Metacrimenes, Komai & Sakiyama, 2023

Komai, Tomoyuki & Sakiyama, Hiroki, 2023, A new genus and new species of palaemonid shrimp (Decapoda: Caridea), associated with the deep-sea crinoid Metacrinus rotundus (Echinodermata: Isocrinidae), from Suruga Bay, Japan, Zootaxa 5380 (3), pp. 227-246 : 229-230

Metacrimenes n. gen.

[New Japanese name: Torinoashi-yadori-ebi-zoku]

Type species. Metacrimenes fenestra n. sp., by monotypy and present designation.

Diagnosis. Body weakly compressed laterally. Rostrum well developed, reaching beyond distal end of antennular peduncle; lateral carina low but sharply delimited; dorsal margin dentate, posterior-most (epigastric) tooth without basal suture; ventral margin unarmed. Carapace smooth; inferior orbital angle produced into triangular process; antennal spine marginal; supraorbital spine absent; hepatic spine present, not reaching anterolateral carapace margin, without basal suture. Pleon smooth, posterodorsal margin of third somite not particularly produced; first to fifth pleura broadly rounded. Telson with 2 pairs of dorsolateral spiniform setae; posterior margin with 3 pairs of markedly unequal spiniform setae. Cornea well developed, globular. Antennular peduncle basal article with 2 distolateral spines. Antennal scaphocerite well developed; basicerite with ventrolateral distal spine. Epistome unarmed. Fourth thoracic sternite without median projection. Mandible without palp. Maxillula with bilobed palp. Maxilla with simple palp; basial endite deeply bilobed. First maxilliped with slender palp; exopod with well-developed caridean lobe and flagellum; epipod bilobed. Second maxilliped with normal endopod; exopod flagellum-like; podobranch absent. Third maxilliped with ischiomerus fused with basis; exopod greatly reduced to bud-like lobe; arthrobranch absent. First pereopod with fingers unarmed on occlusal margins. Second pereopod equal and similar from right to left, not particularly elongate; articles unarmed; chela cylindrical, occlusal margins of fingers entire, unarmed. Third to fifth pereopods each with biunguiculate dactylus. Uropod with protopodite unarmed posterolaterally; exopod with minute posterolateral spine accompanied mesially by much longer spiniform seta. Gills comprise of pleurobranchs on fourth to eighth thoracomeres (first to fifth pereopods).

Remarks. The greatly reduced exopod of the third maxilliped links Metacrimenes n. gen. to Mesopontonia (represented by seven Indo-West Pacific species: M. brevicarpus Li & Bruce, 2006 ; M. brucei Burukovsky, 1991 ; M. gorgoniophila Bruce, 1967 ; M. gracilicarpus Bruce, 1991 ; M. kimwoni ; M. monodactylus Bruce, 1991 ; M. verrucimanus ) and Waldola Holthuis, 1951 (represented only by the type species W. schmitti Holthuis, 1951 from the eastern Pacific), but in these genera the third maxilliped exopod is absent (cf. Holthuis 1951; Bruce 1967, 1991, 1996; Li & Bruce 2006; Park et al. 2020), while it is a rudimentary (“bud-like”) lobe in Metacrimenes n. gen. Furthermore, Metacrimenes n. gen. differs from the latter two genera in having a well-developed antennal spine on the carapace; the hepatic spine located in its usual position; the posterior-most tooth of the dorsal rostral series not clearly separated from the preceding teeth; and with the second pereopods being equal in length and similar in shape. In Mesopontonia and Waldola , the antennal spine is absent; the hepatic spine is located at a submarginal position near the anterolateral margin of the carapace; the posteriormost tooth of the dorsal rostral series is distinctly separated from the preceding teeth and located at the epigastric position; and the second pereopods are more or less unequal in size and dissimilar in shape ( Holthuis 1951; Bruce 1967, 1991, 1996; Li & Bruce 2006; Park et al. 2020). Furthermore, Waldola differs from Metacrimenes and Mesopontonia in the poorly developed exopod on the first maxilliped and the absence of an exopod on the second maxilliped ( Holthuis 1951), both of which are well developed in the other two genera. The absence of ventral teeth on the rostrum distinguishes Metacrimenes n. gen. and Waldola from Mesopontonia as in all species of Mesopontonia , the rostrum is armed with ventral teeth ( Bruce 1967, 1991, 1996; Li & Bruce 2006; Park et al. 2020).

The phylogenetic analysis based on the concatenated dataset derived from the mitochondrial 16S rRNA and COI sequences shows a clade cconsisting of five taxa, Metacrimenes fenestra n. sp., Paraclimenes gorgonicola , Mesopenaeus kimwoni , M. verrucimanus , and Brucecaris tenuis (bootstrap support 87.5%). Metacrimenes fenestra n. sp. and Paraclimenes gorgonicola were recovered in sister position, however, with low bootstrap support ( Fig. 8 ). Paraclimenes Bruce, 1995 [represented by two species: P. gorgonicola and P. setirostris ( Bruce, 1991) ] differs from Metacrimenes in the absence of an antennal spine on the carapace; the hepatic spine in a submarginal position; the distinct epigastric position of the posterior-most tooth of the dorsal rostral series; the possession of a well-developed, flagellum-like exopod of the third maxilliped; and with second pereopods being distinctly unequal in length and dissimilar in shape ( Bruce 1991; Park & Fransen 2021). Brucecaris Marin & Chan, 2006 [represented only by the type species B. tenuis ( Bruce, 1969) ] differs from Metacrimenes n. gen. in having a well-developed, flagellum-like exopod on the third maxilliped and the greatly elongate fingers of the second pereopods.

Among the above mentioned taxa, only Metacrimenes fenestra n. sp. and Brucecaris tenuis are crinoid associates ( Marin & Chan 2006). Species of Mesopontonia , Paraclimenes and Waldola are associated with gorgonians (Cnidaria) ( Holthuis 1951; Park et al. 2020; Park & Fransen 2021).

Etymology. An arbitrary combination of two generic name, the host crinoid genus Metacrinus and the palaemonid genus Periclimenes Costa, 1844 , to which the new taxon is superficially similar. Gender: masculine.

Figures

FIGURE 8. Maximum-likelihood (ML) tree constructed from concatenated data sets of the mitochondrial COI and 16S rRNA genes of Metacrimenes fenestra n. gen., n. sp. and closely allied palaemonid taxa (log likelihood -10197.759547). Numbers at nodes are bootstrap values; only values>80% are shown. The outgroup (Brachycarpus biunguiculatus) is omitted.