Hymenodoridae, Lunina & Kulagin & Vereshchaka, 2024

Lunina, Anastasiia, Kulagin, Dmitry & Vereshchaka, Alexander, 2024, The taxonomic status of Hymenodora (Crustacea: Oplophoroidea): morphological and molecular analyses suggest a new family and an undescribed diversity deep in the sea, Zoological Journal of the Linnean Society (Zool. J. Linn. Soc.) 200 (2), pp. 336-351 : 349-350

Morphological traits in Hymenodoridae

The clade Hymenodoridae is robust in all analyses and represents a deep-sea branch of Oplophoroidea . Although juveniles of Hymenodora may occasionally occur in the mesopelagic zone, especially in high latitudes ( Jónsdóttir 2014), and even contribute to the stomach content of Leach’s storm petrel Hydrobates leucorhous (Vieillot, 1818) (Steele and Montevecchi 1994), adults live mainly in the bathypelagic (Vereshchaka et al. 2019) and abyssopelagic zones (e.g. H. chacei ; Wasmer 2022). The whole family Hymenodoridae is supported by three synapomorphies, which are linked to mouthparts ( Fig. 5C ). Hymenodora shows omnivorous behaviour, with a strong tendency towards carnivory ( Kreibich et al. 2010), and the synapomorphies linked to the mouthparts are likely adaptive to this type of feeding in the bathypelagic realm. The simplified second maxilla and modified first maxilliped might be more efficient for consuming small deep-sea copepods that contribute significantly to the diet of Hymenodora ( Kreibich et al. 2010) .

Within the clade, Hymenodoridae show further specialization for feeding through two nearly opposite traits. Hymenodora possesses a subovoid terminal segment of the second maxilliped attached diagonally, which is likely unsuccessful in catching large and fast-moving planktonic animals ( Fig. 7E ), whereas Sclerodora has a specialized subtriangular terminal segment attached to the penultimate segment transversely and bearing robust terminal setae ( Vereshchaka et al. 2021: fig. 4E), which is adapted for catching actively moving prey.

Another pair of alternative traits in Hymenodoridae is linked to the carapace and pleon strengthening: Sclerodora has a firm integument, whereas the integument of Hymenodora is membranous, with the characteristic reticulum of carinae and sulci making the carapace more rigid. Similar, but not homologous, compensatory strengthening of the carapace (a possible response to increasing carapace loads) has been observed in other Oplophoroidea , such as Meningodora and Notostomus ( Lunina et al. 2021) .

The clade Hymenodoridae also shows a morphological trait linked to the telson, a spinose endpiece in the H. acanthitelsonis + H. chacei clade; this is a homoplasy found also in the clade Oplophoridae ( Systellaspis and Janicella ). All known specimens of H. acanthitelsonis and H. chacei have been collected during near-bottom hauls and are probably benthopelagic. Given that Janicella and most Systellaspis are also benthopelagic ( Vereshchaka 1995), we suggest that the spatulate endpiece on the telson might serve in bedding into sediments.

Morphological traits within the pelagic clades H. frontalis + H. gracilis and H. glacialis are linked to further strengthening of their membranous carapace (an additional crescent-shaped sulcus in the H. glacialis clade) and to various strategies of passive defence (number of pre- and postorbital rostral teeth) and escape behaviour (proportions of the carapace and rostrum). In fact, pelagic decapods may use the rostrum as a passive defensive structure (long, acute and spiny) and/or as a rudder during backwards escape flips (active protection) ( Lunina et al. 2019). Our PCAs ( Fig. 6 ) show that the rostral angle and the numbers of post- and preorbital teeth are three major factors explaining morphological diversification within Hymenodora . With the exception of H. frontalis , which has an unusually long rostrum, these morphological modifications can be recorded using statistical analyses only.

Overall, in contrast to their unusually high molecular divergence, Hymenodoridae show very low morphological diversity, which is likely linked to a homogeneous deep-sea habitat. Remarkable traits can be found only in the likely benthopelagic species, H. acanthitelsonis and H. chacei , whereas evolution of definitely pelagic clades is coupled mainly with feeding specialization (mouthparts), strengthening of the membranous carapace (carinae and sulci) or the proportions of the body. The last of these is the only group of characters that mirrors the cryptic speciation found here through molecular methods.

Figures

Figure 5. Phylogenetic analyses of Oplophoroidea.A, molecular tree (six-gene analysis). Only supported clades are shown (analysis 3). Statistical support is indicated as Bayesian posterior probabilities (black, below branches) and maximum likelihood bootstrap (blue, above branches). B, morphological tree (analyses 4 and 5; trees were similar, and supports are divided by an oblique stroke character if they differed). Only supported clades are shown. Statistical support is indicated as Bremer support (black, below branches) and maximum likelihood bootstrap (red, above branches). C, common morphological synapomorphies (see coding in Supporting Information, Table S4).

Figure 6. Principal component analysis of seven morphological characters of the clade Hymenodoridae. A, molecular clades within Hymenodora glacialis and Hymenodora gracilis combined (analysis 6). B, molecular clades within H. glacialis and H.gracilis analysed separately (analysis 7). The x- and y-axes are eigenvalue scales; the morphometric dataset is presented in the Supporting Information (Table S5); only four most significant factors are shown.

Figure 7. Morphological characters distinguishing three families of Oplophoroidea.A–E, Hymenodoridae (Hymenodora frontalis, St. 7497 d271) mandibles (A), second maxilla (B), proximal endite of second maxilla (C), first maxilliped (D) and second maxilliped (E). F–J, Oplophoridae (Systellaspis debilis, 179RT 23.1) mandibles (F), second maxilla (G), proximal endite of second maxilla (H), first maxilliped (I) and second maxilliped (J). K–O, Acanthephyridae (Acanthephyra quadrispinosa Kemp, 1939, AMK87 7277-D13) mandibles (K), second maxilla (L), proximal endite of second maxilla (M), first maxilliped (N) and second maxilliped (O). All scale bars:1 mm.