Researchers have identified two new species in the genus Bogidiella, small, shrimp-like amphipods that live in subterranean aquatic environments, from cave waters on the Ryukyu Islands, a subtropical island chain in southwestern Japan. These creatures are clear products of their underground, underwater environment, often foregoing pigmentation and eyes, elongating their appendages, and reducing the size of their swimming legs, called pleopods. Hiroshima University Professor Ko Tomikawa of the Graduate School of Humanities and Social Sciences and his collaborators describe the two amphipod species previously unknown to science in a study published in Zootaxa in June 2026.
Amphipods, specifically those in the family Bogidiellidae, can be found in often overlooked environments, such as narrow connecting waterways and groundwater. These aquatic habitats also vary widely, from freshwater and brackish water to seawater-influenced systems such as anchialine waters, which are connected to the ocean through underground passages.
The new species in this study are Bogidiella isajii and Bogidiella painushima. B. isajii was collected from an anchialine undersea cave on Minamidaito Island, while B. painushima was found in freshwater flowing through a cave on Ishigaki Island. Cave waters like these are closely tied to groundwater environments, which can support specialized life adapted to darkness.
"Groundwater has long been used in human life as a source of drinking water and for agricultural purposes, but in recent years, the conservation of the groundwater environment has become a major challenge," said Tomikawa, corresponding author of the study.
B. isajii is named after Mr. Yoshitaka Isajii, its discoverer. Specimens of the species measure approximately 3.8-4.3 mm long, about the size of a sesame seed. The species is described as lacking the inner ramus, which is a branch-like appendage on its pleopods. Typically, amphipods have three pairs of pleopods, each with two branches. The rami armature (hardened structure of the exoskeleton; provides shape and utility) of the uropods ("legs" closest to the rear of the animal) in pairs 1 and 2 is also absent. Additionally, the telson, or what most people know as the fan-like structure on the tail of a shrimp (though in these species it is more of a flap), is a different shape than seen in other members of the genus, and appears to be wider than it is long. The telson generally is a useful part in distinguishing amphipod species.
B. painushima's name is derived from "Painu-shima," an Okinawan term meaning "southern island." Specimens of the species measure approximately 1.4–3.2 mm long, making them slightly smaller than B. isajii. Although morphologically similar to other species in the genus, B. painushima differs in features of the head, antennae, telson, and gnathopods, appendages used in feeding and grooming. Among these differences are the shape of the antennal sinus, a recess in the head where the antenna attaches, and the size of the gland cone on the second segment at the base of the second antenna. The armature of the telson is also distinct, and differences in the gnathopods are another defining characteristic of the species.
Samples were taken to determine the genetic sequences present in these species for future studies.
Based on the surprising diversity found in groundwater and anchialine environmental samples, Tomikawa and his team would like to conduct more comprehensive surveys to further uncover some of the biological mysteries groundwater might hold. Additionally, research towards the evolutionary history of the new species and species in these environments in general is necessary to understand what biodiversity looks like in subterranean aquatic areas. Once this is achieved, conservation efforts towards groundwater environments can be enacted.
Conserving biodiversity, especially in areas unthought of by most people, is a larger goal of the study. Researchers hope that through exploration, education, and awareness, a more thoughtful society can be realized and the deterioration of natural areas due to human activities can be curbed.
The research team also included Hiro Yoshimura and Mushi Ugomeki of the Graduate School of Humanities and Social Sciences at Hiroshima University, and Michitaka Shimomura of the Seto Marine Biological Laboratory at Kyoto University.
The Japan Society for the Promotion of Science helped support this research.
