Inhabitants of Kelp Forests

• On the Holdfast
• On the Sea Floor
• Within the Kelp Canopy
  

There is one marine production, which from its importance is worthy of a particular history. It is the kelp, Macrocystis pyrifera. This plant grows on every rock, from low-water mark to a great depth, both on the outer coast and within the channels…The number of living creatures of all Orders, whose existence intimately depends on the kelp is wonderful. A great volume might be written, describing the inhabitants of one of these beds of seaweed….I can only compare these great aquatic forests of the southern hemisphere, with the terrestrial ones in the inter-tropical regions. Charles Darwin, 1845

Kelp forests world-wide support very high levels of marine biodiversity. This is because of their unique three dimensional nature and the complex structure of these forests. Like trees in a forest, Giant Kelp plants provide shelter and habitat for an enormous number and diversity of animals, including, fish, molluscs (sea snails), bryozoans (lace corals), polychaetes (worms), crustaceans (crabs, isopods, amphipods), echinoderms (sea urchins, seastars) and sponges.

Kelp forests provide habitat in several ways: as settlement habitat for larvae, and as food and shelter for adults either living among the kelp fronds, or living directly on the kelp plant itself. On the seafloor, a wide range of plants and animals occur in the low-light, sheltered environment created beneath the kelp canopy.

Cross section of a typical Giant Kelp
forest in Central California showing distribution
of common species and microhabitats
associated with the forest
(from Foster and Schiel 1985).

On the Holdfast
The rootlike holdfast of Giant Kelp, particularly the decaying portions, shelter an entire community of animals, including small crustaceans (isopods, amphipods), crabs, sea urchins, polychaetes, ophiuroids (brittle stars), small fish and eels, hydroids, bryozoans, gastropods (molluscs) and sponges. In California, about 770 animal species have been recorded living in kelp forests. Of these, more than 150 different species shelter in the kelp's holdfast alone. Approximately 23,000 individuals (from 9 invertebrate phyla) have been recorded living in 5 Giant Kelp holdfasts alone.

 

On the Sea Floor
Outside the holdfast, on the seafloor, sponges, tunicates, anemones, cup corals and bryozoans are probably the most commonly occurring sessile animals with kelp forests. Low turfing fleshy algae and encrusting and articulated coralline algae also dominate due to their high tolerance to low light and high wave energies conditions.

Like forests on land, the growth of understorey plants and turf algae and the germination of new plants, is suppressed by the low light beneath the dense kelp canopy. However, when a gap is created in the canopy (ie. through storms removing plants), the increased light stimulates the vigorous growth of understorey plants (including juvenile Macrocystis). Mobile organisms which live on the seafloor, include Blacklip Abalone (Haliotis rubra) and lobster (Jasus edwardsii) which live in rocky crevices. Small abalone feed on encrusting algae while large abalone and lobsters feed on drift algae.

Echinoderms also commonly live and feed on the seafloor under kelp forests and include, sea urchins, such as Rodgers Sea Urchin (Centrostephanus rodgersi) and Heliocidaris erythrogramma, the Feather Star (Cenolia sp.), sea stars (Pateriella regularis, P.calcar, Uniophera sp.), the Eleven-armed Sea Star (Coscinasterias calamaria), and Biscuit Stars (Tosia sp.).

Within the Kelp Canopy
In addition to the bottom dwelling species, a large number of animals occur within the kelp canopy. Some live on the kelp plant itself, such as isopods, seastars, sea urchins, sea snails and bryozoans. While other species reside in the water column of the kelp forest, closely associated with kelp plants. These species include octopus, cuttlefish, seahorses and fish, and diverse assemblages of planktonic species such as jellyfish, crustaceans and fish larvae.

Common fish species include wrasse (Notolabrus tetricus, N.fucicola, Pictilabrus laticlavius, Pseudolabrus psittaculus), Bridled Leatherjacket (Acanthaluteres spilomelanurus), Shaws Cowfish (Aracana aurita), Butterfly Perch (Caesioperca lepidoptera), Blotch Tailed Trachinops (Trachinops caudimaculatus), the Common Bullseye (Pempheris multiradiata) and the Weedy Sea Dragon (Phyllopteryx taeniolatus).

Many commercially important fish and invertebrates find shelter and food within the Giant Kelp forest. Lobsters and abalone live and feed on the rocky reefs under the kelp canopy. Fishes such as Stripey Trumpeter (Latris lineata), Bastard Trumpeter (Latridopsis forsteri), bream (Acanthopagurus australis), blackfish (Girella tricuspidata) and snapper (Pagrus auratus), feed on the algae and small crustaceans on the reef beneath the kelp.

Seastars and soft-spined urchins make their home climbing among the kelp fronds. The sea urchin, Holopneustes purpurescens, lives in 'nests' which it makes by pulling the fronds of the kelp plants around itself. The nests provide protection and shelter from predators, as the urchin has very small spines. The nests also provide shelter to the herbivorous snail, Phasianotrochus eximius, which would be vulnerable to predation on the open surface of the kelp.

Another animal commonly found on kelp plants is the lace coral or bryozoan, Membranipora membranacea, whose coral skeleton forms large, white, lacy patches on the kelp frond. Membranipora also provides both shelter and food for nudibranchs (sea slugs) from the family Coramidae, which have evolved an effective camoflague against predators - a white network pattern on their bodies to exactly match the pattern of the bryozoan skeleton.

The kelp canopy is also a major habitat for marine life. When growth is vigorous, the kelp forest is crowned by a dense surface canopy. The canopy acts as a nursery for juvenile fishes, attracting swirling schools of small bait fish and predatory fish. The buoyant kelp canopy also provides a resting place for seabirds and seals. Beneath the canopy, the kelp forest also provides seals with shelter from predators.

The Giant Kelp plant is not only an important habitat, but an essential source of nutrients for many animals. For example, limpets ingest broken pieces of kelp from offshore beds and detritivores ingest microscopic pieces of kelp after it has broken down in the surf. On the seafloor, large numbers of filter-feeders or suspension feeders (such as sponges), feed on kelp detritus. This abundance is also probably due to the concentrated settlement of planktonic larvae beneath the kelp canopy, which has accumulated because of the reduced water flow (compared to more exposed hard substrata).