Five new Tetragoniceps copepod species described from Korean waters

A morphological study of benthic harpacticoid copepods collected in Korean waters identified and described five new species of the genus Tetragoniceps: T. concavus, T. spinulifer, T. brevisetosus, T. unisetus and T. protuberans. The findings establish the first confirmed record of Tetragoniceps in the northwestern Pacific.

The specimens came from five sampling stations around Korea during a biodiversity study of benthic harpacticoids in intertidal and sublittoral habitats. Researchers used SCUBA diving, a 0.1 m² Smith-McIntyre grab sampler and the Karaman-Chappuis method. Samples were sieved through a hand net with 64 μm mesh and immediately preserved in 95% ethanol or fixed in 10% formalin.

Benthic organisms were separated from sediment using Ludox centrifugation. The researchers then sorted Tetragoniceps specimens with a stereomicroscope and preserved them in 80% or 95% ethanol. They examined and illustrated the material with a drawing tube mounted on a BX53 microscope equipped with differential interference contrast. Dissected appendages were mounted on H-S slides in a 1:5 lactophenol-glycerol mixture and observed from both sides, with slide edges sealed using nail varnish.

Descriptions followed the terminology of Huys et al. (1996), while the setal armature formulae for thoracic legs were adopted from Lang (1934). Type material for four species was deposited in the Marine Biodiversity Institute of Korea, and the type material for T. protuberans was deposited in the Honam National Institute of Biological Resources. The study also presented a revised comparison of diagnostic characters and an updated identification key covering all recognized Tetragoniceps species.

All five species were assigned to Tetragoniceps because they combined an elongate first antennulary segment bearing a strong, acute process at its outer distal corner with a non-foliaceous female fifth leg. They also shared a distinctive pattern in the second through fourth swimming legs: one inner seta occurred on the first and second exopodal segments, while the third exopodal segments carried six, five and seven elements in the second, third and fourth legs, respectively. The authors reported that this combination had not been observed in previously described congeners.

The new species were differentiated principally by appendage setation, fifth-leg structure and caudal-ramus form. T. concavus has a cylindrical female caudal ramus about 2.6 times as long as wide, with a slightly concave dorsomedial surface and a constriction near the insertion of seta VII; its male caudal ramus is more elongate, about 6.5 times as long as wide. T. spinulifer has relatively short female caudal rami, 1.6 times as long as wide, which narrow across the posterior two-fifths and bear long spinules along the inner and ventroposterior surfaces.

T. brevisetosus is distinguished by extremely short inner setae on the second exopodal segment of the second through fourth swimming legs and by comparatively short apical setae on the corresponding endopods. Its female fifth-leg exopod has a length-to-width ratio of 4.3, while the male exopod has an elongate distal extension and reduced armature. T. unisetus has a comparatively small process on its first antennulary segment, one apical seta on the distal segment of the mandibular exopod, two setae on the maxillular exopod and five setae on the female fifth-leg exopod. Its male second-leg endopod bears a conspicuous process adjacent to the long apical seta.

T. protuberans has a cylindrical female caudal ramus about three times as long as wide, with a conspicuous protuberance on the ventroposterior margin at the outer distal corner. Its female fifth-leg exopod has a length-to-width ratio of 3.1, and its male exopod has an elongate spinous distal extension. Unlike the other four newly described species, T. protuberans was collected from a brackish habitat where a river meets the sea.

Re-examination of original descriptions and illustrations led the researchers to interpret three stout inner elements on the female fourth-leg third exopodal segment as inner elements in all known Tetragoniceps species. The study also highlighted sexual dimorphism in the modified outer distal corner of the male fourth-leg second exopodal segment and in the loss of the distal-most inner seta from the male fourth-leg third exopodal segment. Similar modifications occur in several other tetragonicipitid genera, but the authors said their broader taxonomic and phylogenetic significance requires further evaluation. The research was based on preserved specimens and morphological evidence, and it documents species diversity and distribution rather than ecological abundance or population trends.

Subject of Research: Morphological description of five new Tetragoniceps copepod species from Korean waters

Article Title: Five new species of Tetragoniceps Brady, 1880 ( Copepoda , Harpacticoida , Tetragonicipitidae ) from Korea

Article References: Cho, K., Seo, J., Kim, J. G., & Lee, J. (2026). Five new species of Tetragoniceps Brady, 1880 (Copepoda, Harpacticoida, Tetragonicipitidae) from Korea. ZooKeys, 1293, 141-186. https://doi.org/10.3897/zookeys.1293.206992

Image Credits: AI Generated

DOI: 10.3897/zookeys.1293.206992

Keywords: Tetragoniceps, harpacticoid copepods, new species, Korean waters, benthic biodiversity, taxonomy, morphology, northwestern Pacific, species identification, Five, species, Brady