Cydippida is an order of
comb jellies
Ctenophora (; : ctenophore ) is a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), and they are ...
. They are distinguished from other comb jellies by their spherical or oval bodies, and the fact their tentacles are branched, and can be retracted into pouches on either side of the
pharynx
The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...
. The order is not
monophyletic
In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria:
# the grouping contains its own most recent co ...
, that is, it does not form a single
clade
In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
.
Anatomy
Cydippids have bodies that are more or less rounded, sometimes nearly spherical and other times more cylindrical or egg-shaped; the common coastal "sea gooseberry," ''
Pleurobrachia'', has an egg-shaped body with the mouth at the narrow end.
From opposite sides of the body extends a pair of long, slender tentacles, each housed in a sheath into which it can be withdrawn.
Some species of cydippids have bodies that are flattened to various extents, so that they are wider in the plane of the tentacles.
The tentacles are typically fringed with tentilla ("little tentacles"), although a few genera have simple tentacles without these side-branches. The tentacles and tentilla are densely covered with microscopic
colloblasts that capture prey by sticking to it. These are specialized
mushroom
A mushroom or toadstool is the fleshy, spore-bearing Sporocarp (fungi), fruiting body of a fungus, typically produced above ground on soil or another food source. ''Toadstool'' generally refers to a poisonous mushroom.
The standard for the n ...
-shaped cells in the outer layer of the epidermis, and have three main components: a domed head with
vesicles (chambers) that contain adhesive; a stalk that anchors the cell in the lower layer of the epidermis or in the mesoglea; and a
spiral thread that coils round the stalk and is attached to the head and to the root of the stalk. The function of the spiral thread is uncertain, but it may absorb stress when prey tries to escape, and thus prevent the colloblast from being torn apart.
In addition to colloblasts, members of the genus ''
Haeckelia'', which feed mainly on
jellyfish
Jellyfish, also known as sea jellies or simply jellies, are the #Life cycle, medusa-phase of certain gelatinous members of the subphylum Medusozoa, which is a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animal ...
, incorporate their victims' stinging
cnidocyte
A cnidocyte (also known as a cnidoblast) is a type of cell containing a large secretory organelle called a ''cnidocyst'', that can deliver a sting to other organisms as a way to capture prey and defend against predators. A cnidocyte explosively ...
s into their own tentacles — some cnidaria-eating
nudibranch
Nudibranchs () are a group of soft-bodied marine gastropod molluscs, belonging to the order Nudibranchia, that shed their shells after their larval stage. They are noted for their often extraordinary colours and striking forms, and they have b ...
s similarly incorporate cnidocytes into their bodies for defense. The tentilla of ''
Euplokamis'' differ significantly from those of other cydippids: they contain
striated muscle, a cell type otherwise unknown in the comb jellies; and they are coiled when relaxed, while the tentilla of all other known ctenophores elongate when relaxed.
''Euplokamis tentilla have three types of movement that are used in capturing prey: they may flick out very quickly (in 40 to 60 milliseconds); they can wriggle, which may lure prey by behaving like small planktonic worms; and they coil around prey. The unique flicking is an uncoiling movement powered by contraction of the
striated muscle. The wriggling motion is produced by
smooth muscle
Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non- striated, so-called bec ...
s, but of a highly specialized type. Coiling around prey is accomplished largely by the return of the tentilla to their inactive state, but the coils may be tightened by smooth muscle.
There are eight rows of combs that run from near the mouth to the opposite end, and are spaced evenly round the body.
The "combs" beat in a
metachronal rhythm rather like that of a
Mexican wave.
From each balancer in the statocyst a ciliary groove runs out under the dome and then splits to connect with two adjacent comb rows, and in some species runs all the way along the comb rows. This forms a ''mechanical'' system for transmitting the beat rhythm from the combs to the balancers, via water disturbances created by the cilia.
Families
Cydippida contains 13 families:
*
Aulacoctenidae
*
Bathyctenidae
*
Cryptocodidae
*
Ctenellidae
*
Cydippidae
*
Dryodoridae
*
Euplokamididae
*
Haeckeliidae
*
Lampeidae
*
Mertensiidae
*
Pleurobrachiidae
*
Pukiidae
*
Vampyroctenidae
There are additionally
incertae sedis
or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...
genera including ''
Duobrachium''.
See also
*
References
{{Authority control
Ctenophore orders