Bodies of Jelly - Jellyfish
Above: a scyphistome feeding. This polyp-stage larva developed from a planula which attached to the rock.
Cnidarians are of course worthy of study in their own right,
but they have also been extremely useful model systems in
biology and biomedical sciences. They have shed light on the
evolution and development of animals, as well as the functioning
of nervous systems (they were among the first nervous systems
studied in depth) and they have even inspired robotics due to
the efficiency of their locomotion. They are also important
ocean predators. It is a pity that funding for this 'grass
roots' science is increasingly hard to obtain, since those who
control the finances want research to be increasingly justified
by a narrow, and often commercial scope. By asking scientists to
justify their research by immediate economic or medical value,
we are in danger of missing out on important basic
understanding. There should always be some funding set aside for
Types of Scyphomedusae
For those who are
curious, the following is a detailed account of selected
scyphozoans, to give the interested reader a better feel for
Note: the positions
of organs and appendages in medusae are described according to the
radius they lie along. In Aurelia, the four perradii
are inline with the four stomach or gastric pouches
(gastric pockets), the corners of the square mouth and the four
oral arms (the oral arms may branch). The four interradii
are situated between the perradii and align with the septa
(the tissue in between the four gastric pouches) and the
subumbrellar funnels. The adradii are situated between the
perradii and the interradii. Additionally, a structure may be centripetal
(towards the centre of the bell) or centrifugal (towards
the margin) or on the margin (marginal). For appendages, proximal
refers to the basal part of the appendage near to the main
body, distal to the other end further from the main body.
Moon jellyfish (Aurelia aurita)
Somaeostomesinclude the Moon Jellyfish (Saucer Jellyfish) Aurelia aurita ( = Aurellia aurita). The medusa of this jellyfish reaches up to 25 cm in diameter. The bell is divided into 8 principal velar lappets alternating with 8 marginal sensory appendages (rhopalia and associated structures) one between each pair of velar lappets. Short hollow tentacles arise at the bell margin (just above the bell edge) and these alternate with an equal number of small marginal lappets. Each marginal sensory appendage or rhopalium is situated between two narrowed marginal lappets, called rhopaliar lappets. Each is also equipped with an ocellus (simple eye) of ectodermal origin at its proximal end, which faces outwards and also a pigment-cup eye of endo- and ectodermal origin on the inner surface, facing inwards towards the oral arms. Many somaeostomes lack ocelli, however. Typical of scyphomedusae, a hood of tissue covers the rhopalium on the upper (exumbrella or outer) side. This hood is equipped with an exumbrella pit lined by sensory epithelium whose cells have sensory cilia. This is situated proximally on the top of the hood or just behind (above) it. Typically, a similar ciliated pit is located on the subumbrella surface at the base of the rhopalium, a subumbrella pit (I am not certain if this latter occurs in Aurelia). In fact, there are two exumbrella sensory pits in Aurelia: an outer pit as just described (at the top of the hood) and an inner pit on the proximal end of the rhopalium just beneath the hood. The end of each club-shaped rhopalium is filled with weighty crystals (statoliths) and acts as a gravity sensor ('organ of equilibrium'). The rhopalia are situated on the inter- and perradii.
The corners of the four-sided central mouth protrude as four thick gelatinous oral arms (mouth arms). Each oral arm has a gutter or trough along its length on the inner (centripetal) surface facing the mouth. The edges are frilly, giving 8 frilly edges in total, and lined by small tentacle-like protuberances. The oral arms are perradial. The four horseshoe-shaped interradial gonads open via canals on the stomach floor where eggs pass out from the mouth to the oral arms where they are brooded in brood pouches formed by the frilly margins until they develop into free-swimming ciliated planulae which then escape.
Jellyfish typically generate vortices as they swim. locomotion in Aurelia aurita has been studied in detail. Contraction of the bell generates a toroidal starting vortex. The recovery stroke (bell expansion) generates a stopping vortex of opposite rotational sense. Starting and stopping vortices merge in a side-wards oriented supervortex which induces flow of surrounding sea water into the subumbrellar cavity, from which food is captured by the tentacles and oral arms, and also downstream to produce thrust. With the correct spacing, the generation of such vortices by pulsatile swimming can generate as much as 150% of the thrust of a continuous jet. Trainling vortices also form in the centre of the toroidal vortices, as water is accelerated, but these reduce efficiency. Pulsed jets produce fewer trailing vortices than continuous jets, so pulsed jetting also increases efficiency (i.e. reduces drag and reduces energy expenditure). Jellyfish swimming may not especially fast or powerful but it is remarkably energy efficient.
Lion's-Mane Jellyfish (Cyanea capillata)
Similar vortices also occur in the Lion's-Mane Jellyfish (Cyanea capillata) which also serve to drive food towards the mass of tentacles. Cyanea is also a somaeostome. This species occurs in several 'varieties' which are sometimes designated as separate species. This is a North Atlantic coldwater jellyfish. The varieties are distinguished by geographical distribution, size and colour, but these are variable and not stable characteristics. The Arctic form, Cyanea capillata var. arctica will be described in detail and occurs off the North American coast, north of Cape Cod, in summer. This is the largest known species of jellyfish reaching a bell diameter of 230 cm (though such giants are not common). The bell is usually a rich brown and yellow but very variable in colour. The muscles and tentacles are a rich rosin or yellow colour. The margin of the lenticular (lens-shaped) bell is divided into 8 main lobes by 8 deep adradial clefts. A median cleft in each main lobe results in 16 lobes, but these are further divided by smaller notches, one on either side of each median cleft, to give a final total of 32 lappets. The 8 median clefts (per- and interradial) each contain a hollow sensory rhopalium covered above by a web of tissue between adjacent lappets. The club has a ventral swelling covered in wart-like protuberances and papillae then extends into a distal tube ending in a knob containing statoliths. There are no ocelli. There are 8 adradial crescents on the subumbrella, midway between the margin and centre of the bell, with the horns facing outwards, bearing 5 concentric rows of tentacles in each. the innermost row contains the oldest and longest tentacles. There are about 800 tentacles in total. The tentacles are hollow and highly contractile, extending for a length of about 25 times the bell's diameter when fully extended (suggesting a maximum length of about 58 m (but more typically up to 37 m).
Four long perradial oral arms extend from the four-cornered mouth which is situated in the centre of the subumbrella. The edges of these arms are greatly folded to form curtain-like oral fringes, hanging down beneath the bell and about as long as the bell is wide.
Each of the four interradial gonads resides in a complexly-folded pouch on the subumbrella floor. Clusters of gastral cirri (gastric filaments) project from the bases of the gonads into the stomach cavity. The coronal muscle forms a ring centrifugal to the gonads and is about as wide as one 8th the bell radius. It is made up of 16 trapezoidal circular muscle blocks, those 8 in the rhopaliar radii being about half as wide as the other 8. The stomach is a central lenticular chamber giving off 16 radial pouches on its outer side and numerous branched canals which ramify throughout the bell without anastomosing. The 8 pouches in the rhopaliar radii are about half as wide as the other 8. The canals are continuous with the hollow interiors of the tentacles and rhopalia stalks and with the gonads. There is no radial canal as there is in Aurelia.The stomach has deep clefts in its aboral floor.
The eggs of Cyanea capillata var. arctica are orange and pass from the ovary into the gastric cavity, to which the gonads are connected, and out through the mouth to be brooded in the folds of the oral arms and escape as planulae. The planulae eventually settle via their anterior end and open a mouth at their posterior. The scyphistoma initially develops 4 tentacles, then later 15-20 tentacles in total and may put out stolons from which secondary scyphistomae may develop. after a variable amount of time, but as short as 18 to 20 days post-attachment strobilation occurs. The young ephyra are only 3.5 mm in diameter and have a 4-cornered mouth and a lenticular stomach with 16 radiating pouches. The four lips will eventually develop into oral arms. The young medusa, about 7 mm in diameter, rarely surfaces and will spread its oral fringes out over the bottom and sides of an aquarium and is largely sedentary. The young medusae and scyphistomae feed on protozoa, starfish larvae and mollusc larvae.
The variety ferruginea is thought to be a variant of the arctica and is found off the North Pacific coasts of America and Asia and reaches 45 cm in diameter with a yellow/orange bell. The stomach and their radial pouches are light brown, the gonads yellow and the tentacles reddish.
Cyanea capillata var. capillata has a bell diameter usually between 50 and 120 cm. A young medusa at 13 mm in bell diameter has about 7 tentacles in each adradial cluster, the middle one being the longest and oldest. By a diameter of 86 mm this has increased to 63 tentacles in each cluster. The bell, palps (oral arms) and tentacles are reddish or yellow-brown and the gonads a red or rose colour. This form occurs in the English Channel, North Sea, and coast of Norway in summer and autumn. Although this form may wash ashore on the British isles, the largest species of jellyfish found off the coasts of Britain is Rhizostoma pulmo since the largest arctic form of Cyanea capillata does not occur in this region. This explain why the largest species of Britain's coasts is often said to be the Lion's-Mane Jellyfish. The variety postelsii is thought to be a local variety of capillata found in the North Pacific to the Aleutian Islands to Oregon.
Cyanea lamarckii (= Cyanea capillata var lamarckii)
The taxonomy of Cyanea capillata is not settled.The definition of an 'animal species' is a population of animals that can interbreed to form fertile offspring (or else reproduces asexually). To what extent the forms of Cyanea interbreed is not known. If they can interbreed, then they are strictly varieties or subspecies; if not then they are distinct species. Cyanea lamarckii (The Bluefire Jellyfish or Blue Jellyfish) has a blue, whitish or violet bell (occasionally yellowish) and oral arms, the bell being darkest at the centre. It typically reaches a bell diameter of about 15 cm. It has about half as many tentacles as the Lion-Mane's Jellyfish (var. capillata) and larger gonads for its size.
A mystery solved?
I once found a blue 'Cyanea capillata' that measured about 1 m in bell diameter stranded off the coast of wales. There are occasional reports of other similar strandings. This could be a very large Cyanea lamarckii or a blue form of Cyanea capillata (var. capillata) or perhaps they do interbreed? In any case, it was a remarkably beautiful specimen.
Cyanea capillata var. fulva (= Cyanea fulva)
A small yellowish form, found off the North American coast from south of Cape Cod to the Carolina coast. The bell diameter rarely exceeds 20 cm.
Cyanea capillata var. nozakii
Similar to var. fulva, but milk-white and found in the inland Sea of Japan.
Cyanea capillata var. versicolor
A pink form and perhaps a southern variety of Cyanea fulva. found off Cape Hatteras (North Carolina) to south Florida.
Above: the rhizostome medusa Mastigias (based on Hyman, 1940, The Invertebrates). The mouth arms bear frilly mouths and end in terminal appendages. Printable version.
Rhizostome medusae have no marginal tentacles and have numerous mouths on 8 fleshy adradial branched oral arms. The lips of the mouths are bordered by minute mobile tentacles. The mesogloea is tough and these jellyfish often reach a large size. The following description applies to the mediterranean Rhizostoma pulmo. Rhizostomes are generally tropical water jellyfish, but Rhizostoma pulmo extends far into temperate waters and is the largest jellyfish found off the coast of the British isles (this is var. octopus = Rhizostoma octopus) and is also called the Barrel jellyfish or Dustbin-lid Jellyfish on account of its size with the bell frequently reaching 60 cm in diameter.This variety is found along the coasts of France, England, Scotland, Belgium, Holland and Germany. Rhizostoma pulmo has 80 lappets (96 to 112 in var. octopus in which the lappets are also more pointed). The bell is pyriform and higher than a hemisphere and usually no more than 15 cm, in diameter, but occasionally as much as 60 cm (swarms of medusae of the larger size have occurred off the coast of England). The exumbrella bears stinging or nettling-warts, giving it a granular texture. There are 8 rhopalia, each bearing an orange-coloured mass (statoliths) but lacking ocelli. There is a triangular ciliated exumbrella sensory pit above the rhopalium and a subumbrella ciliated sensory pit on its lower surface. Each rhopalium is flanked by a pair of narrow lanceolate (pointed and elongated) rhopalar lappets. There are 8 velar lappets in each octant (64 in total) and 16 rhopalar lappets giving (16 + 64) 80 lappets in total.
In the centre of the subumbrella is is the arm-disk, a raised ring of tissue supporting the massive oral arms. The young medusa usually loses the primary central mouth, but sometimes this persists in the centre of the arm-disk. The arm-disk is 4-sided proximally, but widens distally to an 8 to 16-sided disc bearing 8 pairs (16) short scapulets which are frilly appendages concealed within the subumbrella space bearing fringed secondary mouths on their upper convex edges. From the ring of scapulets and arm-disk hang the 8 oral arm. There is a proximal unfrilled upper section to the arms and a more proximal or lower frilly section. Each frilly segment bears 3 longitudinal wings, so this region of the arms is Y-shaped in cross-section, with a central arm canal and the two arms of the Y directed outwards (centrifugally). These frilly wings bear numerous mouths whose lips are fringed by a row of short flexible knobbed tentacles armed with nematocysts and mucus-secreting cells to aid in food capture. Each arm bears a distal pendant club-shaped structure, the terminal club, an appendage of unknown function which is triangular to 3-rayed in cross-section with a central canal. These appendages are thinnest in their middle and are widest proximally (or distally in var. octopus). the upper arm section is longer than the winged part (shorter in var. octopus). The arms are about as long as the bell is wide.
In some rhizostomes, including Mastigias, the four genital sacs fuse centrally to form a single cruciform chamber situated between the arm-disk and the stomach and not connected to the stomach. The four subgenital pits also fuse into a cruciform subgenital porticus below the stomach. In this case the arm-disk is reduced to four thick perradial columns alternating with gaps into which each chamber of the genital sac opens via a genital pore (genital ostium) in each lobe of the subgenital porticus. The stomach is cruciform (giving off 4 perradial pouches). In Rhizostoma, the genital sacs open into the stomach and gametes can then be shed via the secondary mouths (see below).
The interradial gonads are invaginated (as in Aurelia) and not protruding (as in Cyanea) and beneath each is a narrow subgenital pit in the subumbrella. The gonads are enclosed in genital sacs and connect to the stomach. The cruciform (cross-shaped) stomach gives out 16 radial canals, 8 to the rhopalia and 8 to the bell margin. The outer halves of these canals are joined together by a network of anastomosing canals. The powerful swimming coronal muscle consists of 16 deltoid blocks of circular muscle alternating with radial canals. There is no distinct ring canal, but the interconnecting canals are widest along the innermost edge of the network. Four canals extend into the arm disk and fork to give 8 canals, one extending into and along each oral arm and sending out branches to the scapulets. (This pattern of canals is modified if the central primary mouth persists in the adult).
The disc mesogloea is milky to creamy-yellow to rusty-yellowish and translucent. the marginal lappets are cobalt blue/blue/violet and the sensory clubs of the rhopalia tips are orange (due to orange statoliths). The frilled mouths are orange/yellow/brown-yellow as are the terminal clubs on their outer surface. The gonads are yellowish. This form is abundant in the Mediterranean in summer.
Presumably, vortices generated by bell pulsations drive feeding currents towards the secondary mouths on the oral arms and scapulets (when the latter are present).
Above: Rhizostoma pulmo from the Mediterranean (based on: Mayer, 1910. Medusae of the world vol. 3). Note the distal ends of the frilly scapulets just visible beneath the bell. These are born on arc-shaped appendages that branch off from the arm-disc/arm-base mass and arc upwards and outwards and bear frilly mouths on their upper convex edges. These effectively catch any food carried up into the bell (presumably by vortices generated by bell contraction). Note also the distal frilly mouth-bearing region of the oral arms and the orange pigment on the outer club of the arm appendages.
The life-cycle of the northern European variant of Rhizostoma pulmo, that is Rhizostoma octopus, has recently been reported (Holst et al. 2007. DOI 10.1007/s00227-006-0594-8). In this form (but apparently not the Mediterranea Rhizostoma pulmo in the strict sense) the males and female medusae can be distinguished by the colour of their gonads, visible through the translucent bell: brown in females when the eggs are ripe, and whitish-blue in males, when these are also ripe. (Rhizostoma is, like most jellyfish, dioecious, that is the sexes are separate.) The released eggs (each one-tenth of a millimeter in diameter) are not brooded in the oral arms but sink to the bottom of the water column (at least in an aquarium tank) and develop into planulae after two days. The planula is light brown and up to 0.15 millimeters in length and has a flagellated ectoderm (the planula is 'ciliated'). The anterior end is broader and equipped with nematocysts. The planulae swam for one to five days, under controlled conditions, rotating about their long axis as they do so (presumably the flagella are arranged in spiral rows along the length of the planula). The planula then attaches and develops into the scyphistoma. The scyphistoma initially has four primary tentacles and at 12 days post-attachment measured 0.5 millimetres in height and after two years, 2.3 millimeters but with up to 24 tentacles each up to 20 mm in length. A small fraction reproduced asexually, chiefly by budding off podocysts (from the foot of the attached scyphistoma). Presumably, a new scyphistoma 'germinates' from each podocyst. The spherical podocysts were up to 0.5 millimeters in diameter and encased in a chitinous shell. During strobilation the tentacles are resorbed and up to 5 discs are formed, each disc detaching to become an ephyra.
The coronate medusae (crown jellyfish) are less familiar to the casual observer beach comber, as they mainly occur in deep waters, though Nausirhoe, Linuche and Atorella occur in surface waters. These jellyfish are also mostly small, most being less than 5 cm in diameter, though some are medium-sized and reach 15 cm in diameter. They are characterised by the coronal groove: a circular groove on the exumbrella which demarcates the upper bell from the lower bell, as if the jellyfish was wearing a crown (corona). The bells are conical, domed or flattened with a scalloped edge.Printable version.
The lower bell is a circlet of pedalia separated by radial grooves. (Pedalia are sometimes thought of as 'tentacle feet' but some bear a rhopalium instead). The radial grooves are inline with the marginal lappets. Each pedalium bears either a single solid tentacle or a rhopalium. Each rhopalium is situated in a groove between a pair of lappets. In coronate medusae the rhopalia may or may not bear ocelli. The rhopalia, tentacles and pedalia are in multiples of four. For example, Nausithoe and Linuche (Thimble Jellyfish) have 8 rhopalia, 8 tentacles and 16 pedalia and is found in the Bahama-Florida region and similar waters. Periphylla periphylla (= Periphylla hyacinthina) the Helmet Jellyfish, has 16 lappets, 16 pedalia, 4 rhopalia and 12 tentacles and a purple dome-shaped bell. It emits red flashes of bioluminescent light. Bioluminescence is typical among coronate medusae and is used to startle potential predators. Note that in each case the number of tentacles + rhopalia equals the number of pedalia.
The four-angled mouth has a simple border (no oral arms etc.) and is borne on the end of a pendant cylinder called the manubrium. Four subumbrella funnels surround the mouth. These are funnel-shaped pits in the subumbrella of coronate medusae (and also in Cubomedusae and Stauromedusae, see below) but only occur in young stages of somaeostomes and rhizostomes, being replaced in the adults of these forms with the four subgenital pits below the gonads. The subumbrella funnels have no known function. Hyman (1940) suggests they may have a respiratory function, though they could also be secretory. One would certainly expect sea water to circulate within them as the bell pulsates, however, they may serve a mechanical function facilitating bell contraction? Some coronate medusae lack these structures (e.g. Atolla bairdii). The central stomach is located high in the bell and is divided peripherally into four gastric pouches lined by gastric filaments (also called digitelli, these are small subumbrella tentacles, typically armed with nematocysts in jellyfish and used in food processing they secrete digestive enzymes and help phagocytose the products of digestion and perhaps guard the entrance to the jellyfish from invaders). The gastric filaments are arranged in rows called phacellae. The filaments project into the stomach and are borne on four gelatinous dividing walls or septa which occupy the space between the gastric pouches. Each septum (there are four) is triangular, with the narrow apex pointing inwards towards the stomach and expand outside the central stomach to form the claustrum, a dividing membrane which narrows the conduits connecting the gastric pouches to the rest of the canal system to narrow slits called gastric ostia. These gastric ostia connect the stomach to the 'coronal stomach' a wide ring canal or sinus in the lower half of the bell, which gives off canals into the pedalia and marginal lappets. The septa connect to the exumbrella at four points, called the septal nodes or cathammata. The powerful ring of coronal muscle which contracts the bell is formed of 16 deltoid blocks of circular muscle in Nausirhoe. Excretory openings, connecting the gastrovascular system to the outside, are present in some species, e.g. there may be numerous openings or pores on the subumbrella where tentacular canals cross the ring canal, when the latter is present.
The life-cycle is poorly known in many coronatae (and jellyfish in general) but in Linuche the eggs are shed into the sea where they develop into swimming planula. There are 8 crescentic or U-shaped gonads on walls of the septa. The scyphistome of Nausithoe is known and consists of a clustered colony of trumpet-shaped polyps, several branching from one or more common basal stems. Each polyp has an expanded oral end fringed with short tentacles. The scyphistome is found in several habitats, including inside sponges. It undergoes polydisk strobilation to produce several ehphyrae.
Nausithoe has the following arrangement:
Atolla has a more complex form. The bell is flat and discoid with a lens-shaped corona. In Atolla bairdii there is a smooth circular ridge beneath the coronal groove, beneath which is a another shallow groove separating the ridge from the pedalia. There are two alternating rings of pedalia, the upper ring bearing tentacles (one solid tentacle per pedalium), each of the lower a rhopalium instead. The arrangement in Atolla bairdii is as follows:
Above: Atolla bairdii. Note the upper row of tentacular pedalia and the lower alternating row of rhopalar pedalia and the marginal lappets. Printable version.Other well-studied genera genera include Periphylla, Atorella and Periphyllopsis.
Above: Atolla wyvielli (based on: Mayer, 1910. Medusae of the world vol. 3). Printable version.With characteristic wide radial notches or furrows in the lenticular crown. Up to 73 mm diameter with 22 to 28 tentacles. Note the small marginal lappets.
4. Cubomedusae (Box
Box Jellyfish are sometimes included in the Scyphozoa, along with the somaeostomes, rhizostomes and coronate jellyfish. Others consider them separate from the Scyphozoa (true jellyfish), Hydrozoa (hydroids), Anthozoa (anemones and true corals) as the Cubozoa. In a number of regards they are more advanced than other scyphozoans and I currently regard them as a highly evolved group of scyphozoans. They occur in warm tropical and subtropical waters, in harbours and bays and also in the open sea, but chiefly in shallow water. Occasionally they are found washed ashore on temperate coasts. Genera include Carybdea (illustrated below), Tripedalia, Chirodropus, Chironex (Chironex flickeri is the Sea Wasp) and Chiropsalmus (Fire Medusa). Given that some have lethal or potentially lethal stings, such as Chironex and Chiropsalmus, the name Sea Wasp is often applied to all the cubomedusans.
The bell is very translucent and colourless or blueish. Generally only the tentacles generally have appreciable colour. Most are small (2 to 4 cm in bell height) but some are moderately large at 10 to 25 cm in height with tentacles up to about 3 m in length. The bell can pulsate very fast at 120 to 150 per minute.
The cubomedusan has a cuboidal bell with four flattened (perradial) sides and a simple margin (i.e. with no lappets). However, the bell margin folds in to create a shell-like rim called a velarium. The four corner edges are interradial and bear, just above the margin, a tentacle or tentacle cluster. Each tentacle is borne on a tough blade-like pedalium. There are four pedalia and tentacles in Carybdea, one per corner. Tripedalia has three pedalia/tentacles in each corner whilst Chiropsalmus has a thick pedalium in each corner which branches into smaller pedalia, each of which bears a tentacle. these 'hand-like pedalia give Chiropsalmus the common name of Four-handed Box Jellyfish. Each tentacle bears rings of nematocysts.
The four rhopalia are perradial, with one in the midline of each side face of the box towards the bell margin, situated in a niche and just above a thickened gelatinous fold, called a frenulum. The four frenula support the velarium. The gastrodermis contributes to the tentacles, velarium and rhopalia. there is a short quandrangular manubrium hanging down from the top of the bell deep inside the deep subumbrella cavity. This bears the mouth which opens into the central stomach at the top of the bell. Four subumbrellar funnels encircle the base of the manubrium. The four gastric pouches are perradial (one in each face) and connected to the central stomach by a gastric ostium or pore passing through septa on either side. Each of the four septa extend from the exumbrella to the subumbrella and each carries a pair of attached sheet-like gonads along the height of the bell. Each septum encloses one subumbrellar funnel. In Chiropsalmus and Chirodropus each gastric pouch evaginates on its upper surface to give a pendant subumbrellar sac, hanging down within the subumbrella cavity. Where each septum joins the central stomach, it gives off a U-shaped bunch of gastric filaments. Each gastric pouch gives off a canal to each rhopalium which forks at each perradial fold (the perradial fold is an extension of tissue continuous with the frenulum) and exits an opening in the septum at the bell margin to form a ring canal. A canal is also given off to each tentacle and blind canals or sinuses supply the velarium.
Each rhopalium has a gravity-sensing statolith and ocelli looking inwards into the subumbrella (note that the bell is highly translucent). In Carybdea each rhopalium has 6 eyes, 2 larger eyes equipped with lens and thus more sophisticated than the ocelli of other schyphozomedusae, as well as 2 pairs of pigment-cup ocelli. Box Jellyfish are strong and very maneuverable swimmers and it has been suggested that they use their visual system to track shoals of fish, fish being their chief prey. The nervous system of cubomedusans is also more advanced, paralleling that in some hydromedusae, by possessing a definite nerve ring near the bell margin, which loops upwards to contact each rhopalium. Jellyfish might not have anatomical brains, consisting of aggregated neuronal ganglia, but their neural nets can process information. The presence of lens-bearing eyes suggests that cubomedusans are extracting some positional information about objects in their visual field. This intensive image processing apparently pre-occupies their nerve nets during active hunting and periods of hunting are followed by periods of sleep, perhaps enabling the nervous system downtime for housekeeping maintenance. This is a convincing argument for the evolution of sleep.
Little is known about cubomedusan reproduction and development. A free-swimming planula develops from the egg and attached to form the polyp (a scyphostoma or cubostoma) which has been observed to completely metamorphose into a single medusan, without strobilation, prompting some to reclassify the cubomedusans as Cubozoa rather than as Scyphozoa. However, Strachler-Pohl and James, 2005 (Mar. Bio. 147: 1271-1277) observed an additional optional second mode of development in Carybdea marsupialis, in which the polyp metamorphoses into the medusa, but leaves behind part of itself as a remnant, suggestive of monodisc strobilation (when a strobila buds off a single ephyra as occurs in some schyphomedusae). This lends weight to the idea that the cubomedusans are a specialised and highly evolved offshoot of scyhphomedusae, whether we decide to classify them separately or not.
The stauromedusaeare also sometimes classed separately from other jellyfish, but appear to be derived from other scyphomedusae. Stauromedusae live upside-down and attached to the substrate via either a stalk (pedicel or peduncle) ending in an adhesive foot (pedal disc) or via an adhesive spot in the center of the exumbrella in those stalkless species. These flower-like forms are small to medium in size, approaching several cm in diameter. They are usually green or brown in colour, but pink, orange, blue and violet forms also occur. Some absorb chromoplasts from their algal substrates through the pedal disc to assume the same colour as vegetation they are attached to. They occur in colder coastal waters, in bays and sounds, etc. Some forms of the genus Lucernaria have been found at abyssal depths and at hydrothermal vents (Lutz et al. 1998, Deep-sea Research II 45: 329-334). They cannot swim, but some can move in a hydra-like fashion (see: hydrozoa) whilst others are permanently attached to their substrates (seaweeds, shells, rocks, etc.) by secreting a hard chitinous adhesive.
Above: Haliclytus; note the central mouth surrounded by four oral lobes and the attachment stalk ending in an adhesive disc and the 8 arms bearing bunches of 100 to 200 hollow tentacles, each tentacle with a terminal knob (equipped with nematocysts). The disc is 20 to 30 mm in diameter. A gonad is visible extending into each arm. Printable version.
These jellyfish are sessile medusae and trumpet or goblet-shaped and resembling polypoids. This is an interesting example of the twists and turns evolution can take to find a niche. Jellyfish are particular amongst cnidarians for having sexual mature free-swimming medusae and sessile polypoid larvae. Here we have an adult form that has reverted to a polypoid-like existence, but is of course a sexually mature adult. Haliclystus expands orally with the subumbrella uppermost and tapers aborally to the stalk (pedicel or peduncle). The bell margin is either circular and fringed with tentacles, or 8-sided and drawn into 8 adradial arms (or 4 bifurcated arms as in Kishinouyea) each ending in a bunch of short capitate tentacles (capitate = clubbed or equipped with a terminal knob). Depending on species (and presumably age of the individual) there may be between 20 and several hundred tentacles per cluster.
In each perradius and interradius there is a marginal body (x 8 in total) called a rhopalioid. This is not a true rhopalium, since it lacks the specialised rhopalial sensory systems, but is a reduced tentacle which serves an adhesive function and is also called an anchor or colletocystophore. At least, the anchor consists of glandular tissue, but Hyman (1940) could observe no use for them in Haliclystus, the anchors did not respond to touch and the tentacle clusters are used for anchorage following detachment and during locomotion. Thus, the function of the 'anchors' remains enigmatic. Each is a horseshoe or oval-shaped adhesive cushion.The coronal muscle, no longer required for swimming, is reduced to a thin marginal band. There are radial epidermal muscle fibres and both the subumbrella and exumbrella nerve nets are well developed.
The four-cornered mouth (flanked by four small oral lobes) opens into a short quadrangular manubrium, which is surrounded by four subumbrellar funnels at its base and opens into the central stomach. The central stomach gives off four perradial lobes (gastric pockets or gastric pouches) separated by 4 interradial septa. Each septum encloses one of the subumbrella funnels which are interconnected through pores in the septa, called septal ostia, to form a ring sinus which gives off canals to the rhopalioids, arms and tentacles. Where the septa joins the central stomach it gives off gastric filaments. The septa also contain septal muscles and manufacture nematocysts. The 8 gonads (or 4 horseshoe-shaped pairs) are elongated bodies borne on the faces of the septa and each gonad extends into one arm. The septa extend to the pedal disc and may fuse in the center to divide the stalk canal into 4 separate canals, as in Haliclystus. Some forms, such as Craterolophus and Halimocyathus, have 4 claustra: longitudinal partitions between the septa along much of their length. These divide off the outer compartment of each gastric pouch for most of the animal's length, but the claustra do not reach to the base of the bell and here the outer compartments are in direct communication with the rest of the stomach. Haliclystus lacks claustra. Haliclystus has been observed to feed on small crustaceans which it catches with its tentacles and then bends the arm inward to deposit the food on the manubrium / oral lobes.
Other forms differ: Lipkea has no anchors and no terminal knobs on its tentacles. Depastrum has no rhopalioids, 16 tentacle clusters and a simple bell margin.
Gametes are shed through the mouth at night, where fertilisation is apparently external. The fertilised eggs develop into non-ciliated vermiform planulae (vermiform = worm-like) that creep along before attaching at their anterior ends, developing a mouth and sometimes budding out secondary larvae as planula-like stolons. The polyp metamorphoses directly into the adult. Some consider the adult to be a modified scyphistoma, but it is perhaps more likely a modified medusa that failed to detach from the strobila following monodisc strobilation. interestingly, Haeckel (1880) described four specimens of three genera of medusae he classified as stauromedusae: Tessera, Tesserantha and Tessaria (though he considers that the three may be different developmental stages of the same species) but which were free-swimming and not sessile. These lacked attachment stalks, but did have a hollow tubular protuberance in the center of the exumbrella. The tentacles were solid and lacked terminal knobs. The reasons for classifying these as stauromedusae are not clear. To my knowledge these forms have not been reported by any other biologist and they remain an enigma.