Amazing Acetabularia - the limits of unicellular life?

Acetabularia mediterranea in shallow water along the fringes of Jezero Malo on Mljet

Acetabularia acetabulum

What comes to mind when you think of a single-celled organism? Something microscopically small like an amoeba or an alga perhaps, with a single nucleus surrounded by cytoplasm and a cell wall? 

How about a complex structure with a stem and a shallow bowl-shaped cap in a delicate eau-de-nil, from one to ten centimetres tall? 

It seems incredible but that's acetabularia, a marine alga which has a single nucleus controlling a gigantic cell with a base rhizoid or root section, a long stem and an umbrella of branches fused into a cap. 

The nucleus sits in the rhizoid section of the cell and if the cap is removed, the cell regenerates a replacement. Furthermore, if the caps of two acetabularia of different species are exchanged, each transplanted cap changes its form to that typical of its new host.

Jezero Malo - the smaller of the two sea lakes

We discovered acetabularia in June, on a visit to Mljet, a beautiful island off the Dalmatian Coast of Croatia (I discovered the charms of Mljet forty-seven years ago in 1970!). 

The western tip of the island has two large turquoise sea lakes, cradled by Karst limestone hills, cloaked in forests of Aleppo pine and linked to the Adriatic by a narrow tidal channel. The warm, sheltered fringes of these lakes provide ideal condition for a morning swim and the growth of these pretty and delicate pioneers of the plant kingdom.