What are they?

Hydromedusae are the medusae stage of Hydroids classified under the Cnidarian Class Hydrozoa. Unlike the Scyphoszoans or “true jellyfish”, such as moon (Aurelia aurita) or lion’s mane (Cyanea capillata) jellyfish, you are unlikely to spot hydromedusae swimming about as these are tiny, often microscopic, jellyfish. Gelatinous zooplankton are estimated to have a global Carbon biomass of 38,300,000 metric tons, of which hydromedusae are the largest and most diverse group. In comparison, the Great whales such as the Humpback and Blue whales have on average 33 tons of Carbon . With over 800 species these tiny predatory jellyfish are typically only a couple of millimetres up to a few centimetres. For example, a common species found in Ireland Lizzia blondina has an umbrella height of only 2mm when fully grown. On the other end of the scale, the bioluminescent crystal jellyfish (Aequorea victoria), found in the Northeast Pacific, can grow up to 6-10 cm in bell diameter.

In the plankton, the hydromedusae can be both diverse and incredibly abundant. In British waters, there are 100 known species of hydromedusae, however many of these are deep water species and are not routinely sampled by researchers. In Irish waters, studies have revealed that there is often up to 20 different species of hydromedusae in one location with densities of several hundred per m3 of water. 


Hydromedusae are predators that feed on other small organisms in the plankton, such as copepods and nauplii. Most are ambush predators. This means they rely on prey swimming within reach of their tentacles which contract on contact, bringing the prey towards the jellies mouth. The specialized Cnidarian stinging cells, called cnidae, may be used to immobilise prey through venom, sticky glue-like substances, or ensnaring barbs. Hydromedusae themselves can be predated on as well. Some have few predators, while others are a food source for many species like  fish or even other larger Cnidarians. There has been some evidence to suggest some species, particularly the smaller ones, are omnivores and feed on small algae, diatoms, and protists. This area is poorly understood however, and so the role of hydromedusae in the marine food web may be underestimated. 

Another major importance of hydromedusae is that Green Fluorescent Protein (GFP) was isolated from the hydromedusae Aequorea Victoria, known as the crystal jellyfish. The discovery of GFP revolutionised the fields of molecular biology, immunology and biochemistry. It is used to tag proteins so that their expression can be monitored. The discoverers of GFP were awarded the Nobel Prize in Chemistry in 2008.

Hydromedusae can be a major problem species for the aquaculture industry. Thanks to their small size, they can easily get into salmon cages for example. This can result in gill disorders in the salmon from hydromedusan stings. Other species such as Ectopleura can attach to the nets or cages, causing a major fouling problem.

Life Characteristics

Hydromedusae are only one stage, the medusan stage, in the life cycle of hydrozoa. The medusae live in the plankton and typically only survive for a few days, although some of the larger species can live for a few months. Most hydromedusae are meroplanktonic meaning they spend only part of their life cycle in the water column. The non-planktonic phase is the hydroid where this colonial poly is typically benthic and sessile, attached to the sea floor, though some are specialised to attach to substrates such as fish scales or clumps of algae. Hydromedusae asexually bud off the colony as either male or female and produce gametes which then form planula larvae that will settle as another hydroid. Alternatively, some species are holoplanktonic with only the medusan stage and so spend their entire life cycle in the plankton.

By Daragh Brown