Invertebrates

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Hydroids


Kingdom: Animalia

Phylum: Cnidaria

Class: Hydrozoa


Common name(s): Hydroids or Hydrozians

General Details

The Hydrozoa is a subgroup of cnidarians containing approximately 3700 species. It is a diverse group with a variety of life cycles, growth forms, and specialized structures. Like many cnidarians, hydrozoans have both polyp and medusa stages in their life cycle. They are distinguished from other groups by their complex life cycle, by the growth of medusae from buds rather than strobilae or from metamorphosis, by the presence of a velum inside the bell of the medusa, and by the production of gametes from ectodermal, rather than endodermal, tissue. Most hydrozoans are marine, and hydrozoan species are found in nearly every marine habitat type; a very few species live in freshwater. Most hydrozoans form colonies of asexual polyps and free-swimming sexual medusae. Colonies are usually benthic, but some, notably the siphonophores, are pelagic floaters. Colonial polyps often have some division of function, with certain polyps specialized for defense, feeding, or reproduction. Most hydrozoans are predators or filter-feeders, though a few have symbiotic algae (zooxanthellae), in the same way that other other groups of cnidarians do.


Better-known hydrozoans include Portuguese man-o-wars (Physalia physalis), the freshwater genus Hydra, fire coral (Milleporidae), and by-the-wind sailors (Velella velella).


Descriptive Details

Most hydrozoan species have a planktonic larval stage called a planula. Planulae are radially symmetric ovoids, often covered with flagellate cells for swimming. They may be very simple embryos or have cells differentiated into several types. Planulae most often settle onto a benthic substrate and develop into a polyp.


Polyps are radially symmetric, and may be urn-shaped, conical, cylindrical, or club-shaped. In most species they are only a few millimeters tall, though the largest grow up to many centimeters, and one, Branchiocerianthus imperator can be 2 meters tall. At their base hydrozoan polyps have basal disks or elongate processes for attaching to substrate, or they may be attached to other polyps. Often there will also be connections here to hollow tubes (called stolons) that connect the polyp to others in its colony, and allow the exchange of food between polyps. Above the base is a ring of contractile cells called the sphincter. These can contract to isolate the contents of the polyp from the stolons, preventing undigested food from entering the stolons. Above this is the gastric column, which usually contains a digestive chamber with a single opening, a mouth at the apex of the column. A ring of tentacles is attached to the column below the apex and above the sphincter. The number, shape and size of tentacles varies greatly, but there are usually between and 8 and 50 on a single polyp (some have many more, and some specialized polyps may have fewer). Most colonial hydrozoans are polymorphic, with different structures reflecting different functions. Some are armed with large spines tentacles for defense but have no mouth, some have tentacles and functional mouths for feeding, and some are only reproductive, with no tentacles or mouth, and produce medusae (see below) or gametes.


Like all cnidarians, hydrozoans have special ectodermal cells called cnidocytes, each containing a single intracellular structure called a cnida (aka nematocyst). Cnidae are unique to the Cnidaria. Each cnida, when triggered by a mechanical or chemical stimulus, shoots out a tiny hollow tube at high speed. Some cnidae are is equipped with sharp spines, and/or venomous or acidic compounds, but some are adhesive and have neither spines nor toxins. Hydrozoans use different types of cnidae to capture prey, to repel predators, and to attach to substrate.


Sizes

mm's to 200 cm


Habitat

Hydrozoans are found in nearly all marine habitats, except perhaps heavy surf zones. They are most abundant and diverse in warm shallow waters, probably as a reflection of food abundance. The small number of freshwater species occur in both lotic and lentic habitats, and are more abundant in eutrophic and mesotrophic waters.


Distribution

Hydrozoans are found in nearly all marine habitats, except perhaps heavy surf zones. They are most abundant and diverse in warm shallow waters, probably as a reflection of food abundance. The small number of freshwater species occur in both lotic and lentic habitats, and are more abundant in eutrophic and mesotrophic waters.



Data provided by:

ADW (Animal Diversity Web) / Accessed December 2014 

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