final hydroids paper

This topic submitted by jennifer miller ( at 5:12 PM on 6/11/04.

Jen and Michele explore Lighthouse Cave in the Bahamas

Tropical Field Courses -Western Program-Miami University

Jennifer Miller

Originally I choose hydroids, as my topic because I figured it would be the least likely to be chosen by others. But shortly into my research, I found that this tiny, tentacle, invertebrate creature is an interesting marine organism. I found myself captivated by everything about them; from their form, to what they eat, and how they reproduce. Although they are often mistaken for a plant, these small and easily overlooked sea jellies can be dangerous
Hydroids are a type of marine organism most commonly found in the Atlantic, Pacific, and Indian oceans with the exception of the hydra which is found in freshwater. There, they can be found both inshore and offshore. While hydroids are mostly found in warm and tropical waters, in reality, they can withstand temperatures as low as –6 degrees. Hydroids are small only measuring up to 5cm long, and 1cm in diameter. There are over 3000 species of hydroids. Hydroids belong to the Kingdom Animalia and are part of the Phylum Cnidaria. They are categorized in the Class Hydrozoa and Order Hydroida. There are two major suborders of hydroids
• Suborder Anthomedusae. The polyps are the dominant phase and sometimes there is no medusa at all. The polyp is not protected by the exoskeleton, which stops at the base of the polyp. (Also known as ‘gymnoblastic hydroids’, or Athecata).
• Suborder Leptomedusae. The polyps are always colonial, with specialized reproductive and feeding individuals. Most lack a medusa. Exoskeleton cups protect both the polyp, and the specialized gonad structures. (Also known as ‘Calyptoblastic hydroids’, or Thecata).
The hydroid has two discretely different body forms, polyp and medusa. One is a sessile organism (polyp) and the other is free swimming (medusa). Both body forms are similar in construction; both consist of a cylindrical body surrounding a digestive cavity, with a single opening, the mouth, at one end surrounded by tentacles. The body wall is composed of three layers of tissue. Thin layers called endoderm and ectoderm are the outside and inside layers, with a layer of jellylike material, called mesoglea in between. Hydroid mesoglea, in the polyps, is basically the glue holding the internal and external tissues together. Hydroids have no head, tail, front or back. There is, however, a distinct upper and lower surface. These are identified as oral (the surface with the mouth) and aboral (the opposite end). They are invertebrates, which means they have no backbone. They do not have “systems” such as digestive or nervous systems. They do have a digestive region or gut, also known as a gastrovascular cavity, but they lack digestive organs. Though they do not have a nervous system they do have nerves and sensory cells that are arranged in a disperse arrangement called a nerve net. They have only one opening into the digestive region, which serves as both the mouth and anus. Hydroids are totally lacking a brain.

Morphology of a polyp
Hydroids are actually colonies of individuals called polyps (benthic generation). The polyp tends to be elongated, with a thin body wall. It is attached to the ocean bottom or other surface by the aboral, its tentacles pointing upwards. In many species the polyp, or hydroid, stage is colonial. This means that as new polyps are created by budding, they will remain attached to a branching common stalk, often hardened with nonliving material, forming a plantlike structure called a hydroid colony. The polyps that make up the colony can have specific functions such as those that are feeding polyps (Gastrozooids) whose job it is to catch food and those that are reproductive polyps (Gonozooids) that produce the medusa. The individual polyps are microscopic, but the feathery colony grows to be up to 12 in. tall.

Hydroid anatomy

The medusa stage (pelagic generation) looks like a jellyfish. It tends to be rounded, bell shaped, with a thick body wall containing much mesoglea (jelly). It moves by swimming with pulsating beats that occur when tissues around the bell contract and force water out the bottom. The medusa swims with the mouth side down and the tentacles dangling. This “jellyfish” stage is so small that it usually goes completely unnoticed. The medusa is free swimming, can be male or female, and lives only a few months. It’s during this stage that reproduction happens.

Sample of a hydroid medusa

The passing through of two stages makes the life cycle of the hydroid somewhat complex. In order to build up their colony the hydroids reproduce by budding. The polyps have no sexual orientation (asexual) but can reproduce by budding off new polyps or medusa. The medusa can be either male or female and produce gametes (sperm or egg) from gonads on the undersurface of the bell. These gametes shed into the water until they fuse to form a zygote (fertilized egg), which grows up to be a planula larva that attaches to the substrate and becomes a polyp. The polyp then becomes part of a colony and the cycle continues. Though all the medusa that buds off of one colony are the same gender, the colonies must be genetically male or female. When a polyp reproduces a new little polyp, instead of medusa, the new polyp stay attached until they develop into a mature polyp. Then the mature polyp drops away and settles to the ground where they grow a new colony. The type of life cycle can vary between different colonies. There are some hydroids that lack a colony building polyp stage, while others lack the medusa stage. The hydroids life cycle is short but they grow very quickly which allows them to rapidly colonize different substrata.

Hydroid lifecycle

Hydroids are carnivores, using their nematocyst-laden feeding tentacles, positioned along their “branches” to sting and catch passing shrimp, worms and copepods. They also eat by collecting particles out of the water (suspension feeding). Hydroids have a simple digestive system comprising basically of a sac with one opening. Because they have only a single opening, waste must pass through the mouth, the same way the food was brought in. Hydroids use these same tentacles as defense against predators. The predators of hydroids include a wide range of fish, crustaceans and mollusks like sea slugs. Many of these sea slugs eat hydroids and steal their weapons.

Nematocyst is the term used to describe the microscopic stinging cells that are found in the tentacles of the hydroids. These stinging cells are dangerous to humans that come in contact with them. Hydroid stings are common among people who clean boat bottoms and scuba divers that brush up against a colony. Usually victims say they feel a prickly sensation followed by instantaneous pain after being stung. Most hydroid stings produce immediate red bumps that can remain itchy and painful for hours while the rash can last up to 10 days. In some cases more severe sting reactions are blisters, swelling and hives. Although there is no cure for the sting, it’s important that you immediately rinse with water (salt or fresh) to wash away any adhering nematocysts. Other than that all you can do is put ice on it for pain and hydrocortisone ointment for the itch.


Hydroids are small but mighty when it comes to surviving in the ocean. They have a unique body form, no brain, and are small enough to be overlooked. They have been mistaken for a plant and have a complex life cycle. Still one thing that is important for us to know is that the same tentacles that they use to catch their prey can also harm us. So despite their small size and delicate appearance, hydroids can deliver a poisonous and powerful sting. We need to understand that these marine organisms are pretty to look at but dangerous to touch.

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