Gutweed

Overview

Gutweed (Ulva intestinalis) is a common, bright grass-green seaweed characterized by its inflated, tubular fronds that often resemble intestines. This cosmopolitan species is highly adaptable, thriving in diverse environments ranging from rocky marine shores to brackish estuaries and even some freshwater habitats.

Appearance

The thallus of this alga consists of hollow, unbranched or sparsely branched tubes that grow from a small, discoid holdfast. These fronds typically reach 10–30 cm in length and 6–18 mm in diameter, though they can be highly variable in size. The tubes are often irregularly constricted or "pinched" along their length, giving them a crumpled, intestine-like appearance. When submerged, the fronds stand upright, but they may become detached and form floating masses buoyed by trapped gas.

Distribution & Habitat

Ulva intestinalis has a nearly worldwide distribution. It is frequently found in the high intertidal zone, including tide pools, seepage areas, and sheltered coastal regions. It is remarkably tolerant of varying salinity levels, allowing it to flourish in areas with significant freshwater runoff, such as estuaries, salt marshes, and ditches. It commonly attaches to hard substrates like rocks, cobbles, and wood, but can also exist as a free-floating organism.

Biology & Behavior

As a summer annual, this seaweed often experiences rapid growth during warmer months. It is a photoautotroph that plays a role in providing shelter for various small marine creatures within its moist, dense mats during low tide. The species reproduces through an alternation of generations, where sexual and asexual phases are morphologically identical. Toward the end of the season, the fronds may decay, often turning a bleached white color.