Rare phytomyxid infection on the alien seagrass Halophila stipulacea in the southeast Aegean Sea

Abstract

Phytomyxids (Phytomyxea) are obligate endosymbionts of many organisms such as algae, diatoms, oomycetes and higher plants including seagrasses. Despite their supposed significant roles in the marine ecosystem, our knowledge of their marine diversity and distribution as well as their life cycles is rather limited. Here we describe the anatomy and morphology of several developmental stages of a phytomyxid symbiosis recently discovered on the petioles of the alien seagrass Halophila stipulacea at a locality in the southeast Aegean Sea. Its earliest stage appeared as whitish spots already on the youngest leaves at the apex of the newly formed rhizomes. The infected host cells grew in volume being filled with plasmodia which resulted in the formation of characteristic macroscopic galls. The plasmodia eventually cleaved into spores and in the final stage, the dark brown to black galls contained mature resting spores often arranged in duplexes and possessing thick verrucous outer walls. Based on its anatomical and morphological features the phytomyxid was identified as Plasmodiophora cf. halophilae. Little is so far known about the origin and eco-physiological functioning of this rare symbiosis. However, seemingly similar infection patterns were reported in 1913 on Halophila ovalis from Java and in 1995 on H. stipulacea from Sicily. Since H. stipulacea is of Indo-Pacific origin and no phytomyxid infection was observed on the co-occurring autochthonous seagrasses Cymodocea nodosa or Posidonia oceanica, we hypothesize that this phytomyxid might have co-migrated into the Mediterranean through the Suez Canal.