Making Use of Relicts: Brisingid Seastars Aggregate on Hydrothermally Inactive Sulfide Chimneys Near Black Smokers

Daniel Woods,Christopher R German,Michael J Cheadle,Cindy L Van Dover,Barbara E John

doi:10.3389/fmars.2022.774628

Abstract

When hydrothermal activity ceases at black-smoker chimneys on mid-ocean ridges, populations of associated invertebrates hosting chemoautotrophic endosymbionts decline and then disappear, but the chimneys can persist on the seabed as relicts. Suspension-feeding brisingid seastars colonize hydrothermally inactive (relict) chimneys on the East Pacific Rise (EPR), though their distribution relative to available hard substrata and proximity to hydrothermal activity is poorly documented. In this study, brisingid abundance on sulfide and basalt substrata was assessed along an ∼3,700 m ROV Jason II transect at the summit of Pito Seamount (SE Pacific; ∼2,275 m). Brisingids were non-randomly distributed, with highest densities (up to ∼300 m–2) on relict sulfides chimneys near active black smokers. Brisingids were relatively uncommon on basalt substrata, and absent on black smokers. We infer that both relict sulfide structures and proximity to black smokers play key roles in the maintenance of dense brisingid populations on Pito Seamount and in similar environments on the EPR. Our observations suggest that experimental introduction of “artificial” relict chimneys providing microtopographic relief could test whether such an approach might mitigate potential impacts of mineral extraction on populations of suspension-feeding invertebrates.

Highlights

High-temperature hydrothermal activity in the deep sea typically forms metersto 10 s-of-meters-high, 3-dimensional structures composed of precipitated sulfide minerals rich in copper, iron, zinc, and other metals (Petersen et al, 2016)
We provide a descriptive account of brisingid distributions, using tests of a posteriori hypotheses to assess whether brisingid seastars (i) aggregate on relict sulfides, especially sulfide chimneys with microscale relief (∼0.5–15 m), (ii) are relatively uncommon on basalt substrata, (iii) are absent on black smokers, and (iv) are scarce in the nearby Pito Deep, where hydrothermal activity is absent (51 km northwest of Pito Seamount, Figure 1)
Apart from an occasional gorgonian whip coral (n = ∼11 total), two seastars (2 different species), and invertebrate taxa associated with black smoker complexes, other benthic megafauna were inconspicuous along the transect

Summary

Introduction

High-temperature hydrothermal activity (black smokers) in the deep sea typically forms metersto 10 s-of-meters-high, 3-dimensional structures (chimneys) composed of precipitated sulfide minerals rich in copper, iron, zinc, and other metals (Petersen et al, 2016). Long-duration hydrothermal activity and large accumulations of mineral precipitates at relatively low spatial frequencies are known from intermediate-, slow-, and ultraslow- spreading mid-ocean ridges (Rona et al, 1993; Halbach et al, 1998; Karson et al, 2015), as well as offaxis sites on the fast-spreading East Pacific Rise (EPR) (Hekinian and Fouquet, 1985; Fouquet et al, 1996). On the EPR itself, the spatial frequency of vent fields with high-temperature (∼350◦C) black smokers is relatively high, on the order of ∼4 fields per 100 km (Chen et al, 2021), and the duration of venting activity is short (decadal scale or less; Lalou and Brichet, 1982; Karson et al, 2015)

Methods

Results

Conclusion