Anemones, Hydras [Cnideria]

These animals use microscopic stinging cells, called cnidoblasts, on their tentacles and body surfaces for capturing prey and defending themselves.

Coiled inside each cnidoblast is a thread-like tube, called a nematocyst, armed with hooks, poison, or a sticky substance. When a bristle on the cell surface is triggered by physical or chemical stimuli, the nematocyst flies out to harpoon, inject or lasso whatever disrupted the bristle. A new cnidoblast then grows to replace the spent one.

Another feature of cnideria is radial symmetry of body parts around a mouth. If you were to divide their body in equal portions, like cutting a pie, the resulting parts would be mirror images of each other.

But the classes of cnideria have very different body types, resulting in very different lifestyles. The anemone is a polyp, or cylindrical body, which attaches to surfaces with its base and faces its mouth and tentacles up like flower petals. The jellyfish is a medusa, a bowl- or saucer-shaped body, with its mouth and tentacles facing down. (The larvae of some jellyfish species also have a sessile polyp phase.) Jellyfish float in the currents. The third class, which includes the hydroids, have many individual polyps and/or medusae functioning in a colony as one organism.

Cnidarians, like sponges, have outer and inner layers separated by a translucent, jelly-like mass. Yet they also have simple tissues. A network of nerve and receptor cells form a nerve net, which operates without the central control of a brain. Contracting cells also form muscle tissue.

Most cnidarians are carnivores. Once the stinging tentacles fringing the mouth capture prey, they draw it into the mouth. This leads to a sac, called the gastrovascular cavity, where enzymes break the food down. Cells on the lining engulf the nutrients. Waste is expelled back out of the mouth.

Hydrozoans
[Hydrozoa]
Most marine hydroids grow as colonies of individual polyps and/or medusae, functioning as one organism. There are free-swimming colonial species; other species form colonies that attach to submerged surfaces.

Free-swimming colonial hydroids, such as the jellyfish-like Portuguese man-o-war, are rarely found north of Cape Cod. But attached hydroid colonies can be found in the marine areas of Great Bay Estuary and the coast beyond.

The individual polyps, known as zooids, typically excrete stiff, branching tubes with cups. Each zooid extends its mouth and ring of tentacles from a cup to snag zooplankton. The tubes, called stems, are connected, allowing the polyp colonies to share food. The delicate stems and cups of these colonies vary extensively in shape, ranging in many cases from a few millimeters to several centimeters in size. Others species grow from a crust, rather than stems, and the zooids form a fine fuzz on surfaces. Look on snail shells to see if you can spot the bright pink fuzz of a snail fur hydroid.

Most hydroid colonies produce reproductive polyp buds, which break off and grow into tiny jellyfish, or hydromedusae. These reproduce sexually by releasing eggs and sperm, which if fertilized, settle down to form the next polyp generation. They populate the new colony with daughter polyps.

anemones, corals
[Anthozoa]
Anthozoa, meaning “flowering animals,” aptly describes this class of cnidaria in which the medusae (jellyfish) generation is absent. As polyps, these sedentary organisms attached to surfaces and spread their stinging tentacles into the water for food and protection.

Several species of the solitary anemones live along the coast and in the lower part of Great Bay Estuary. Corals are the colonial anthozoa found in tropical waters.

Anemones
Anemones are hard to spot since they tend to attach themselves to rock crevices, under piles of seaweed and to other remote surfaces in the lower intertidal zone. Identification is sometimes a challenge, since they withdraw their tentacles and take on a blob shape at low tide or when disturbed.

Anemones are not stuck to a surface for life, like other sessile organisms, such as the barnacle. They can glide slowly by contracting the pedal disk on their lower side. When they decide to attach again, they’ll secrete a sticky substance from this disk.

Here are two anemones you can look for:

Striped Anemone
Diadumene lineata
These have olive-colored bodies with bright orange or white stripes. The polyp grows to three-quarters of an inch long. Look for them among the mussel beds and gravel, or on dock pilings and wharfs. The anemones pictured at left were actually hanging upside down in a tidal pool crevice. The pool was on a slab of bedrock in the intertidal zone between the Portsmouth Yacht Club and the U.S. Coast Guard dock in New Castle.

Frilled Sea Anemone
Metridium senile
Adults have up to a thousand tentacles and are colored white, orange or yellowish-brown, and sometimes pink. Those living inshore are smaller than their offshore maximum size, which is four inches tall and three and a half inches wide. Look for them in tide pools and among rocks crevices and dock pilings of the lower intertidal zone.

jellyfish
[Scyphozoa]
The mouth and surrounding tentacles of these soft, bell- or saucer-shaped cnidarians hang down in the water as they slowly pulsate through the water. The muscle contractions along the outer edges of the medusa expel water, propelling it along.

Offshore species of jellyfish prey on larger animals like fish, paralyzing and pulling them with their tentacles up into their mouths.

Jellyfish entering Great Bay Estuary have to be able to tolerate varying salinity levels.

Moon Jelly
Aurelia aurita
The moon jelly can handle salinities as low as 16‰. This translucent jellyfish has symmetrical design on its upper surface, like four abutting horseshoes, which are the gonads. It grows to 10 inches wide, with a relatively short fringe of numerous tentacles. The moon jelly feeds on plankton. They are more apt to be seen in the fall.

Copyright © 2006 Barbara Driscoll.