First quantitative assessment of deep-sea acorn worm (Enteropneusta) in PMN-rich zones of the Central Indian Ocean Basin

Abstract

The deep-sea harbors a diverse array of organisms playing crucial roles in the ecosystem yet remains largely unexplored. Enteropneusta, commonly known as acorn worms, are a key component of the deep-sea benthos as they play a major role in surficial bioturbation and nutrient cycling. However, knowledge about their distribution, abundance, and behavior in these profound depths is scarce. This study provides the first quantitative assessment of enteropneusta (acorn worms) in the Central Indian Ocean Basin at depths greater than 5200 m. The data were collected using high-resolution seafloor imagery captured by an Autonomous Underwater Vehicle (AUV), which is pre-programmed to carry out surveys independently—unlike Remotely Operated Vehicles (ROVs), which require continuous control by operators aboard the surface vessel. In this study the AUV analyzed an area of 0.5 km², revealing 2955 enteropneust fecal trails and 13 acorn worms (12 Tergivelum sp. and single Allapasus sp.). The study examined trail characteristics, including shapes, area coverage, and quality, as well as acorn worm morphology. Spiral trails dominated, with no significant preference for rotational direction. Trail quality was ranked from 1° to 3°, with most trails being 2° and 3° quality. This pioneering work provides crucial insights into deep-sea ecosystem functioning in the region, informing responsible decision-making regarding potential PMN exploration activities. This report also a first kind of its quantitative approach of enteropneusta among all the potential PMN sites in the global ocean. The findings contribute to our understanding of deep-sea biodiversity and ecological dynamics in the Indian Ocean.