281 © The Author(s) 2020. Published by Oxford University Press on behalf of The Linnean Society of London. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com Zoological Journal of the Linnean Society, 2021, 193, 281–294. With 3 figures. Geochemistry drives the allometric growth of the hydrothermal vent tubeworm Riftia pachyptila (Annelida: Siboglinidae) NADEZHDA RIMSKAYA-KORSAKOVA 1, * , , DIEGO FONTANETO 2 , SERGEY GALKIN 3 , VLADIMIR MALAKHOV 1 and ALEJANDRO MARTÍNEZ 2, 1 Department of Invertebrate Zoology, Faculty of Biology, Lomonosov Moscow State University, 119234 Moscow, Russia 2 Molecular Ecology Group (MEG), Water Research Institute (IRSA), National Research Council of Italy (CNR), 28922 Verbania Pallanza, Italy 3 Laboratory of Ocean Benthic Fauna, Shirshov Institute of Oceanology of the Russian Academy of Science, 117218 Moscow, Russia Received 23 May 2020; revised 15 September 2020; accepted for publication 15 October 2020 The tubeworm Riftia pachyptila is a key primarily producer in hydrothermal vent communities due to the symbiosis with sulphur-oxidizing bacteria, which provide nourishment to the worm from sulphides, oxygen and carbon dioxide. These substances diffuse from the vent water into the bloodstream of the worm through their tentacular crowns, and then to the bacteria, hosted in a specialized organ of the worm, called a trophosome. The uptake rates of these substances depend on the surface/volume relationship of the tentacles. We here describe two morphotypes, ‘fat’ and ‘slim’, respectively, from the basalt sulphide-rich vents at 9 °N and 21 °N at the East Pacific Rise, and the highly sedimented, sulphide-poor vents at 27 °N in the Guaymas Basin. The ‘fat’ morphotype has a thicker body and tube, longer trunk and smaller tentacular crowns, whereas the ‘slim’ morphotype has shorter trunk, thinner body and tube, and presents longer tentacular crowns and has a higher number of tentacular lamellae. Given the dependence on sulphides for the growth of R. pachyptila, as well as high genetic connectivity of the worm’s populations along the studied localities, we suggest that such morphological differences are adaptive and selected to keep the sulphide uptake near to the optimum values for the symbionts. ‘Fat’ and ‘slim’ morphotypes are also found in the vestimentiferan Ridgeia piscesae in similar sulphide-rich and poor environments in the northern Pacific. ADDITIONAL KEYWORDS: East Pacific Rise – Guaymas Basin – lamellae – morphometrics – obturaculum – sulphides – tubes – tentacles – Vestimentifera. INTRODUCTION The discovery of the unique fauna of hydrothermal zones of the oceans was one of the most important events in marine biodiversity of recent decades (Corliss et al., 1979; Bright & Lallier, 2010; Hilário et al. , 2011). The submersible DSV Alvin found large, red, tube-dwelling worms rising up to several meters above the seafloor around the vent smokers (Corliss & Ballard, 1977; Ballard & Grassle, 1979; Corliss et al., 1979). These worms, formally described as the vestimentiferan annelid Riftia pachyptila Jones, 1981 (family Siboglinidae), are today amongst the best-known and most characteristic species of hydrothermal vent communities (Rouse, 2001). Riftia pachyptila is gutless and it obtains nourishment solely through a highly efficient symbiosis with chemoautotrophic bacteria, allowing the worm to reach up to 3 m in length at growth rates of 160 cm per year (Thiébaut et al., 2002). Populations of R. pachyptila are key ecosystem engineers of hydrothermal vent communities, especially in the Pacific Ocean rifts (Scott & Fisher, 1995; Shank et al., 1998), where they reach a large biomass responsible for most of the primary production in the ecosystem (Lutz et al., 1994; Thiébaut *Corresponding author: E-mail: nadezdarkorsakova@gmail. com Downloaded from https://academic.oup.com/zoolinnean/article/193/1/281/6048373 by guest on 18 March 2023