[IIOE2-EA03] Saint-Paul and AmSterdam Islands Biodiversity Assessment (SPASIBA)

Lead Investigator :

1. Laure CORBARI, Museum National d'Histoire Naturelle, UMR 7205 ISYEB, 55, rue Buffon 75005 Paris, France
   laure[dot]corbari[at]mnhn[dot]fr

Other Key Participants:

1. Stephane HOURDEZ - Co PI cruise , LECOB, UMR 8222, CNRS-UPMC, Observatoire Oceanologique de Banyuls, France
   hourdez[at]obs-banyuls[dot]fr
2. Thomas SAUCEDE - Co PI cruise , UMR 6282 Biogeosciences, Universite Bourgogne Europe, France
   thomas[dot]saucede[at]u-bourgogne[dot]fr

Period of Project: end 2027 - early 2028 (exact dates should be confirmed soon by the French fleet)

Brief description of the Project:

The Saint-Paul and Amsterdam (SPA) islands are among the most isolated islands on the planet. Due to their remote geographical location, the SPA area encompasses a wide range of geomorphological features, both in coastal and bathyal zones, suggesting a unique diversity of habitats and associated benthic organisms. SPA is situated at the convergence of distinct benthic biogeographical provinces, suggesting diverse and contrasting biogeographical affinities. The last scientific oceanographic cruise took place nearly 40 years ago, and the available specimen collections are now unsuitable for genetic analyses that align with current systematic practices. The benthic environments of the SPA islands have been little explored over time and across depths, and biological data remain fragmentary. Based on the limited existing samples, affinities with South African fauna have been proposed, but alternative biogeographical hypotheses exist and need to be tested based on new samples and genetic data. The various hypotheses regarding dispersal, colonization, or endemism mechanisms in this remote region may be biased due to insufficient sampling. Therefore, an understanding of biogeographical patterns must now rely on genetic data.

To address the knowledge gaps regarding shallow and deep benthic environments of the SPA islands and to answer questions related to connectivity, biogeography, and endemism, the SPASIBA mission is structured around complementary scientific objectives:

• To explore and acquire new data on the benthic biodiversity (fauna/flora) of the SPA region, establish a taxonomic inventory, and gather new information by exploring new sites and habitats at greater depths (up to 3000 m);
• To describe and analyse species assemblages by studying community dynamics and species habitat preferences (e.g., kelp forests, seamounts, shallow hydrothermal zones, abyssal plains);
• To study benthic connectivity through phylogeographic analyses to determine the level of genetic isolation and geographic endemism between the two islands, and by comparison, with more distant regions such as those located in the southern Indian Ocean (e.g., Crozet, Kerguelen, Walters Shoal, South Madagascar, South Africa).

Region(s) of study:

The exploration zones were defined on the basis of existing biological data (Zones 1 and 2, which include sampling stations from Beurois and the MD50 cruise, as well as areas exploited by fisheries). These areas will be resampled in order to address current knowledge gaps. The operations will target both shallow-water and deep-sea habitats, focusing on geomorphological structures that are already known but at greater depths than previously explored, as well as on areas that have never been studied (i.e., Zone 3).

Overview of selected zones (blue/yellow squares) with positions of the selected seamounts (yellow stars).

• Zone 1 constitutes the main focus of the expedition (16 days on site). It encompasses a wide range of environments, from shallow coastal waters to depths of 3,000 m, and includes numerous seamounts-some of which are subject to fishing activities, while others remain pristine-as well as extensive abyssal plains.
• Zone 2 was selected primarily because of the presence of the large Cap Horn seamount and several additional seamounts located on the opposite side of the Amsterdam fracture (4 days on site). Focus could be made in the area close to Cap Horn seamount where hydrothermal activity has been recently reported (January 2025, OHA-Geodams project).
• Zone 3, in contrast, has never been investigated biologically. Although this area has already been mapped, revealing numerous seamounts (notably the 'Chains of the Dead Poets'), it will be the first time that dedicated biological sampling is carried out there. Their isolated position, northeast of the SPA plateau, offers the opportunity to compare benthic assemblages. Sampling of their summits (ranging from 500 to 1,000 m depth, comparable to those of the SPA area) will generate new biological data and contribute to a better understanding of connectivity patterns within the region.

The sampling program will be structured around two main exploration axes, each requiring specific sampling strategies:

• An inventory of coastal benthic biodiversity (Zone 1, blue squares), through diverse diving samplings (quadrats, brushing techniques, and underwater suction samplers; eDNA), complemented by photo/video transects (diving and SCAMPI transects below 100 m depth) for habitat characterization. Particular emphasis will be placed on habitats dominated by Macrocystis/Laminaria and coralline calcareous algae, habitat-forming invertebrates (bryozoans and cnidarians), as well as the shallow-water active hydrothermal site located within the crater of St. Paul.
• An inventory of deep-sea benthic biodiversity (Zones 1-3, yellow squares and stars,) encompassing a variety of geomorphological features such as banks, seamounts, slopes, and abyssal plains. These surveys will employ a range of sampling devices, including beam trawls, Waren dredges, and suprabenthic sledges. Deep sea habitats will be documented by an integrative methodology coupling photo/video (SCAMPI transects) with benthic samplings performed by towed gears (Hanafi-Portier et al. 2021).