Copepods are among the most abundant animals on Earth, tiny crustaceans that teem in oceans, lakes, and rivers, but a remarkable number of species have abandoned the light entirely. The genus Elaphoidella alone contains 219 described species and subspecies worldwide, and its members have been found everywhere from ordinary ponds to springs, karst aquifers, and deep cave systems. Many are stygobitic — obligate inhabitants of groundwater — with pale bodies, reduced eyes, and slender limbs adapted for squeezing through water-filled crevices. Southeast Asia has proven a hotspot for new discoveries, particularly Thailand, where 13 new Elaphoidella species have been described in recent years. Vietnam, by contrast, had recorded only five species before this find, a gap that researchers attribute to insufficient sampling of groundwater microhabitats rather than a genuine scarcity of animals.

The new species was collected in March 2024 by Ngoc-Son Tran of Duy Tan University during a survey of copepod diversity in the mountain districts of Quang Nam Province. The sampling site sits in hilly terrain at roughly 90 meters above sea level, on the Vu Gia stream in Bac Tra My District. To reach the animals, the team used the Karaman-Chappuis method, a classic technique for sampling the hyporheic zone: a hole about one meter from the stream was dug down to the water table, and the water that seeped in was filtered through a fine 60-micrometer mesh. The captured specimens were immediately fixed in 4 percent formaldehyde solution before being dissected and mounted on microscope slides for detailed morphological analysis.

Under the microscope, the holotype female — a completely dissected adult deposited in the zoological collection of Duy Tan University in Da Nang — revealed a body of striking precision. The animal measures between 361 and 477 micrometers, with a mean of 420 micrometers, and has an elongated, subcylindrical form that tapers toward the rear. Its eight-segmented antennules, one-segmented antennal exopod, and the structure of its five pairs of swimming legs confirmed it belongs squarely within Elaphoidella, and more specifically within Group VIII, the sewelli group, as defined by the German copepod specialist Karl Lang in 1948. The anal operculum, a lid-like plate at the rear of the body, carries 38 to 41 fine spinules along its free margin — one of several characters that proved decisive in separating the new species from its relatives.

The closest match to the Vietnamese newcomer is Elaphoidella brancelji, a cave-dwelling species described from Thailand in 2021. Both species share a serrated dorsal margin on the rear body segments, smooth inner margins on their caudal rami — the paired, oar-like appendages at the tip of the abdomen — and similar armature on the first and third through sixth swimming legs. But the differences are unmistakable. In the new species, the caudal rami run parallel and are only about 1.2 times as long as they are wide, whereas in E. brancelji they diverge slightly and are twice as long as wide. The first segment of the first swimming leg’s inner branch reaches only half the length of the corresponding outer segment in the new species, compared with two-thirds in its Thai cousin. The second inner branch of the second swimming leg carries three setae in the Vietnamese animal but four in E. brancelji, and the baseoendopod of the fifth leg bears a small prominence at the insertion of one seta that is entirely absent in the Thai species.

Equally telling are the contrasts with Elaphoidella vietnamica, the only other Vietnamese Elaphoidella described as new to science, found in 1967 in a cave lake in northern Vietnam. The new species has a bell-shaped copulatory pore, two unequal setae on the sixth leg, and a swollen base on one of its caudal setae — all features that differ from E. vietnamica’s narrow, slit-like pore, single seta, and unswollen seta. With the addition of E. quangnamensis, six Elaphoidella species are now recorded from Vietnam, all of them collected from subterranean waters, whether cave pools, phreatic zones, or, in this case, the hyporheic zone of a river.

One mystery stands out: no male of the new species has ever been found. Only female specimens were collected, and all efforts to locate males were unsuccessful. This is not an isolated case. In Thailand, stygobiotic Elaphoidella species such as E. isana and E. namnaoensis have been collected in large numbers of females from many localities, yet no males have ever been discovered. Researchers raise the possibility of strict asexual reproduction, or at least parthenogenesis, in which males are extremely rare and reproduction proceeds predominantly without them. Parthenogenesis is often observed in groundwater-dwelling populations, where limited nutrient availability and constrained mobility may favor such a strategy, offering an adaptive advantage in colonizing or persisting in these isolated habitats — and potentially explaining how Elaphoidella species have achieved such a widespread global distribution.

The discovery also underscores how much of Vietnam’s subterranean copepod fauna remains unexplored. Of the 26 copepod species now known from the country’s subterranean waters, 19 are stygobitic and five are interstitial, and 15 species — 62.5 percent — are considered endemic to Vietnam. Yet the hyporheic zone has been chronically overlooked in Vietnamese copepod surveys, with only five species reported from such habitats to date. Most previous research has focused on cave pools and lakes, driven by Vietnam’s rich karst topography and abundance of caves. The country’s dense river network, with its high ecological diversity in the hyporheic zone, offers strong potential for discovering more stygobiotic copepods and increasing the total number of recorded species.

For now, Elaphoidella quangnamensis stands as the second Elaphoidella species known to be endemic to Vietnam, a stygobitic copepod probably exhibiting parthenogenesis, with its male unknown so far. The parthenogenetic condition might be confirmed if further, more widespread research shows that males are indeed entirely absent. In the meantime, the tiny crustacean serves as a reminder that some of the planet’s most isolated ecosystems — the dark, water-filled spaces beneath our rivers — still hold species that no one has ever seen, waiting for a shovel, a sieve, and a patient microscope to bring them into the light.

Subject of Research: New hyporheic copepod species Elaphoidella quangnamensis discovered in central Vietnam

Article Title:  Elaphoidella quangnamensis sp. nov. ( Copepoda , Harpacticoida , Canthocamptidae ), a new hyporheic species from central Vietnam

Article References: Tran, N.-S., Phung, K. C., & Watiroyram, S. (2025). Elaphoidella quangnamensis sp. nov. (Copepoda, Harpacticoida, Canthocamptidae), a new hyporheic species from central Vietnam. Subterranean Biology, 53, 13-29. https://doi.org/10.3897/subtbiol.53.161072

Image Credits: AI Generated

DOI: 10.3897/subtbiol.53.161072

Keywords: copepod, Elaphoidella quangnamensis, hyporheic zone, groundwater, Vietnam, stygobitic, new species, biodiversity, parthenogenesis, Subterranean Biology, Elaphoidella, quangnamensis