Kelp builds underwater forests without being a plant at all
Giant kelp can add half a metre in a single day and tower up to 80 metres, forming dense forests in cold coastal seas. It photosynthesises and has stalks and blades, yet kelp is not a plant. It is a brown alga that invented its tree-like body independently, arriving at similar shapes by a completely separate evolutionary road.
The order Laminariales holds about 30 genera. Kelps need nutrient-rich water between roughly 6 and 14 degrees Celsius, which confines them to shallow temperate and Arctic oceans. Fossils once suggested they appeared in the Miocene, but kelps were already growing in the northeastern Pacific at least 32 million years ago. Their lineage dates to around 189 million years, far younger than vascular plants, which arose some 419 to 454 million years ago.
The resemblance to land plants is convergence, not kinship. Instead of roots, a holdfast grips the seabed; instead of a stem, a flexible stipe coated in slimy mucilage rather than a waxy skin; instead of leaves, blades. American species such as bull kelp carry gas-filled floats that lift the blades toward sunlight. Giant kelp even moves nutrients through trumpet-shaped sieve elements that echo plant plumbing, though a 2015 study found this network does not yet scale with size the way trees do, hinting that it is still evolving.
Shape is remarkably flexible. Members of one species growing in calm water can look quite unlike relatives battered by surf, while unrelated kelps in the same rough spot may look alike, a mismatch that has made classification tricky. Bull kelp can alter blade shape to change drag and catch more light. A 2020 comparison of 14 species found reinforcing traits such as stiffness evolved repeatedly along gradients of wave exposure.
Floats also help kelp travel. Detached plants form rafts that drift far and seed new coasts, carrying hitchhiking species with them; low genetic diversity suggests giant kelp colonised subantarctic waters only recently. Grazers can undo it all, and sea urchins can strip forests into barren ground. People have long used kelp: coastal Indigenous peoples wove fishing nets from bull kelp, Japan eats kombu, and until the 19th century burned kelp ash was a source of soda ash, a meaning the word itself once carried.
Source: Kelp
