Evolution of cave suspension feeding in Protodrilidae (Annelida) ALEJANDRO MART  INEZ,KIRSTEN KVINDEBJERG,THOMAS M. ILIFFE &KATRINE WORSAAE Submitted: 9 March 2016 Accepted: 17 June 2016 doi:10.1111/zsc.12198 Mart  ınez, A., Kvindebjerg, K., Iliffe, T. M., Worsaae, K. (2016). Evolution of cave suspen- sion feeding in Protodrilidae (Annelida). — Zoologica Scripta, 00,1–13. Protodrilidae belongs in a lineage that until now entirely consisted of deposit-feeding, highly adapted interstitial annelids. Except for a pair of anterior palps, all protodrilids lack appen- dages, parapodia and chaetae; and have slender bodies adapted to glide between the sand grains by ciliary motion. The first exception to these characteristics is Megadrilus pelagicus n. sp. inhabiting the water column of the anchialine La Corona cave system in Lanzarote. Its morphology and evolutionary history are here investigated by combining observations from in vivo video recordings and advanced microscopy with phylogenetic analyses. Our studies revealed a unique pelagic, suspension feeding behaviour attained by its long ciliated palps in combination with an autopomorphic dorsal ciliated keel and several longitudinal and trans- verse ciliary bands. Phylogenetic analyses recovered Megadrilus pelagicus n. sp. nested within Protodrilidae indicating that its unique traits are derived within the family. These traits are traced in the tree topologies in correlation to cave colonization. The evolution of these traits can be functionally explained by the different demands of a pelagic suspension feeding strat- egy compared to the ancestral deposit-feeding guild of the family. The origin of this suspen- sion feeding strategy was presumably favoured by the partial isolation of the anchialine ecosystem, connected to the sea only through the highly porous volcanic subterranean bed- rock. This crevicular connection limits the amount of predators and turbulence in the cave, but allows continuous water flow into the system carrying organic particles, which is the main source of food when photosynthetic primary production does not occur and sedimenta- tion is limited. These conditions may select for pelagic suspension feeding as the most feasi- ble life-strategy in anchialine caves, which the dominance of pelagic, suspension feeding crustaceans and annelids in anchialine cave assemblages may also reflect. For species of ancestrally deposit-feeding lineages entering the cave system, such as the annelid families Protodrilidae and Nerillidae, an adaptive-shift from interstitial to crevicular habitats seem- ingly correlates with dramatic morphological changes and speciation. The dramatic changes observed in these primarily interstitial lineages compared to their relatives, point to alterna- tive adaptive evolutionary pathways related to ecological fitness contrary to the previously proposed theories focusing on geological or stochastic processes. Corresponding author: Alejandro Mart  ınez, Marine Biological Section, University of Copenhagen, Universitetsparken 4, DK-2100 Copenhagen, Denmark. E-mails: amartinez.ull@gmail.com Kirsten Kvindebjerg, Marine Biological Section, University of Copenhagen, Universitetsparken 4, DK-2100 Copenhagen, Denmark. E-mails: louisekvindebjerg@hotmail.com Thomas M. Iliffe, Department of Marine Biology, Texas A&M University at Galveston, Galveston, TX 77553-1675, USA. E-mail: iliffet@tamug.edu Katrine Worsaae, Marine Biological Section, University of Copenhagen, Universitetsparken 4, DK-2100 Copenhagen, Denmark. E-mail: kworsaae@bio.ku.dk Introduction Marine interstitial environments constituted by small spaces between sand grains are inhabited by the most diverse ani- mal community on Earth from a phylogenetic point of view (Rundell & Leander 2010). Members of most of the extant metazoan phyla occur in the tiny interstices, including miniaturized members of macroscopic phyla (Giere 2009; Worsaae et al. 2012) as well as exclusively meiofaunal lineages, some of them with unclear phylogenetic affinities (e.g. Loricifera, Gastrotricha, Diurodrilus, Lobatocerebrum) ª 2016 Royal Swedish Academy of Sciences 1 Zoologica Scripta