Two Eyeless, Sesame-Sized Crustaceans Found in Japan's Ryukyu Groundwater Hint at Hidden Underground Life

On Ishigaki Island, freshwater runs through a cave, and in it live animals that measure between 1.4 and 3.2 millimeters. The largest of them approach the size of a sesame seed. They have no eyes and no color. In June 2026 they were formally described as a new species, Bogidiella painushima.

The description came from Ko Tomikawa, a professor at Hiroshima University, and his collaborators, published in the journal Zootaxa. The university put out a news release on Aug. 17. It covers two new species, both amphipods, a group of small crustaceans related to shrimp, and both placed in the genus Bogidiella.

The second species, Bogidiella isajii, is a little larger, at about 3.8 to 4.3 millimeters. Scientists collected it from an anchialine cave on Minamidaito Island, a kind of underground habitat where the water is connected to the sea through passages below the surface. So the two animals come from different kinds of underground water, one freshwater and one linked to the ocean. That suggests the subterranean habitats of the Ryukyu Islands can support specialized communities under quite different conditions.

According to Hiroshima University, both species are eyeless and unpigmented. Their appendages are elongated, and their swimming legs, called pleopods, are reduced. These are traits associated with life in subterranean water, where sunlight is essentially absent. In permanent darkness, eyes become less useful over evolutionary time, and animals in such places may lean on other sensory structures to find food and move around. The same pattern has turned up repeatedly in cave-dwelling organisms around the world. The family these amphipods belong to, Bogidiellidae, is known from overlooked waters including groundwater and narrow underground waterways, ranging from freshwater to brackish and seawater-influenced settings.

The researchers identified the two species by examining their anatomy closely and comparing it with other members of Bogidiella. The find expands what is known about a group that lives where people rarely can reach.

The habitat is arguably the larger part of the story. Groundwater systems are hard to study because they cannot be surveyed on foot or watched from above the way forests, rivers and coastlines can, which makes their biodiversity easy to underestimate. Tomikawa, the corresponding author, noted that groundwater has long mattered to people as drinking water and as water for agriculture, and that conserving groundwater environments is an increasingly important challenge, as quoted in an Eureka Alert report. Seen that way, groundwater is not only a supply but an ecosystem.

The discovery fits a wider pattern in the island chain, which stretches through southwestern Japan toward Taiwan and mixes land, freshwater, coastal and underground environments. Japan's Environment Ministry identifies cave and spring systems on Miyakojima as important wetlands, because underground rivers are well developed there and spring-fed caves support rare crustaceans and other aquatic species. Separately, the D-ARK project has used remotely operated vehicles to explore deep-sea karst caves around the Daito Islands. JAMSTEC reported in 2026 that researchers found new and rare organisms there, including species found nowhere else in Japan.

The finding does not prove that the Ryukyu Islands hold countless undiscovered underground species. What it shows is that familiar landscapes can hide biodiversity researchers have yet to document. The work could eventually help scientists understand how isolated groundwater systems shape evolution, and how subterranean species move, diversify and survive in the dark. For now, the two animals remain nearly invisible, in caves most people will never see.