The Hydrozoa, the newest clade, can exhibit a life cycle with both stages present, polyp and medusa. The polyp is usually the dominant stage. They however exhibit the greatest diversity in life cycle.
We will look at three sspecies to illustrate differences in life cycle. The first is a species belonging to the genus Clytia which exhibits the "typical" life cycle and larvae of a hydrozoan.The colony consists of feeding hydranths, or gastrozooids, that were produced from one larva that settled and gave rise to a polyp that reproduced asexually. At certain times of the year, gonozoids are produced which give rise to medusae or the sexual reproducing stage of the life cycle. Medusae produce sperm and eggs. Fertilized eggs give rise to planula larvae.
1. Clytia sp.
Planula larvae
Below is diagram of the lab colony developing and releasing Clytia medusae. Note the simple structure of their medusae to that of a scyphozoan.
Interestingly, sex determination is influenced by temperature. While some colonies produce medusae of only one sex, others will produce male medusae when kept at low temperatures (around 16 °C) and mainly females at 18- 24°C (Carré & Carré, 2000*).
Female medusa on left and three male medusae on right.
2.Tubularia sp.
Not all Hydrozoan species exhibit this typical life cycle. One of the common alternatives encountered in marine species is to retain the gonozooid and/or medusa. These are often reduced and no longer produces a free swimming stage. In some colonies modified polyps are produced which act essentially as gonads. This happens in species of Tubularia.
Life cycle-----------------------------------------------Larvae before and after release from gonophore
Tubularia produces beautiful feeding polyps or gastrozooids in shades of pink and orange. View the film showing a feeding polyp, and then the video showing a colony with feeding polyps in all different stages of producing what is term gonophores or highly modified gonozooids, essentially which act as gonads. Highly modified larvae develop in the fertilized female gonophore and produce a swimming larvae that eventually gives rise to a polyp.
A colony of Tubularia sp.
3. Hydractinia
Hydractinia is the extreme example of polyp polymorphism. Here a colony forms of independent polyps that take on many functions. Some act as feeding individuals,gastrozooids, providing through stolons or connections to other individuals the nutrition for the colony. Others take on reproductive function, producing sperm and eggs. Some colonies produce both male and female gonozooids. Still others become defensive in function. Spines and lashing dactylozooids discourage other organisms from invading and settling in the colony.
The colonies live on shells of hermit crabs.
In previous semesters students have mapped distribution of the different types of polyps by videotaping the colony and then plotting distributions.
Students were asked to determine where there were high densities of gastrozooids,
gonozooids,
or defensive zooids as spines and the rarer dactylozooids.
Here are some of the maps obtained by groups of students.
Featured arthropod, no excuse me, cnidarian for the lab.
Every semester a new creature that even Dr. Feaver over the years has not seen appears. Two years ago is is an unknown species of creeping hydromedusae. These are unique hydrozoans. Because a citing of these critters is so unusual, we will examine one creeping hydromedusae, instead of our usual arthropod addition to the lab.
Unknown species of Hydromedusae
A species of Hydromedusae, probably a species of Eleutheria, a hydrozoan group that sometimes has a reproductive stage that is a "walking or creeping" medusa. The relationships of this group is debated, as is what species should be included. It is not known, since the species was not identified, if a polyp stage is also present.
This first video was suppose to be one of species that produces individual solitary and single bright red polyps, until the Filmer noticed the strange creature to the far right.
A close up of that hydromedusa. Most hydromedusae have a polyp stage, some do not.
Planula larvae
