Largest single-celled organism

The largest single-celled organism is Caulerpa taxifolia, a unicellular species of fern-like green seaweed that is native to tropical waters of the Caribbean Sea and the Pacific and Indian oceans. This fronds of this alga can regularly grow to a length of 20–60 cm (8 in–2 ft), though there are extreme cases of it reaching up to 2.8 m (9 ft 2 in). Despite being just one cell it has evolved a range of structures that both in outward appearance and function imitate those of multicellular plants.

Caulerpa's structures include a stolon (stem) that runs along the surface of the substratum upon which is growing, plus a series of leaf-like fronds and root-like holdfasts evaginating from the stolon. The holdfasts serve to anchor the cell in position and absorb phosphorus.

C. taxifolia has gained the notorious nickname of "killer algae" owing to its proliferation of the ocean floor (at the expense of other plants and organisms such as coral) in non-native regions, particularly in the Mediterranean Sea where it was accidentally introduced in the 1990s. Indeed, it has been included in the IUCN's list of the 100 Worst Invasive Species.

One of the next largest known singled-celled organisms is Valonia ventricosa, a bubble alga, so-named because of its single cell's spherical shape, and which can attain a diameter of around 5.1 cm (2 in). Also known as the sea grape or sailor's eyeballs on account of its orb-like form, V. ventricosa inhabits subtropical and tropical ocean waters worldwide, and its single cell is coenocytic in structure (i.e., it contains multiple nuclei and chloroplasts, for although it constitutes just one single cell, its cytoplasm is delineated into several distinct sections, or domains, all linked via cytoplasmic bridges.)