightly stained with Methylene Orange for taxonomical analysis. The specimen was observed and analyzed in whole and then dissected with sharpened needles; the appendages were examined as temporary mounts in glycerin and later sealed with Entellan® as permanent mounts. Drawings were prepared using a camera lucida mounted on an E-200 Nikon compound microscope with Nomarski DIC at magnifications of 400 and 1000×. Terminology of the body parts and appendages followed Huys and Boxshall (1991); abbreviations used in this work are: EXP = exopod, ENP = endopod. Body length of the copepod was measured from the anterior margin of the rostrum to the posterior margin of the caudal rami. The polychaete and the copepod are deposited in the collection of the Florida Museum of Natural History (FLMNH). Diagnosis. 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.
Oral cone ( Fig. 2I) with usual structure of asterocherids, produced into siphon-like distal portion, reaching insertion of leg 1.
Etymology. 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.
Habitat. 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.
Remarks. 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 Table 1. Comparison of characters of three species of Monocheres. Based on Johnsson & Bustamante (1997), Stock (1966), and present data. with protuberant papilla-like chitinous process and two setae with cluster of short setules with low rounded process, two setae and row of setules Postero-lateral corners of preanal somite without denticle with denticle with denticle First segment of antennary endopod shorter than basis longer than basis shorter than basis 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. Ecological comments. 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.