Contents lists available at ScienceDirect Aquatic Botany journal homepage: www.elsevier.com/locate/aquabot Root hair anatomy and morphology in Posidonia oceanica (L.) Delile and substratum typology: First observations of a spiral form Agostino Tomasello a,1 , Rosaria Perrone a,b,1 , Paolo Colombo a , Maria Pirrotta a,b, ⁎ , Sebastiano Calvo a a Department of Earth and Marine Sciences, University of Palermo, Viale delle Scienze, Ed. 16, I-90128 Palermo, Italy b CoNISMa URL Palermo, Via Archirafi n. 18, I-90123 Palermo, Italy ARTICLE INFO Keywords: Anatomy Posidonia oceanica Type of root hair Substratum ABSTRACT The morpho-anatomical root hair features of P. oceanica ramets collected in meadows settled on different sub- strata (sand, matte and rock) were analysed. On each substratum, nine plagiothropic rhizomes each one com- posed by 3–6 interconnected short shoots were collected between April and May 2016 at 10 m of depth. On sand and on rock, the adventitious roots showed two distinct tubular and spiral-shaped hairs, clustered in yellowish- gray gelatinous pads. Tubular root hair tips were dactiliform and generally attached to grains of rock fragments. Moreover, a sub-circular swelling zone occurred. On matte, root hairs did not form gelatinous pads, were very short and had a simple distal portion. The root hair anatomy and morphology described here provides new information on the P. oceanica root system that can express a remarkable root hair polymorphism. 1. Introduction The seagrass Posidonia oceanica (L.) Delile forms the most common, productive and widespread meadows of the Mediterranean Sea (Gobert et al., 2006), likely representing the marine ecosystem with the highest levels of biodiversity (Hemminga and Duarte, 2000). It is able to grow on different types of substrata, including sand that is easily penetrable by the roots; rock, in which the very sturdy roots are able to enter through crevices (Mazzella et al., 1993) and matte, a typical self-built terraced formation consisting of intertwined rhizomes, roots and sedi- ment (Boudouresque and Meinesz, 1982). Several studies have high- lighted the importance of substratum features for the distribution and colonization process of P. oceanica (De Falco et al., 2008; Marbà and Duarte, 1997) as well as its morphology and growth dynamics (Di Maida et al., 2013), root development and architecture in particular (Alagna et al., 2013; Alagna et al., 2015; Balestri et al., 2015). Different strategies of root adaptation to different substrata have been docu- mented at different levels. For example, it was observed that biomass allocation was more or less unbalanced between root and leaf bundle structures, according to substratum type (Di Carlo et al., 2007). More- over, variations in root structure, orientation and diameter can be af- fected by substratum typology, at least during the first stages of clone development, as reported in a detailed study on root topology (Balestri et al., 2015). Recently, adhesive root hairs in P. oceanica seedlings have been discovered and described (Badalamenti et al., 2015), while knowledge on these structures in adult shoots are still quite limited. In this study, the morpho-anatomical root hair features of P. oceanica shoots collected in meadows settled on different substrata were ana- lysed in order to gain a better understanding of the adaptive mechan- isms of root systems, which enable this species to colonize marine en- vironments. 2. Materials and methods The study was carried out in North Western Sicily (Italy), within the Marine Protected Area of Capo Gallo – Isola delle Femmine, where an extensive P. oceanica meadow grows on three types of substratum. For substratum differentiation, we chose shoots growing on sand, plants settled on visibly exposed rock, and plants growing on matte. On each substratum, nine plagiothropic rhizomes, each composed of 3–6 inter- connected short shoots, were collected between April and May 2016 at 10 m of depth for morpho-anatomical analysis. For rock, in particular, shoots anchored in calcarenitic substratum were sampled using a hammer and chisel. The pieces of rock with anchored shoots were sliced at the labora- tory for better analysis of root development (Fig. 1A). Then integer roots were isolated by chemical dissolution of rock using 1% hydro- chloric acid. https://doi.org/10.1016/j.aquabot.2017.12.001 Received 6 April 2017; Received in revised form 14 December 2017; Accepted 14 December 2017 ⁎ Corresponding author at: Department of Earth and Marine Sciences, University of Palermo, Viale delle Scienze, Ed. 16, I-90128 Palermo, Italy. 1 These two authors contributed equally. E-mail address: maria.pirrotta@unipa.it (M. Pirrotta). Aquatic Botany 145 (2018) 45–48 Available online 15 December 2017 0304-3770/ © 2017 Elsevier B.V. All rights reserved. T