Neoasterocheres Canário, Rocha, Neves & Johnsson, 2017, gen. nov.

Neoasterocheres gen. nov.

Diagnosis. Asterocheridae. Body dorsoventrally flattened. Prosome ovoid or discoid, urosome cylindrical and 4- segmented in female. Antennule of female basically 19-segmented with large aesthetasc on 17th segment; ancestral segments IX–XIII fused; distal 3 segments frequently fused to become 1 or 2 segments. Antenna with 1-segmented exopod and 3-segmented endopod bearing distal claw. Oral cone short or elongated, siphon-like. Mandible consisting of stylet and 1- or 2-segmented palp bearing two distal setae. Maxillule bilobed. Maxilla 2-segmented; distal segment forming curved claw. Maxilliped with 6-segmented. Legs 1–4 with 3-segmented exopod and endopod. Free segment of leg 5 with 2 or 3 setae.

Remarks. Kim (2010) proposed a new definition for Asterocheres, with the main diagnostics characters related to leg setation and number of antennule segments posteriorly to aesthetasc. Thus, the author restricted the number of valid species in Asterocheres and many species were considered as species inquirendae. Since then, new species of Asterocheres were described (Crescenti et al. 2010; Varela 2010, 2012; Kim & Min 2013) and redescriptions were published (Conradi & Bandera 2011; Bandera & Conradi 2013; Bandera & Conradi 2014). Although the new species being described shows the leg setation established by Kim for Asterocheres, the antennule segmentation highlights a relevant difference in the fusion pattern. Neoasterocheres gen. nov. can be distinguished from Asterocheres by having the fusion of antennule ancestral segments IX–XIII resulting in the aesthetasc located on the 17th segment, with 3 distal segments frequently fused to become 1 or 2. Kim (2010) stated that the female antennule is basically 21-segmented because depending on the existence of distal fusions the amount of segments can be reduced to 19, however these fusions occurs between the last 3 segments, which are posterior to ancestral segment XXI. When Kim (2010) established that the aesthetasc is located on segment 18 in Asterocheres, the author excluded the possibility of occurrence of other additional fusions previous to the 18th segment, and disregarded the existence of species presenting fusions of additional ancestral segments.

According to Huys & Boxshall (1991) on the ancestral pattern of a siphonostome antennule, segment XXI is characterized by the presence of an aesthetasc additional to the maximum armature of two setae found in an unfused segment. Therefore, the authors highlighted the presence of a group of setae on ancestral segment IX, thus indicating a fusion involving at least segments IX–XII. This is expressed in a maximum of 8 setae present on the segment and the existence of eight free single segments between this compound segment (IX–XII) and the one bearing the aesthetasc (XXI). The presence of less than eight free segments between these two landmarks indicates the occurrence of further fusions in the section. Besides that, a spine is commonly present on ancestral segment XIV, and may constitute another relevant landmark. In Asterocheres we found a segment between the compound segment (IX–XII) and the segment bearing a spine (XIV), thus indicating that ancestral XIII segment is free. In the new genus the segment with a spine (XIV) is close to the compound segment (IX–XII). The ancestral segment XIII is not free, it is fused with the previous segment (IX–XII). This pattern can be confirmed by the existence of only seven free segments between the original IX–XII and the XXI, and by the absence of any double segment between these two landmarks, which would be indicated by the presence of four setae and a segment with a length twice its regular size.

In addition, the new genus shows characters that differ from the remaining asterocherid genera. Bythocheres Humes, 1998, Cheramomyzon Humes, 1989, Collocheres Canu, 1893, Collocherides Stock, 1971, Dermatomyzon Claus, 1889, Discopontius Nicholls, 1944, Gerulusosacculus Ivanenko & Defaye, 2004, Glyptocheres Humes, 1987, Ophiurocheres Humes, 1998, Meandromyzon Stock, 1989, Rhyncomyzon Giesbrecht, 1895 and Thermocheres Kim, 2010 share an urosomite with 3 post-genital urosomites in the female (Claus 1889; Canu 1893; Giesbrecht 1895; Nicholls 1944; Stock 1971, 1989; Humes 1987, 1988, 1989, 1998; Ivanenko & Defaye 2004; Kim 2010) instead of two as observed in the new genus. The tergites of the third pedigerous somite are not expanded posteriorly over rest of prosome in Neoasterocheres gen. nov. as it is seen in Phyllocheres (Humes 1996b). Both Discopontius Nicholls, 1944 and Meandromyzon coronatum Stock, 1989 have 2 post-genital segments like Neoasterocheres n. gen., but Discopontius has a 2-segmented P4 endopod and M. coronatum has a 1- segmented mandibular palp, both characters diverging from those observed in Neoasterocheres n. sp. (Nicholls 1944, Stock 1989).

Chelacheres has a claw-like element distally on the antennary endopod (Stock & Humes 1995), which is absent in the new genus. Neoasterocheres gen. nov. differs from Cephalocheres Kim, 2010, Humesimyzon Kim, 2010 and Mimacheres Leigh-Sharpe, 1934 once it does not show reduction or absence of the antennary exopod to a single seta (Leigh-Sharpe 1934; Kim 2010) or the 2-segmented condition as described in Humescheres Kim, 2005 (Kim 2005). The mandibular palp is 1-segmented or absent in Asteropontella Stock, 1989, Asteropontius Thompson & Scott, 1903, Asteropontopsis Stock, 1987, Gascardama Kim, 2010, Hetairosyna Humes, 1991, Hetairosynopsis Humes, 1996 and Stenomyzon Kim, 2010 (Thompson & Scott 1903; Stock 1987, 1989; Humes 1991, 1996a; Kim 2010) and 2-segmented with 1 seta in Parasterocheres Humes, 1996 (Humes 1996b) while a 2- segmented palp with 2 distal setae is found in Neoasterocheres gen. nov. Hetairosynella Kim, 2010 possesses five setae on the inner maxillulary lobe (Kim 2010) instead of four as in Neoasterocheres n. gen. Stockmyzon Bandera & Huys, 2008 shows a five-segmented maxilliped (Bandera & Huys 2008) differencing from Neoasterocheres gen. nov. The new genus can also be distinguished by its possession of a biramous P4 with 3-segmented exopod and endopod. This condition diverges from Cletopontius Thompson & Scott, 1903, Cyclocheres Kim, 2010, Cystomyzon Stock, 1981, Discopontius Nicholls, 1944, Kolocheres Johnsson, 1999, Obesiella Ridewood, 1903, Oedomyzon Stock, 1981, Peltomyzon Stock, 1975, Siphonopontius Malt, 1991 and Tuphacheres Stock, 1965 (Ridewood 1903; Thompson & Scott 1903; Nicholls 1944; Stock 1965, 1975a, 1981; Malt 1991; Johnsson 1999a; Kim 2010). In addition, Neoasterocheres gen. nov. has two inner setae on the second endopodal segments of all legs, thus diverging from Cecidomyzon Stock, 1981, Gomumucheres Humes, 1996, Hermacheres Stock, 1987, Indomyzon Ummerkutty, 1966, Inermocheres Boxshall, 1990, Onychocheres Stock & Gooding, 1986, Psilomyzon Stock, 1965, Scottocheres Giesbrecht, 1897, Sinopontius Giesbrecht, 1897 and Tychomyzon Humes, 1991, all with the second endopodal segments of at least one leg armed with one inner seta (Giesbrecht 1897; Ummerkutty 1966; Stock & Gooding 1986; Boxshall 1990; Humes 1991, 1996b; Stock, 1965, 1975a, 1981, 1987). Neoasterocheres gen. nov. differs from Acontiophorus Brady, 1880, Asterocheroides Malt, 1991, Asteropontoides Stock, 1975, Hammatimyzon Stock, 1981, Mesocheres Norman & Scott, 1905, Orecturus Humes, 1992, Paracontiophorus Eiselt, 1961 and Parasteropontius Johnsson, 1999 by carrying five elements on the third endopodal segment of leg 4 (Brady 1880; Norman & Scott 1905; Eiselt 1961; Stock 1975b, 1981; Malt 1991; Humes 1992; Johnsson 1999b). Laperocheres Ivanenko, 1999 lacks an inner seta on the first exopodal segment of legs 1–4 (Ivanenko 1999) while these setae are present in Neoasterocheres gen. nov. Neoasterocheres gen. nov. can be distinguished from Monocheres Stock, 1966 by the presence of a free one-segment of leg 5 (Stock 1966a) and additionally differs from Doropontius Thompson & Scott, 1903 which possess a two-segmented leg 5 (Thompson & Scott 1903).

Further, some of the species treated as valid Asterocheres by Kim (2010) share the antennulary fusion pattern herein defined as belonging to Neoasterocheres gen. nov. So, we propose that six species, i.e., A. enewetakensis Humes, 1997, A. dysideae Humes, 1996, A. rotundus Malt, 1991, A. scutatus Stock, 1966 and A. serrulatus (Humes, 1996) are transferred to the new genus. Asterocheres humesi Varela, 2012 was described posteriorly to Kim´s work and also have this fusion pattern in the antennule (Varela 2012) and should be in Neoasterocheres gen. nov.

Etymology. The name ‘ Neoasterocheres ’ is a combination of ‘neo’ (from the Greek adjective meaning ‘new’) and Asterocheres, referring to the similarities between the new genus and its congener.

Type species by original designation: Neoasterocheres breviseta sp. nov.