Dead man fingers algae

Geographic Origin and Habitat: Codium fragile, sometimes nicknamed "Dead Man's Fingers" or "Green Fleece," is native to the northwestern Pacific Ocean (Japan, Korea, China), but due to human maritime traffic, it has become a cosmopolitan alien species, colonizing the Atlantic, North American, and Mediterranean coasts. Its natural habitat includes sheltered tide pools, shallow rocky bottoms, and estuaries. It moderately tolerates organic pollution and settles on hard substrates or bivalve shells in areas with weak to medium hydrodynamic energy.

Taxonomy and Genetics: It is a chlorophyte green macroalgae belonging to the Codiaceae family. This species also features the peculiar coenocytic structure, meaning a single gigantic cytoplasm not partitioned by cell walls (a multinucleated single-celled super-organism). The various subspecies (like ssp. *tomentosoides*) are distinguished by minute anatomical differences in the microscopic tubular filaments that make up the thallus.

Physical Structure: It lacks true roots, instead having a spongy, strongly adherent basal disc from which a fleshy and heavily branched thallus rises. The branches are dichotomous (always splitting into two in a "Y" shape), cylindrical, and as thick as a human finger. Internally, the structure is not solid but composed of interwoven filaments (utricles) that give the plant a soft and compressible texture.

Color and Texture: It colors the aquarium a dark, somber green, almost bottle green. Its most fascinating feature is its surface texture: to the touch, it is extremely spongy, velvety, and fuzzy (hence the name *tomentosoides*). This softness is due to the fine tips of the utricles protruding on the surface, protecting the algae's body from desiccation during low tide cycles.

Lighting and CO2: It grows slowly in the wild and is very versatile in the aquarium. It does not require the blinding lighting of coral reef tanks, instead preferring moderate to low-medium light (PAR 80-150). Under excessively intense light (especially under powerful LEDs), the upper branches can bleach (photolysis) and quickly become covered with nuisance algae. It does not require any carbon dioxide supplementation.

Nutrition and Substrate: It obligatorily requires a stable anchor (a rock or a large shell), unable to survive buried in the sand. Like all macroalgae, to maintain thick, fleshy branches it needs constantly detectable nitrates and phosphates in the water column, although its absorption rates are extremely mild compared to the rapid Caulerpa or Chaetomorpha.

Water Chemistry: Being a species that braves intertidal cycles and migrations in nature, it is extremely resistant to thermal and chemical swings. It thrives at temperatures between 18°C and 26°C. It will tolerate periods of slight hyposalinity (brackish water), although in the long term in an aquarium it requires normal density parameters (1.024-1.026) and an alkaline pH to preserve the spongy turgor of its branches.

Space Management and Placement: Thanks to its very placid growth and highly peculiar aesthetics, it is one of the rare green macroalgae recommended for the main display alongside corals, or for thematic aquariums dedicated to seahorses (which love to use the Codium's "fingers" as a holdfast for their tails). It should preferably be placed on the bottom, in areas with moderate to low flow, away from blinding direct light.

Trimming: Its lethargic growth requires minimal maintenance interventions (rarely more than once every two months). Pruning is performed by firmly pinching the base of the bifurcation ("Y") with your fingers and pulling, or using sharp scissors. Being coenocytic, it will bleed green cytoplasm, but in minimal quantities and with a low toxic impact compared to other species. The removed branch will hardly attach spontaneously elsewhere without being glued or tied to a support.

Risks and Diseases: It does not pose the catastrophic collapse (meltdown) risk of Caulerpas. The main problem in an aquarium is that its spongy, velvety surface is a natural magnet for suspended detritus and cyanobacterial algae. If the water flow is insufficient, the Codium will become covered with a brown film or filamentous microalgae (epiphytes) that will slowly suffocate it until it rots. Regularly blowing its fronds with a turkey baster averts this asphyxiation.