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 quite extreme, from huge jellyfish down to abundant tiny medusae from the hydroids or planula larvae of the anemones. Much of the plankter members will depend on the method of reproduction. As a group they are complex and extremely varied such that the web site cannot cover them in any detail.
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. The medusa found in the class Hydrozoa are referred to as hydromedusae to distinguish them from the jellyfish. On the shore they form colonies of tiny polyps attach to substrates (e.g. seaweed/rock). The hydroid exception is the Siphonophores (see Velella velella often called By-The-Wind-Sailor).
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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, 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 and the Atlantic sub- tropics. See below.
Hydrozoa is a class within the Cnidaria phylum and these colonial hydroids are Siphonophores
For Jellyfish Click Here
For Ctenophores Click Here
Hydrozoa
Obelia, a common hydroid attached to seaweed. This is a feeding phase and produces a sexual medusa that swims off into the plankton
Swimming medusa of Obelia
There are many different forms of hydromedusa and some are only known from their planktonic stage.
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Sarsia sp
Most are around 1-2mm across but this varies enormously.
The beautiful Amphinema with a top-knot!
Eutima, has been feeding on diatoms.
A ventral view of a possible medusa of Clythia. With a closeup (x200) of one of the statocyst
Floating Colonial Hydrozoans - Siphonophores
The "body" of polyps froma Muggiaea x100
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.
Closeup of the caught copepod in the Muggiaea bell
The photo of Muggiaea under the microscope was magnified nearly x100 but is incomplete. Many samples when collected can damage the float and jelly bells (so not obvious), called nectophores, which hold them buoyant. The second photo here shows a 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 "body" 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.
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, especially in the spring.
These are examples of typical planulae: highly ciliated with a long tuft of sensory hairs at the anteriour end.
The tube sea anemones are a unique group of anemones found in soft sediment and Cerianthus lloydi is one of the most widespread species. Soon after the planula develops it passes through several metamorphic phases including an antipathula stage. The posterior of the planula develops 4 ciliated lobes/"pods" and these propel it through the water as a quite bulbous structure with a slit for a mouth between them. It stops and ejects waste cellular material slowly out of the mouth to become a more slimmed-down version which over several hours expands the four ciliated "pods" that eventually become tentacles. Over several weeks they grow more of these and everything elongates ready for settling in the mud. It looks "spidery" and some authors refer to this as an arachnactis stage. Size: 600 microns across the right specimen.
An early antipathula larva and a later "spider" form, the arachnactis
Jellyfish
Jellyfish adults are part of the megaplankton and the most common species around the UK coast is the Moon Jellyfish Aurelia aurita. Often in the summer and autumn they can reach high densities. The fine tentacles around the fringe of the bell have large numbers of sting cells for collecting zooplankton but generally do not effect humans. In the centre of the bell are 4 gonads. Planula larvae are produced in the second half of the year when they can be common in the plankton. The planula produces a small polyp stage (scyphistomae) on the shore which bud planktonic larvae called an ephyra from the top that grow over three months in the spring to form an adult.
Moon Jellyfish Aurelia aurita *
Compass Jellyfish Chrysaora hysoscella
A late stage ephyra of a jellyfish approximately 5-6 mm across. This is likely to be the larva of the Mauve Stinger Pelagia noctiluca which is characterised by having the brown warts on the bell. These are full of sting cells for both protection and food collection.
Moon Jellyfish Aurelia aurita
* Photo thanks to Bruno Kelzer
Ctenophora - the Sea Gooseberries or Comb Jellies
Predominantly a spring and summer occurrence, they can be very common around the Haven although very difficult to see as they are so transparent. Two species are found on our coast. The most likely to be seen is Pleurobranchia pileus and on occasions the very large Beroe. The former is around a centimetre across with two trailing tentacles for catching prey like copepods. The ctenes or combs are the columns which can produce colour (interference patterns) as they beat for locomotion.
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
