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Jellies - Cnidaria and Ctenophores

There is a surprising amount of gelatinous material in the plankton stemming from a number of sources but especially the spectacularly diverse phylum Cnidaria. Comprising sea anemones, jellyfish and hydroids the size can be extreme as well. From huge jellyfish down to abundant tiny medusae from the hydroids much of the plankters depend on the method of reproduction. This page concentrates on the microscopic forms that appear in the Dale samples. Sea anemones are large polyp creatures living on the shore and although they can clone asexually they may sexually produce a larva (planula) which disperses in the plankton. A small medusa is typically from a class called Hydrozoa where colonies of tiny polyps attach to substrates (e.g. seaweed/rock). The exception are the Siphonophores (see Velella velella often called By-The-Wind-Sailor). A close relative of the cnidaria is the Ctenophora a small phylum, commonly called Sea Gooseberries or Comb Jellies they are voracious carnivores of copepods

Velella velella By-the-wind-sailor

Velella velella, trapped in a rockpool

Like the infamous Portuguese Man O'war Velella is not a jellyfish but a hydrozoan and forms a floating colony of polyps. Hanging down from the float are tentacles with sting cells to catch prey. The stiff sail blows it across the surface. Like other hydroids this produces a planktonic medusa. There are times when the shoreline goes blue with them washed in from the Gulf Stream from the Atlantic sub- tropics. See below.

Hydrozoa is a class within the Cnidaria phylum and these colonial hydroids are Siphonophores


Medusa jelly
The hydroid Obelia. Small colony of polyps

Obelia, a common hydroid attached to seaweed. This is a feeding phase and produces a sexual medusa that swims off into the plankton

Medusa Obelia

Swimming medusa of Obelia

obelia medusa

There are many different forms of medusa and some are only known from their planktonic stage.

Medusa bell
Medusa bell of a hydroid, Sarsia

Sarsia sp

Medusa of poss Eutima species stack1.jpg

Most are around 1-2mm across but this varies enormously.

Medusa of the hydroid Eutima hydroid with diatoms in gut

 Eutima, has been feeding on diatoms.

A possible example of Clythia. With a closeup (x200) of one of the statocyst

medusa statocyst
Medusa of the hydroid Clythia hydroid

Floating Colonial Hydrozoans - Siphonophores

Siphophore Muggiaea Calyophora

The most well known Siphonophore is Physalia physalis Portuguese man o'war. All these floating colonies of hydroids are quite spectacular and varied. Muggiaea atlantica is a Calyophorid species, very different and tiny in comparison to the giant Physalia.  The "polyp body" is barely 0.5mm in length and consists of an elongated stem which forms the base polyp. This moves by extension and contraction. Attached to the bottom are other polyp individuals and have the typical sting cells found in the Cnidaria. Under the microscope any creature moving passed can be stung with a long "string" attached to the harpoon that pierces the prey, pulling it slowly back to the body. 

Copepod caught by Siphophore Muggiaea Calyophora bell

Closeup of the caught copepod in the Muggiaea bell

Siphophore Muggiaea Calyophora bell with copepod

The photo of Muggiaea under the microscope was magnified nearly x100 but is incomplete. Most samples are missing the float and jelly bells, called nectophores, which hold them buoyant. The second photo here shows a rare exception when the complete structure has been shot and stacked with x8 magnification to display the outer bell and an inner one (that shows up better). The colony of polyps are in a lower section on the left.  It is slightly complicated by the bigger copepod that it has caught. The distinct oval structure at the top of the inner nectophore seemed to be oil and the canal running from the polyps up to it had distinct lipid moving along, one-way, to the top. 

These first appeared in July and August 2023. By September they were very common.

The entire structure is several millimetres tall. 

Siphophore Muggiaea atlantica Calyophora bell
Siphophore Muggiaea atlantica Calyophora

Further examples of the Siphonophore Muggiaea atlantica.  These were small individuals and the lower photo is one contracted to 220 microns. A tentacle with sting cells is visible at the top.

Anthozoa - the Sea Anemones

Planula larvae of sea anemones. Common in Milford Haven

Planula larva of sea anemone x200
Planula larva of sea anemone x200

Ctenophora  - the Sea Gooseberries

Predominantly a spring and early summer occurrence, mainly Pleurobranchia and on occasions the very large Beroe

Pleurobranchia Sea Gooseberry Ctenophore
Pleurobranchia Sea Gooseberry Ctenophore
Ctenophore larva cydippid sea gooseberry

Cydippid larval stage. Sea Gooseberries have columns of cilia (the ctenes) that provide locomotion and create beautiful rainbow interference patterns. Ctenes are visible in the larvae

Ctenophore larva cydippid sea gooseberry
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