Introduction

The hadal zone is the deepest of the oceanic biozones, comprising areas that lie from 6,000 to almost 11,000 m deep, and make up 45% of the oceans total depth range. Areas that reach these extreme depths are shaped by geological activity and tectonic plate movement, forming unique features such as trenches and fracture zones that are often isolated from each other (Jamieson and Stewart 2021). This isolation, especially when coupled with extreme hydrostatic pressures, makes the hadal zone an intriguing ecosystem for understanding the diversity of organisms and genetic connectivity between ultra deep systems (Jamieson et al. 2010).

Scavenging amphipods are one of the most prominent groups of organisms at hadal depths (Dahl 1959; Wolff 1959; Hessler et al. 1978). These scavengers are in such abundance that they are a key prey resource for larger organisms (Jamieson and Weston 2023; Wainwright et al. 2026). For example, above ~8,200 meters, large teleosts (bony fish) such as snailfish (Liparidae) and cusk-eels (Ophidiidae) are known to prey upon scavenging amphipods (Jamieson et al. 2010; Dasgupta et al. 2024) whereas at deeper depths, the top predators shift to invertebrate groups. Predatory hadal amphipods have known representatives from the families Eusiridae (Lörz et al. 2018; Weston et al. 2024) and Pardaliscidae (Jamieson et al. 2012) and are less frequently caught in baited traps tailored for scavenging species due to their preference for live prey. Baited traps are currently the most common biological hadal sampling method (Jamieson et al. 2013), and as such the resulting lack of captures of predatory amphipods provide a gap in understanding of the true diversity of non-scavenging amphipods (Weston et al. 2024). In situ observations of amphipod predation events by other amphipods have primarily been observed from the genus Princaxelia (Jamieson et al. 2012).

Princaxelia Dahl, 1959 is a pardaliscid amphipod primarily known from hadal trenches across the western Pacific (Kamenskaya 1981). They are distinguished from other pardaliscid amphipods by a large, elongated maxilliped palp that extends over twice the length of the outer plate (Karaman 1974). There are currently five species described within the genus: Princaxelia abyssalis Dahl, 1959 initially described from the Kermadec Trench, with further reports from the Aleutian, Kuril-Kamchatka, Izu Bonin (Izu-Ogasawara), Yap, Japan, Philippine, and Bougenville trenches at depths between 6,435 and 9,530 m (Kamenskaya 1981); Princaxelia magna Kamenskaya, 1977 from the Yap Trench at depths between 7,190 and 7,250 m; Princaxelia jamiesoni Lörz, 2010 initially described from the Japan and Izu-Ogasawara trenches, but further reported from the Kuril-Kamchatka Trench at depths between 7,703 and 9,316 m (Jażdżewska and Mamos 2019); Princaxelia marianaensis Tomikawa & Watanabe in Tomikawa, Watanabe, Tanaka & Ohara, 2021 from the Mariana Trench at 5,683 m depth; and Princaxelia stephenseni Dahl, 1959, the type specimen, from the northwest Atlantic off the coast of Iceland at a depth of 1,505 m (Stephensen 1931). Being a predatory genus, the lack of captured specimens has caused difficulties and uncertainties in the taxonomy of Princaxelia.

This study describes two new species; the first species, Princaxelia malohi sp. nov., is described using five specimens captured between 8,000–8,350 m in the Tonga Trench (SW Pacific Ocean). Supplemental in situ footage from Princaxelia species from this expedition is also provided. The second species, Princaxelia kahat sp. nov., is described from the Mariana Trench (NW Pacific Ocean) at depths between 8,000–8,964 m. These represent the sixth and seventh species described within the genus, with Princaxelia malohi sp. nov., constituting the first identified occurrence of Princaxelia from the Tonga Trench.