Â鶹ÒùÔº

Rare giant shrimp is more widespread than previously believed, new findings reveal

Alicella gigantea, a rarely sighted giant crustacean that inhabits the depths of the ocean and can reach up to 34 cm in length, is not rare after all. This species, though elusive, is one of the most widely distributed species of its kind, inhabiting and thriving in more than half of the world's oceans across the globe, as per a recent Royal Society Open Science by researchers from the University of Western Australia.

Amphipods like A. gigantea are shrimp-like crustaceans known for their narrow frames and a signature hump-backed curl in their posture. Found in nearly every aquatic habitat on Earth, amphipods are one of the most diverse crustacean orders, with over 10,000 known species. They roam the deep sea, from the abyssal depths (3,000–6,000 m) to beyond hadal zones (>6,000 m), but their number significantly decreases as we dive deeper towards the ocean bed.

The first sighting of a 28 cm-long A. gigantea was captured on camera at a depth of 5,304 meters in the abyssal North Pacific, though its identity remained unknown at the time. Subsequent captures eventually confirmed the species as A. gigantea.

For the next two decades, however, the species seemed to have disappeared from human view, giving rise to the perception that this deep-sea giant was exceptionally rare. Furthermore, very few specimens have been recovered, which hinted at a low population density.

Despite its broad distribution across the seven seas, our understanding of A. gigantea remains limited, particularly regarding its demography, genetic diversity, and population structure. To date, only seven studies have provided DNA sequence data.

For this study, the researchers aimed to offer a comprehensive global view of the species to date. The team brought together data from published sources and new expeditions, compiling all 195 records of A. gigantea across 75 locations, spanning 15 seafloor features in the Pacific, Atlantic, and Indian Oceans.

The two mitochondrial genes (16S, COI) and one nuclear gene (28S) of the newly collected specimens were subjected to genetic analysis to get a better understanding of their distribution patterns and evolutionary history throughout geological time.

Genetic analyses revealed exceptionally low divergence among A. gigantea populations worldwide, suggesting strong genetic conservation and supporting the existence of a single, globally distributed species.

The haplotype network, used to visualize genealogical relationships, revealed shared haplotypes—sets of genetic markers located on a single chromosome—and showed minimal differentiation among A. gigantea across regions. The Pacific Ocean was identified as the species' largest potential habitat, with 75% of its area lying within the suitable depth range.

This study establishes that A. gigantea, the infrequently collected supergiant amphipod, is not actually rare, as it occupies 59% of the world's oceans and has an exceptional global range.

The researchers note that by comparing molecular markers from the newly collected specimens to existing datasets, this study lays a solid groundwork for developing a global species hypothesis for future testing.