Invertebrates in the Plankton: Cnidaria & Ctenophora


Hydrozoa are a class of jellyfish in the Phylum Cnidaria.  Many of them show a phenomenon known as an alternating life cycle, which includes a benthic stalked (polyp) form, and free-swimming jellyfish (medusa) form. The polyps may be found attached to docks or floats, while the medusae live in the plankton. The medusa is the reproductive stage--an individual has either male or female gonads. Fertilization takes place in the water column. The fertilized egg develops as a planktonic planula larva, until it settles on a suitable substrate and buds to form a new colony. This colony of polyps will then bud medusae, continuing the life cycle. The medusae feed using tentacles to bring prey to the mouth. The tentacles contain nematocysts, or cells that sting the prey and are also used for protection. Medusae swim via jet propulsion (pulsing of the bell).
Phialidium is an example of a hydroid medusa.
This hydroid will form a planula larva.
Seen here is a picture of the medusa stage in the Obelia life cycle. This organism shows statocysts, sensory structures that are used to detect gravity and thereby maintain vertical orientation. Each Obelia has either male or female gonads and the eggs are internally fertilized.
Sarsia has a lifestyle similar to Obelia, but some species have both asexual and sexual reproduction in the medusa stage. During the asexual stage medusae bud off of the manubrium, the elongated structure surrounding the mouth. The medusa stage is also morphologically distinct from other species in that it only has four tentacles and no statocysts.
Muggiaea is a type of cnidarian called a Siphonophore. Siphonophores are floating or swimming colonies of gelatinous zooids (organisms) that function together. Each individual in the colony performs a different activity and is given a name that defines its role. For example, Muggiaea has one angular nectophore, or bell, which moves the colony by pulsating. The stem of the colony is composed of gastrozooids, or polyps used in feeding, gonozooids are involved in reproduction, and dactylozooids are covered in nematocysts and used for protection. The colony stays buoyant by a drop of oil at the tip of the bell. The swimming bell moves by shooting forward and veering in an arc, which drags the stem and other zooids along with it.  This type of movement creates a fishing net used to capture prey. Siphonophores, along with some other medusa forms and some ctenophores, produce bioluminescence when they move.

Aglantha does not have a true polyp stage and it reproduces sexually with gonads hanging down from the top of the bell. This jelly differs from others in that it does not move by rhythmic pulses, but it drifts in the water and then leaps a few centimeters.

Ctenophores are planktonic organisms that are jelly-like in form but, unlike true jellies, they propel themselves by beating rows of compound cilia in waves.  Their movement is therefore very gentle and gradual.  The compound cilia occur in comb rows, five longitudinal rows on the body. Almost all ctenophores are hermaphroditic. They release their eggs and sperm into the water to be fertilized. However, there are exceptions. Some species self-fertilize, while others do not release their eggs until they are fertilized. The two ctenophores presented here have different methods of feeding.
Pleurobranchia have long tentacles lined by colloblasts, or sticky cells, that grab onto prey when contacted. In a rocking motion, they use tentacles to bring prey to their mouths.
In contrast, Beroe do not have tentacles, so they engulf prey directly into their mouths. They are able to eat organisms that are larger than themselves (e.g. Pleurobranchia) because of their large mouth and pharynx.  The lips of the animal have a remarkable ability to seal up, like a ziploc bag, after capturing prey.

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