A Little Bit More
Hydra are really formidable animals, we’re just lucky they aren’t any larger! Like the jellyfishes (some of which are dangerous to humans), hydra have individual stinging cells that are used both in defense and for capturing prey. Although a hydra’s nematocysts are small, their large number and potent toxins make up for their lack of size. Each nematocyst contains a coiled thread-like filament that if touched, springs out like a harpoon with enough force to puncture a small animal.
The hydras are freshwater creatures (many of their relatives are marine) that are a bit unusual since they live alone and not in colonies. Hydras can move about the bottom or on bits of plants or debris by a kind of head to foot somersaulting motion although they’re mostly stationary. These animals are probably most easily found in quiet bodies of water attached to plants and twigs. Hydra feed most heavily on crustaceans, like daphnia, insect larvae, and small worms. The toxins their nematocysts produce stun their prey and make it easy for them to direct food into their mouth. Hydra are often unintentionally brought into aquariums when plants, rocks, or other underwater materials are added to a fish tank. Although most of the animals in an aquarium are much too large to be affected by resident hydras, small, baby fish are easy prey for hydra so their presence is to be avoided.
Activities
Hydras
Objectives: See live hydra in action
Materials: Hydra from a biological supply house, aquarium to house hydra
Websites for purchasing hydras
Here are some things to look for when your hydras arrive:
1. Do their tentacles wave about as they “feel” for food?
2. Do the individual hydras tend to clump in one area or are they spread out at random?
3. If food is available, do you notice any movement by the hydra to help them catch their prey?
4. Do the hydra respond in some way if you tap on the glass or splash in their water?
5. Do the hydra respond if you place a small object next to their tentacles?
6. Do the hydra respond in some way to an immediate increase or decrease in light?
Key Concepts
Behavior and Regulation, Predator/Prey Relationships
Questions
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Posted by Ruby Tzimeas on June 23, 2006 at 03:50 PM
Q: I live in Hawaii near a shoreline that is full of offshore freshwater springs. I and a few others have been stung by something in the ocean water that we can't see. It seems to get under the looser parts of our bathing suits and leaves swollen, red, itchy welts that last far longer than any insect bite. One person was told by a dermatologist that they are hydra stings. This is definitely taking place in the water, not on the shore. What do you think? Mahalo!
A: I've had some similar experiences myself. When I had my "stings" I remembered that I had been swimming among lots and lots of small jellyfish. Seems they were able to sting on contact but were not strong enough for me to note immediately. Later, my skin reacted to these with small swollen areas similar to what you experienced. Hydra are closely related to jellyfish and sting in the same manner so either there are some jellyfish in your area or the hydra story is correct (although hydra attach to substrate, in my experience and are not free-floaters). If I had to choose, I'd choos the jellyfish as the culprit. Keep in mind that some jellyfish are very difficult to see so you may have overlooked them. Jellyfish can be very numerous in an area at one time and totally absent the next so I hope you'll be able to swim in your area most of the year. My very best to you. Aloha.
Posted by Vinny Moriarty on October 23, 2005 at 09:07 PM
Q: How do fish such as Nomeus gronovii manage to survive in asociation with portuguise man o war. Are they immune to the sting, do they have mucuse that prevents the nematocysts from firing, or do they just avoid being stung though quick movments
A: Good questions on your part - you're obviously thinking clearly about this. I wasn't sure myself so I went online and found a nice site that has good illustrations and discusses the fish's relationship with the Man-o-War. Take a look at www.aloha.com
Thanks for asking.
Posted by martin braun on August 31, 2005 at 12:13 AM
Q: I remember from a course on Zoological anatomy that there is at least one type of ray-finned fish that lives among or near the tentacles of stinging jellyfish and also has the ability to swallow an entire nematocyst which then,(almost right out of a comic book or child's or primitive human idea of the passing of traits by consuming them), is able to have the nematocyst leave the animal's digestive system and to become part of the fish's own defensive arsenal. It's as though one could swallow a gun and, the next day, have a working firearm as part of one's anatomy!
Do you know what this process is called and what the fish species is?
A: You have a good memory. The fish you're referring to is one of the Driftfishes called the Man-o-War fish (Nomeus gronovii). These fish are almost always found in association with the stinging jellyfish called the Man-o-War (Physalia physalia). This is a commensal relationship in which one of two organisms benefits by its association without harming or helping the other. Of course, in this relationship, the fish gains protection from the stinging tentacles although it too can be killed if not careful. Here are a couple of websites to help out too. Thanks for asking.
Posted by Niki Smith on August 19, 2005 at 12:45 PM
Q: What will a freshwater eel do to you and can it hurt you?
A: The American Eel is just a streamlined fish! They do have small teeth and could bite if you caught one and put your hand near its mouth but otherwise, they're totally harmless.
In certain parts of North America, people catch and eat them. Apparently, they're very tasty. Don't confuse these guys with the Moray Eels that are well known for their size and aggressiveness - they're a totally different animal.
Hope this answers your questin. Thanks for asking.
Posted by Deb Branson on May 15, 2005 at 12:59 PM
Q: We have collected hydras in our ponds to observe. Can their sting hurt us?
A: Only if you're a tiny creature. The stinging cells of hydra are much too small to affect humans. Of course, we do know that jellyfish can have stinging cells that are strong enough to penetrate our skin but not the hydra in our local waters.
Glad you're collecting these animals and hope they help you and/or your students learn more about the natural world. If you want to keep them for an extended period, you may want to renew their water with "new" pond water that is full of tiny creatures for the hydra to feed on. Thanks for your question.
Posted by stephanie l. on March 26, 2005 at 08:26 PM
Q: Can hydras be found in all parts of america?
A: Hydra are quite common in ponds, spring brooks, unpolluted streams and rivers, and in the shallow areas of lakes. I'm sure you can find some areas of North America wherethere are none, but they do range across the continent in appropriate habitat. Keep in mind that their population varies over the year so at certain times, like in the spring, their numbers may be extremely high in a given body of water and at other times very low.
Posted by Patrick Fellows, on March 24, 2005 at 01:31 PM
Q: When were eels first discovered? What did people think they were when they discovered them?
A: No one is sure when the first eel was discovered since it happened so very long ago. As the poster points out www.enaturalist.org at first they thought they were a kind of cross between a snake and a fish since that's what they looked like.