Kelp

Kelps are a group of large brown seaweeds belonging to the order Laminariales, which includes roughly 30 different genera. Even though they look plant-like and perform photosynthesis using chloroplasts, kelps are actually stramenopiles, not true plants. They form dense, forest-like thickets in shallow temperate and Arctic oceans, anchored by stalks. These organisms need nutrient-rich water and prefer temperatures between 6 and 14 °C. Some genera, like *Macrocystis* and *Nereocystis*, are famously fast growers, adding up to half a meter daily and eventually reaching lengths of 30 to 80 meters. In the 1800s, the term "kelp" was commonly used for seaweeds that were burned to produce soda ash (mainly sodium carbonate), and it also referred to the ash itself.

The body of a kelp, called the thallus, is made up of flat, leaf-like blades attached to elongated, stem-like stipes. A root-like holdfast secures the kelp to the ocean floor. In American species like *Nereocystis lueteana*, gas-filled bladders called pneumatocysts form at the base of the blades, helping float them toward the surface for better light access. The stipes are covered in a slimy mucilage layer rather than a waxy cuticle.

Growth happens at the base of the meristem, where blades and stipe meet. Grazing can limit this growth; sea urchins, for instance, can turn kelp forests into barren areas known as urchin barrens. The kelp life cycle alternates between a diploid sporophyte and a haploid gametophyte stage. The haploid phase begins when the mature kelp releases spores that germinate into male or female gametophytes. Sexual reproduction then produces a diploid sporophyte, which grows into a full-sized individual.

Seaweeds were once thought to be related to land plants, but they are only very distantly connected. Their plant-like structures—blades, stipes, and sporangia—evolved independently through convergent evolution. Radiometric dating shows that vascular plants appeared around 419–454 million years ago, while the ancestors of Laminariales are much younger, at about 189 million years. Despite these differences, comparisons between kelp and plant structures remain useful, though most similarities stem from convergent evolution.

Some kelp species, including giant kelp, have evolved transport systems for organic and inorganic compounds, similar to the vascular systems of trees. In kelp, this network uses trumpet-shaped sieve elements. A 2015 study of six Laminariales species, including giant kelp, tested whether these sieve elements follow the same allometric relationships seen in plant xylem. The researchers expected similar scaling to minimize pressure gradients, but found no universal allometric pattern across the tested structures. This suggests that brown algae transport networks are still evolving to efficiently fit their environments.

Convergent evolution also occurs within the kelp group itself, leading to niche conservatism—where different species evolve to share similar ecological roles rather than diverging into distinct niches. A 2020 study examined functional traits like blade mass per area, stiffness, and strength in 14 kelp species. It found that many of these traits evolved convergently across the kelp family tree. These traits often correlated with distribution along a wave disturbance gradient, meaning that species in rougher waters shared similar structural reinforcements. This suggests that niche partitioning along wave disturbance gradients is a key driver of divergence among closely related kelps.

Because kelp habitats are often turbulent and varied, plasticity in structural traits has been important for their evolutionary history. This plasticity helps explain the high levels of morphological homoplasy—where different lineages look alike—that make classifying brown algae difficult. Kelp in the same wave regime often share similar blade shapes, while members of the same species from different wave regimes can look quite different. Plasticity most often affects blade morphology, including width, ruffle, and thickness.