Zoologica Scripta Evolution of cave suspension feeding in Protodrilidae (Annelida) ALEJANDRO MARTINEZ,KIRSTEN KVINDEBJERG,THOMAS M. ILIFFE &KATRINE WORSAAE Submitted: 9 March 2016 Martınez, A., Kvindebjerg, K., Iliffe, T. M., Worsaae, K. (2016). Evolution of cave suspen- Accepted: 17 June 2016 sion feeding in Protodrilidae (Annelida). — Zoologica Scripta, 00,1–13. doi:10.1111/zsc.12198 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 Martınez, Marine Biological Section, University of Copenhagen, Universitetsparken 4, DK-2100 Copenhagen, Denmark. E-mails: [email protected] Kirsten Kvindebjerg, Marine Biological Section, University of Copenhagen, Universitetsparken 4, DK-2100 Copenhagen, Denmark. E-mails: [email protected] Thomas M. Iliffe, Department of Marine Biology, Texas A&M University at Galveston, Galveston, TX 77553-1675, USA. E-mail: [email protected] Katrine Worsaae, Marine Biological Section, University of Copenhagen, Universitetsparken 4, DK-2100 Copenhagen, Denmark. E-mail: [email protected] Introduction metazoan phyla occur in the tiny interstices, including Marine interstitial environments constituted by small spaces miniaturized members of macroscopic phyla (Giere 2009; between sand grains are inhabited by the most diverse ani- Worsaae et al. 2012) as well as exclusively meiofaunal mal community on Earth from a phylogenetic point of view lineages, some of them with unclear phylogenetic affinities (Rundell & Leander 2010). Members of most of the extant (e.g. Loricifera, Gastrotricha, Diurodrilus, Lobatocerebrum) ª 2016 Royal Swedish Academy of Sciences 1 Evolution of cave Protodrilidae A. Martınez et al. (Swedmark, 1964; Worsaae & Rouse 2009; Kerbl et al. 2015; absent or limited. Sediments constituting the main habitats Laumer et al. 2015). Despite their paraphyletic origin, inter- of interstitial species are limited to very coarse cinder stitial species share miniaturized and seemingly simple body patches from the erosion of the walls of the cave, which plans (Rundell & Leander 2010) adapted to the size restric- accumulate in certain areas, as well as to a 30 m high tions and physical conditions of the interstitial environment mound of loose sand, Montana~ de Arena, at 750 m from the (Boaden 1964; Webb 1969). Viscous regimen dominates entrance of Tunel de la Atlantida (Wilkens et al. 2009). interstitial water motion, limiting the flow to the upper few Accordingly, the endemic cave fauna is dominated by spe- centimetres of the sediment column (Huettel & Webster cialized swimming or drifting suspension feeders; whereas 2001) and resulting in accumulation of organic matter and marine benthic and interstitial species flourish exclusively often steep vertical gradients of oxygen (Jansson 1967; Webb where benthic trophic resources are available (Wilkens & & Theodor 1968; Huettel et al. 1996; Rusch et al. 2000; Parzefall 1974; Martınez et al. 2009). In fact, this is a gen- Burdige 2006). Emergence from the sediment by interstitial eral trend in most studied anchialine caves; all dominated by species is prevented by the exposure to predators and risk of cave endemic suspension feeding faunal assemblages such as being carried away from their habitat by current. Accord- the crustacean remipedes, thaumatocyprid ostracods, ther- ingly, interstitial species are photonegative and exhibit vari- mosbaenaceans and atyid shrimps among others (Iliffe et al. ous adhesive properties and other adaptations to ensure their 1983; Palmer 1985; Humphreys & Eberhard 2001; Alvarez permanence in the sediments (J€agersten 1952, 1954; Gray et al. 2015; Martınez et al. 2016). These lineages exhibit a 1967; Jouin 1970; Martin 1978a,b; Palmer 1988). high degree of endemism and disjunct distribution in geo- Protodrilidae Hatschek 1888 is a lineage of marine inter- graphically widely separated caves of Bermuda, Bahamas, stitial annelids distributed worldwide (Westheide, 2009). Yucatan, Canary Islands, Christmas Island and Western All protodrilids are externally simple, highly adapted to Australia. This disjunct distribution and the absence of obvi- deposit-feeding while gliding in the interstices of marine ous marine relatives have attracted the interest of zoologists coastal sediments. The 38 described species of Protodrili- and biogeographers, who have proposed two main hypothe- dae are subdivided into six genera (Martınez et al. 2015), ses to explain the origin of these anchialine communities: (i) all sharing the presence of cylindrical and ciliated (sensory a Tethyan origin depending on relictualization and vicari- cilia and motile bands) bodies, without parapodia or chae- ance linked to plate tectonics or changes in the sea level, tae, and paired palps with internal canals connected behind and (ii) active colonization and adaptation of deep sea forms the brain (Jouin 1970; Von Nordheim 1989). Protodrilidae, possibly entering the cave systems through crevicular path- together with Saccocirridae and Protodriloididae constitute ways (Hart et al. 1985; Wilkens et al. 1986, 2009; Iliffe et al. the clade Protodrilida (Purschke & Jouin 1988; Purschke 2000; Koenemann et al. 2009). However, the results from 1990; Di Domenico et al. 2014). This clade, previously molecular phylogenetic analyses on some of these iconic recovered nested within Canalipalpata (Annelida) in light anchialine taxa (Ahyong et al. 2011; Botello et al. 2012; of morphological evidence (Purschke 1993; Worsaae & Hoenemann et al. 2013; Martınez et al. 2013b, 2014; Gon- Kristensen 2005), has been recently placed as the sister zalez et al. 2015) do not always substantiate these traditional group of Aciculata based on molecular phylogenomic anal- hypotheses, showing that the origin of this fauna is more yses (Andrade et al. 2015; Struck et al. 2015). complex than what any single model can predict and high- Interstitial fauna, including several protodrilid species, is lighting that cave colonization and speciation might have abundant in the coastal sediments of Lanzarote, a 12 million followed more diverse adaptive pathways, related to ecologi- years old volcanic island arising from the oceanic crust, cal fitness. This emphasizes the necessity for morphological 110 km off NW Africa. Geological diversity of the island and evolutionary studies on new cave-adapted species in includes very permeable vesicular basalts and scoria which order to understand the collective origin of anchialine allows the infiltration of marine waters into La Corona lava communities. tube, a 22,000 years old anchialine cave at the northern end Despite the ecological differences between the anchialine of the island (Carracedo & Rodrıguez-Badiola 1993; Mari- water column and interstitial environments, a population of noni & Pasquare 1994; Carracedo et al. 2003). The water Megadrilus (Protodrilidae) was discovered within La Corona column of La Corona lava tube is not stratified and is only lava tube (Iliffe et al. 2000; Martınez et al. 2009). We here affected by tidal currents (0.5 – 2.5 m/s), carrying