Neptune’s necklace (Hormosira banksii) is one of the many brown seaweeds endemic to New Zealand and the cooler waters around Australia. Its distinctive fronds, which look like a string of brown beads, are made up of chains of ovoid, hollow segments joined by thin constrictions in the stalk. Small reproductive structures are scattered over each “bead.”
Dense mats composed almost entirely of this one species can be found on seashore rocks. The fronds are attached to the rock by a thin, disk-shaped holdfast. Neptune’s necklace also lives unattached among mangrove roots. The shape of its segments varies according to habitat. They are spherical and about 2cm wide in fronds growing on sheltered rocks, in mussel beds on tidal flats, or in mangrove swamps. Fronds growing on subtidal rocks on moderately exposed coasts have smaller segments that are just 6mm long.

(Photo source)

Having a variety of colorful common names, such as Witch’s Hair, Acid Kelp, and Landlady’s Wig, Desmarestia aculeata is a large seaweed consisting of narrow brown fronds with many side-branches. Its bushy appearance is the reason for it being called landlady’s wig and witch’s hair. The smallest branches are short and spine-like, hence the species name aculeata, which means “prickled.” In summer, the whole plant is covered with delicate branched hairs.

(Photo source)

Spectacular seaweed (Drachiella spectabilis) is rarely seen, except by divers, as it normally grows in relatively deep water and is rarely washed ashore. It also grows in shallower water within kelp forests. It has a thin, fan-shaped frond, split into wedges, that spreads out over the rock and reattaches with small root-like structures called rhizoids. Young plants have a purple-blue iridescence, which is lost as the seaweed ages. Sexual reproduction is unknown in this species and spores are produced asexually.

(Photo source)

The oyster thief (Colpomenia peregrina) gets its unusual name from its habit of growing on shells, including commercially grown oysters. The frond is initially spherical and solid, but as it grows, it becomes irregularly lobed and hollow and fills with gas. Sometimes, this can make it sufficiently buoyant to lift the oyster, which is not attached to the seabed, and they may both be carried away by the tide. This seaweed has a thin wall with only a few layers of cells. The outer layer is made of small, angular cells which contain the photosynthetic pigments that give the oyster thief its brown color.

(Photo source)

This odd green seaweed, known as Sailor’s Eyeball (Valonia ventricosa), looks like a dark green marble, and consists of a single large cell attached to the substrate by a cluster of filaments called rhizoids. Younger plants have a bluish sheen, but older ones become overgrown with incrusting coralline red seaweeds.

This curious little green alga, referred to as Mermaid’s Wineglass (Acetabularia acetabulum), grows in clusters on rocks or shells covered with sand in sheltered parts of rocky coasts within its range. Although it grows to 3 cm, it consists of just one cell.

Many pelagic fish species are attracted to floating objects that provide shelter from predators, currents, and even sunlight. Floating logs and seaweed also provide a meeting point. Mini-ecosystems often develop on and around large drifting logs. Seaweeds and goose barnacles settle, providing shelter and food for crabs, worms, and fish. Shipworms bore into the wood, and their tunnels provide further refuge. Occasionally reptiles, insects, and plant seeds survive and drift on logs, and may eventually be washed ashore to colonize new places, including new volcanic islands.

Floating Sargassum seaweed provides a safe haven for the sargassumfish (Histrio histrio) pictured. More than fifty animal species have been recorded in this habitat.

The California coast is famous for its beds of giant kelp (Macrocystis pyrifera), the largest seaweed on the planet. It forms dense forests just offshore, and in Monterey Bay it outcompetes bull kelp for sunlight in many places, but the latter dominates in more exposed areas. Inshore of these giant species, other smaller kelps thrive. The kelp forests provide a unique habitat. Sea otters, which live among the kelp forests and eat sea urchins, are thought to be important in controlling the urchins, which graze on the kelp. Seagrasses of the genus Phyllospadix are also found in Monterey Bay. Unusually for seagrasses, they can attach to rock and grow in the surf zone or in intertidal pools on rocky coasts. Each year over 140,000 tons of giant kelp are harvested in California for the extraction of alginates, which are used in the textile, food, and medical industries.

In exceptional circumstances, giant kelp can be eighty meters long. The forests are at their thickest in late summer and decline during the dark winter months.

Spectacular Seaweed (Drachiella spectabilis)

Length: 6cm

Location: Western coasts of northwestern Europe

Maerl (Phymatolithon calcareum)

Diameter: 7cm

Location: Coasts of Atlantic islands, Philippines