The hadal zone, defined as ocean depths from 6,000 meters to nearly 11,000 meters, accounts for roughly 45 percent of the ocean’s total depth range. These extreme environments are carved by tectonic activity into trenches and fracture zones that are often isolated from one another, creating natural laboratories for studying how life diversifies under crushing hydrostatic pressure. Scavenging amphipods are among the most conspicuous animals at hadal depths, swarming in such numbers that they serve as a key prey resource for larger organisms. Above approximately 8,200 meters, bony fish such as snailfish and cusk-eels prey on these scavengers, but at greater depths the top predators shift to invertebrate groups, including amphipods that hunt their own kind.

Predatory hadal amphipods are far less frequently captured than their scavenging relatives, largely because baited traps, the most common biological sampling method at these depths, are tailored to attract scavengers rather than live-prey hunters. This sampling bias has left a significant gap in understanding the true diversity of non-scavenging amphipods. Princaxelia, a genus of pardaliscid amphipods distinguished by an elongated maxilliped palp that extends over twice the length of the outer plate, is primarily known from hadal trenches across the western Pacific. Because specimens are rarely caught, the taxonomy of the genus has long been hampered by incomplete material and uncertain diagnostic features.

The first new species, Princaxelia malohi, was described from five specimens captured between 8,200 and 8,350 meters in the Tonga Trench during the 2024 Inkfish Tonga Trench Expedition aboard the research vessel Dagon. The specimens were collected using funnel traps baited with mackerel and attached to free-fall landers, then fixed immediately in high-concentration ethanol and stored at minus 12 degrees Celsius. The specific epithet malohi derives from the Tongan word for mighty or powerful, a nod to the animal’s predatory prowess and the adaptations it must possess to survive at such extreme depths. This species constitutes the first identified occurrence of Princaxelia from the Tonga Trench, a hadal feature that had previously yielded only unidentified predatory amphipod observations.

Key diagnostic features of Princaxelia malohi include the presence of two plumose setae on the inner plate of maxilla 1 and a single strong projection near the base of the dactylus on both gnathopods, the appendages used for grasping prey. The holotype, a mature male measuring 35.1 millimeters, is deposited at the Museum of New Zealand Te Papa Tongarewa. Specimens were imaged using focus-stacked macro photography, dissected appendages were mounted on microscope slides, and taxonomic figures were drawn from composite images using open-source vector illustration software. Length ratios of appendages were derived from images using image analysis software, ensuring precise morphological documentation.

The second species, Princaxelia kahat, was described from specimens collected between 8,000 and 8,964 meters in the Mariana Trench, deployed from the vessels RV Falkor in 2014 and TV Shinyo-Maru in 2017. The name kahat comes from a Chamorro word meaning to hunt stealthily, as if stalking or sneaking, reflecting both the observed predatory behavior of the genus and its habitat in the darkness of the hadal zone. The holotype, a mature male measuring 59.3 millimeters, is deposited at the Smithsonian National Museum of Natural History. This is only the second species described from the Mariana Trench, and it is morphologically very similar to Princaxelia magna, a species whose type specimen has never been located, complicating confident assignment of the new material.

Diagnostic features of Princaxelia kahat include six plumose setae on the inner plate of maxilla 1, three to five projections near the base of the dactylus on gnathopod 1, and four to six projections on gnathopod 2. The species exhibits pronounced sexual dimorphism, with females possessing a slender, setose six-articulate accessory flagellum on the first antenna and a much stronger distomedial peduncular projection on the first uropod than males. Antennae are extremely elongate, extending over 70 percent of body length, with the primary flagellum bearing up to 108 articles. Scanning electron microscopy of Princaxelia malohi revealed a smooth exoskeletal surface rather than the scale-like structures reported for some other pardaliscid amphipods, suggesting that surface sculpturing may become useful as SEM is more widely applied to deep-sea amphipod taxonomy.

DNA barcodes at the 16S rRNA and COI mitochondrial regions were generated for all type specimens where possible, adding to the limited but growing genetic database for the genus. Pairwise genetic distances between the two new species were substantial, ranging from 15.76 to 16.42 percent for uncorrected p-distance at 16S, confirming their status as distinct taxa. The addition of genetic data is particularly important for Princaxelia, for which sequence data remains scarce. At present, only one previously described species has an associated COI sequence, making the new sequences a vital reference point for future work on the taxonomy, diversity, and biogeography of deep-sea pardaliscid amphipods.

In situ video footage from the baited landers captured remarkable behavioral observations. At 8,200 meters in the Tonga Trench, Princaxelia individuals were seen capturing amphipod prey using their anterior appendages, holding prey between the gnathopods and maxillipeds while swimming, then restraining prey against the sediment surface during consumption. The animals adopt a distinctive free-swimming posture in which pereopods 3 and 4 are tucked beneath the body, pereopod 5 is angled upward, and pereopods 6 and 7 trail behind. Large aggregations were observed at baited deployments in both trenches, with up to five individuals simultaneously in the Tonga Trench and up to seven in the Mariana Trench. These observations suggest that Princaxelia is locally abundant where prey densities are high, particularly around food falls or natural aggregations of scavenging amphipods.

The discovery that two Princaxelia species occur within a single trench system provides the first evidence of more than one member of the genus coexisting in the same hadal feature. Princaxelia marianaensis was previously collected from cold seep habitats at 5,683 meters, whereas Princaxelia kahat inhabits boulder-field habitats below 8,000 meters, suggesting possible niche partitioning by depth or habitat type. Below approximately 8,300 meters, where fishes reach their lower depth limit and disappear, Princaxelia may occupy a particularly important predatory role within hadal amphipod communities. The researchers also updated a morphological matrix and dichotomous key to the genus to aid future identification, noting that the number of basal projections on the gnathopod dactyli is likely a diagnostic feature that should be recorded in all future Princaxelia captures. Together, the new descriptions, genetic data, and behavioral observations show that Princaxelia is not simply a rare taxonomic curiosity but a potentially important predator in the deepest food webs on Earth, even as fundamental aspects of its taxonomy, behavior, and ecology remain poorly understood.

Subject of Research: Two new species of Princaxelia (Crustacea, Amphipoda, Pardaliscidae) from hadal depths of the Tonga and Mariana trenches (Pacific Ocean)

Article Title: Two new species of Princaxelia ( Crustacea , Amphipoda , Pardaliscidae ) from hadal depths of the Tonga and Mariana trenches (Pacific Ocean)

Article References: Duffy, G. A., Wainwright, J. A., Gonzalez, B. C., Bond, T., & Jamieson, A. J. (2026). Two new species of Princaxelia (Crustacea, Amphipoda, Pardaliscidae) from hadal depths of the Tonga and Mariana trenches (Pacific Ocean). ZooKeys, 1289, 303-334. https://doi.org/10.3897/zookeys.1289.204655

Image Credits: AI Generated

DOI: 10.3897/zookeys.1289.204655

Keywords: Princaxelia, Amphipoda, Hadal zone, Tonga Trench, Mariana Trench, Deep-sea taxonomy, Predatory amphipods, species, Crustacea, Pardaliscidae, hadal, depths