An assessment of the taxonomic status of the Mediterranean endemic genus Acrodiscus Zanardini (Halymeniales, Rhodophyta)

Antonio Manghisi,Céline Bonillo,Gaetano M Gargiulo,Marina Morabito,M Antonia Ribeira,Line Le Gall

doi:10.5852/ejt.2017.267

Abstract

Acrodiscus Zanardini is a poorly known monotypic endemic Mediterranean genus based on A. vidovichii (Menegh.) Zanardini. Rarely reported, its reproductive structures have remained undocumented, leaving its exact taxonomic position uncertain. Solely on the basis of its vegetative structure, Zanardini provisionally placed it in the family Cryptonemiaceae of the order Cryptonemiales (currently the Halymeniaceae of the Halymeniales), although he was uncertain as to whether the new genus actually belonged to that family or should instead be included in the Gigartinaceae of the Gigartinales (where Meneghini had originally placed it). In the present study we have extensively sampled A. vidovichii and documented its vegetative and tetrasporangial features. As well, we provide molecular-sequence data (COI-5P, rbcL, LSU) that indicate its phylogenetic affinities. We confirm Acrodiscus as a member of the Halymeniaceae and its status as an independent genus. Searches of several institutional herbaria have allowed us to locate and lectotypify Meneghini’s Chondrus? vidovichii by the discovery of his original material now held at the Herbarium Horti Pisani (Pisa, Italy).

Highlights

Acrodiscus Zanardini is a poorly known and persistently enigmatic genus endemic to the Mediterranean Sea
None of our analyses conclusively resolved the family position of Acrodiscus Zanardini, as its alliances changed depending on the reconstruction method
The three supergeneric lineages within the Halymeniaceae emerging from our phylogenies have already been found in previous works (Manghisi et al 2014; Mineur et al 2010; Nelson et al 2014), but morphological/anatomical characteristics unifying the members of the three lineages can still not be precisely specified

Summary

Introduction

Acrodiscus Zanardini is a poorly known and persistently enigmatic genus endemic to the Mediterranean Sea. Based on Chondrus? Species of the genus Acrodiscus by Zanardini (1868) based on vegetative structure and position of the tetrasporangial sori. In the absence of female reproductive structures, it has not been possible to relate Acrodiscus to existing members of the Halymeniaceae owing to the importance of carpogonial and auxiliary-cell “ampullae” in the taxonomy of that family (Chiang 1970; Gargiulo et al 2013; Saunders & Kraft 1996). Halymeniacean vegetative structures are on the contrary quite similar to those of other families within the Gigartinales. Based on the similarity of morphology and presence of tetrasporangial sori, Schmitz (1889) proposed that A. vidovichii should be transferred to the genus Polyopes. Based on the similarity of morphology and presence of tetrasporangial sori, Schmitz (1889) proposed that A. vidovichii should be transferred to the genus Polyopes. Guiry & Guiry (2016) have suggested that Acrodiscus is congeneric with Cryptonemia, it lacks the differentiated stipes and especially the refractive medullary cells typical of that genus (Kraft & Saunders 2014: 165)

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