What are Jellyfish?
The term jellyfish means different things to different people, but in the context of this project we mean free-floating or free-swimming animals belonging to the groups Cnidaria and Ctenophora.
Cnidarians include animals like sea-anemones and corals that live attached to the seabed, as well as the free-swimming jellyfish. They are characterised by a number of features, the most obvious one being the possession of stinging “cells” or cnidoblasts (also called nematocysts) that are scattered throughout their surface tissues: contrary to popular belief not all species of jellyfish have long or trailing tentacles. The cnidoblasts are concentrated in the tentacles and/ mouth appendages and are used to poison or sting prey prior to capture and ingestion. Jellyfish can be thought of as upside down sea-anemones, and like sea-anemones they have muscles that expand and contract and which allow the animal to swim through the water in a pulsed fashion.
Ctenophores by contrast neither possess stinging cells nor well developed muscles, but rather move through the water using plates of cilia (known as ctenes) that are arranged in rows that run all or part of the way along the “length” of the body. Most species have eight ctene rows and the ctenes in a row beat in a sequential manner to propel the animal through the water. Light often reflects from and is refracted by the ctenes as they beat, to give off mini rainbows. Some species have complex tentacles that trail behind the animal and which are used to catch prey. These tentacles are armed with colloblasts (not cnidoblasts), which discharge a glue to ensnare hapless prey.
Similarities between pelagic cnidarians and ctenophores
Although the two groups of animals are actually quite different from each other, their free-swimming representatives (jellyfish) have a number of similarities.
Firstly, they are both members of the plankton. This means that although they are free-swimming they do not have the ability to move horizontally against ocean currents, and must effectively go with the water flow and drift. Nektonic animals, by contrast, are also free swimming but they do have the ability to move horizontally against ocean currents (e.g. squid, most fish, turtles and marine mammals).
Secondly, they are composed almost entirely of water and have a jelly-like consistency. In fact over 95% of the weight of a jellyfish comprises water, the balance being a mix of proteins (mostly) with a few fats and carbohydrates thrown in for good measure. This contrasts with your average fish, which is only 80% water.
Thirdly, they are both carnivores, and feed on a very wide range of prey items using basically similar tactics - they are entangling or cruising predators that deploy tentacles armed with specialised “cells” (albeit of different type) for prey immobilisation.
And then there are a host of similarities in their feeding and other physiologies. Taken together, it is perhaps no surprise that the two groups were previously recognised together in the Coelenterata.
Types of Jellyfish - Cnidarians
The phylum Cnidaria is divided into two major groups, the Medusozoa and the Anthozoa. The difference between the two rests on their mode of life. In the case of Medusozoa there is an alteration of life-history stages (they are said to be metagenic) between a small and generally cryptic bottom-living (or benthic) phase (a polyp) and a conspicuous free-swimming stage (the medusa or jellyfish) that lives in the open water (the pelagos, hence pelagic). The polyp is considered to represent the “juvenile” phase of the life-cycle and produces medusae by a process of asexual budding, whilst the medusa is considered to be the “adult” phase and reproduces sexually in the conventional way. Sexes are generally separate, eggs and sperm are liberated into the water and the fertilised egg develops into a planula larva, which settles on the seafloor to develop in turn into a polyp. In the case of the Anthozoa (sea-anemones and corals etc), there is no alteration of life-history phases and the medusa stage has been lost entirely so that the polyp is responsible for sexual reproduction: eggs and sperm are shed into the water column and a planula larva develops that then settles on the seafloor to produce a new polyp.
The Medusozoa contains three main groups (Hydrozoa, Scyphozoa and Cubozoa), each with a number of sub-groups: their characteristics are detailed below, and a dichotomous key enabling you to identify each major group can be found by clicking here.
Class - Scyphozoa
These are the large jellyfish that get washed up on the beaches and that clog both fishers’ nets and Koeberg’s water intake pipes. They tend to be conspicuous and are sometimes massive in size. They have a fairly thick umbrella that is hemispherical, saucer- or cannonball-like in shape (never cuboidal), which is frequently pigmented: they are almost always opaque. They have a variable number of tentacles at the margin of the umbrella (some have none), and may or may not have appendages or arms around the mouth. They do not have a velum or inward-projecting shelf around the inner, lower margin of the umbrella. Most species have a polyp stage. Scyphozoans are widely distributed in oceanic and coastal waters around the globe. There are approximately 200 species in four orders: 13 species have been recorded around South and southern Africa.
Order - Stauromedusae
Also known as stalked jellyfish, these animals are found attached to the seabed or to other benthic organisms and lack a pelagic phase in their life-cycle. Whilst interesting, they do not form the focus of this project.
Order - Semaeostomeae
These animals are characterised by having tentacles at the margin of the umbrella AND by having arms or appendages surrounding the mouth. Most species have an umbrella-like or saucer-shape umbrella, that is usually pigmented (sometimes highly so) but that is not very thick. Most species display have a polyp stage in their life-cycle, though one (Pelagia noctiluca) has lost its polyp and is entirely pelagic. They tend to be found in coastal waters and shallow seas. Forty-two species in three families: five species have been recorded around South and southern Africa.
Order - Rhizostomeae
These are also known as root mouth jellyfish. These animals lack tentacles at the margin of the umbrella but have often very elaborate, robust and filamentous appendages or arms around the mouth. Most species are hemispherical in shape, and their thick, rubber-like umbrella is opaque and usually (but not always) whitish in colour. All species have a polyp stage in their life cycle. Root-mouth jellies are found in coastal waters and shallow seas. Eight-two species in eight families: six species have been recorded around South and southern Africa.
Order - Coronatae
These animals look quite different from those of the previous two orders, because the umbrella is constricted and has a pronounced groove on the upper surface: they can look a bit like flying saucers. The umbrella is fairly thick and rubber-like; is opaque and often deep red or purple in colour. Coronate medusae have marginal tentacles and although the lips of the mouth may hang below the umbrella, there are no well developed arms or appendages. While some species are known to have a polyp stage in their life-cycle, their biology is poorly known because most species are only found in deep water. Thirty-one species in six families: two species have been recorded around South and southern Africa.
Class - Cubozoa
These are the box jellies. They are always box-like or cuboidal in shape and have one or more opaque tentacles arising from a tentacle-stalk (or pedalium) at each lower corner of the umbrella. Most species are fist-size or smaller, though a few species are much larger. The umbrella is usually of a firm consistency and is translucent; it possesses a velum or inward-projecting shelf around the inner, lower margin of the umbrella. Cubozoans lack appendages or arms around the mouth. Box jellies have a polyp stage in their life cycle. These are considered to be the “bad boys” of the jellyfish world because they include amongst their ranks a number of known human-killers. Box jellies show a relatively restricted distribution in shelf and coastal waters, being largely temperate and tropical. There are 16 species in two families: three species have been formally described from around South and southern Africa, though a third is known from anecdotal evidence.
Family - Carybdeidae
These are box jellies with tentacles that arise on their own pedalium at each corner. Most species have a single pedalium (and hence a single tentacle) at each corner, but not all. Nine species: two species have been recorded around South and southern Africa.
Family - Chirodropidae
These are box jellies with a single pedalium at each corner, but each pedalium may bear many tentacles. Seven species: one species has been recorded around South and southern Africa.
A complicated and very diverse group of animals that includes siphonophores, such as the blue-bottle. Most hydromedusae tend to be fairly small (thumb-nail size) and have a relatively thin and transparent umbrella. The shape of the umbrella is very variable - cuboidal, hemispherical, saucer-like or even bell-like: always with a velum or inward-projecting shelf around the inner, lower margin of the umbrella. Some species may possess highly coloured spots and internal structures, which makes them very photogenic. Tentacles may or may not be present and when present they usually (but not always) arise from small swellings (or bulbs) at the edge of the umbrella. The mouth is suspended on a collar (or manubrium), which surrounds the stomach and can be likened to the clapper of the bell: the stomach in turn leads into a canal system that distributes the products of digestion around the body of the animal. Some hydrozoan species display a life-cycle comprising alternate polyp and medusa stages, but in many others the medusa or the polyp stage may be modified, or even lost entirely. Hydromedusae are found in all seas and oceans around the globe and are a conspicuous component of the plankton.
Although most of these animals are unlikely to be observed by SAJellyWatchers, we include a brief summary of the characteristics of the major groups for completeness.
Class - Automedusa
These are Hydrozoans without a true polyp stage: truly oceanic. Most species have a wide global distribution and many occupy the deep sea: with two pelagic sub-classes.
Sub-Class - Narcomedusae
The umbrella is usually saucer-like, with a thickened central disc and thinner periphery that is divided into deep lobes. The main tentacles emerge at the base of the peripheral lobes, from deep inside the animal’s tissues, although minute tentacles may be present at the margin of the umbrella: tentacular bulbs are absent. The manubrium (mouth collar) is broad, short and thin and does not tend to lead into a well-developed radial canal system basally: supports gonads externally. The life-cycle of these animals can be complicated as some have parasitic larvae.
Sub-Class - Trachymedusae
The umbrella is hemispherical or deeply campanulate (bell-shaped), without peripheral lobes. The mouth and manubrium may be at the end of a greatly elongated stalk. There is a well developed radial canal system, around which the gonads are wrapped.
Class - Hydroidomedusa
Hydrozoans with an alternation of polyp and medusoid stages in the life-cycle: secondary polyp or medusa loss may be observed in some groups. Polyps often occur as colonies of clones, with different individuals assuming different morphologies and functions, so that some polyps will be “responsible” for defence, others for feeding, and others still for reproduction: all are interconnected. With the exception of the siphonophores and some anthomedusae, most species are coastal: few are known from the deep sea. There are five sub-classes, of which only three are sufficiently common to warrant comment.
Sub-Class - Anthomedusa
Medusae are usually bell-shaped, and the gonads are wrapped around the manubrium. The tentacles, if present, arise from the umbrella margin. The polyp of these hydroids tends to lack a protective sheath around the tentacle-bearing hydranth. Not all described polyps produce medusae.
Sub-Class - Leptomedusa
The medusae are usually hemispherical or discoid in shape, and the gonads are wrapped around the radial canals. The tentacles arise from the edge of the umbrella. The polyp stage has a protective exoskeleton around the tentacle-bearing hydranth. Not all described polyps produce medusae.
Sub-Class - Siphonophorae
A siphonophore comprises a mix of medusae and polyps together in a single free-swimming “colony”: they are complicated animals! Neither the medusae nor the polyps of a siphonophore bear any resemblance to the medusae and polyps of other hydrozoans, and they are referred to as medusoids and polypoids respectively. So, a siphonophore comprises a mix of medusoid and polypoid “individuals”, which are all joined together on a stem (or siphosome). The polypoid individuals are responsible for feeding (gastrozooids and dactylozooids - each bearing a tentacle) and floatation (pneumatophore), whilst the tentacle-free medusoid individuals are responsible for movement (nectophores) and sexual reproduction (gonophores): protection is provided by bracts, which are of unknown origin! Siphonophores are found throughout the worlds oceans, and although most are fairly small - some may exceed 30 m in length. They are generally transparent, but some may be brightly coloured.
Order - Cystonectae
These are siphonophores without nectophores but with a fairly large pneumatophore. This group includes the well known blue-bottle, which lives on the sea surface (neustonic).
Order - Physonectae
These are siphonophores with fairly large nectophores and with a small pneumatophore.
Order - Calycophorae
These are siphonophores with nectophores but without a pneumatophore. Most species have two usually dissimilar-looking nectophores, though some may have more.
Types of Jellyfish - Ctenophores
Although the phylum Ctenophora is divided into two main groups, depending on whether tentacles are absent (Nuda) or present (Tentaculata) at any stage in the life of the animal, our understanding of the taxonomy of the group is presently uncertain. Unlike cnidarians, ctenophores do not have an alternation of life-history phases and most are what is known as holplanktic. That is, their members are born into the plankton and die in the plankton. Most species are hermaphroditic, which means that all individuals are both male and female, and many are simultaneous hermaphrodites - both “genders” are fertile at the same time. The implication of this is that an individual can fertilise his own eggs! The fertilised eggs develop into a larval form that may bear little resemblance to the adult. Some species are able to reproduce when only a few days old, which means that populations of ctenophores can grow at an alarming rate. Ctenophores are found throughout the world’s oceans, and although they can reach high abundances close to shore, they are also found in the deep-sea. One hundred and ninety-four species in two Classes: there are no formally described species of ctenophore from around South or southern Africa, although two types are recognised.
Class - Nuda
These are Ctenophores that lack tentacles at any stage of their life. They are generally ovoid or conical in shape, and have a relatively thick, if delicate body. They tend to be fairly large and are transluscent or opaque, and have a very wide mouth at one “end”. Most species have a coastal distribution, and they are thought to feed exclusively on other ctenophores. One Order (Beroidea) is currently recognised globally, with a single species in the region: Beroe cucumis.
Class - Tentaculata
Although these ctenophores possess tentacles at some stage during their early development, these may be lost or reduced on reaching maturity. There are six described Orders of planktic tentaculate ctenophore, each of which has a quite different body form, but only three of these are described here because the others (Thalassocalycida, Ganeshida, Cambojiida and Cryptolobiferida) are very rare. Although only one species is currently identified from around South and southern Africa, others MUST be present so please keep your eyes open and contact us if you find anything unusual.
Order - Cydippida
The body is fairly small, ovoid or spherical in shape, occasionally laterally compressed or cylindrical. Mature individuals retain a single pair of long delicate and branched tentacles that can be withdrawn into protective sheaths within the body. These animals are generally referred to as entangling, sit-and-wait predators that set enormous nets for their prey to swim into. The eight ctene rows are generally of near-equal length. Their bodies tend to be fairly small and translucent, and some think they resemble gooseberries (of the “English” variety) - hence the name sea gooseberry. One species is currently known from the region (Pleurobrachia pileus) although seven distinct families and 70 species have been described globally. Our local sea gooseberry can commonly be seen in harbours.
Order - Lobata
These animals look nothing like gooseberries, although they too are generally ovoid in shape. They either have very reduced tentacles as adults or they have lost them altogether, and they feed by capturing prey by filtering them out of the water as they swim actively through it. The body is expanded into a pair of lobes at the mouth end: the ctene rows are not all of near-equal length. They are transluscent, fairly large and very delicate animals that break-up easily on capture. No species have formally been described from the region, but as five families and 44 species are currently recognised globally, it is just a matter of time before we collect some local representatives. This group contains the notorious Mnemiopsis leidy from the eastern seaboard of the Americas, which was accidentally introduced into the Black and Caspian Seas and which brought the pelagic fisheries there to its knees.
Order - Cestida
Most of us are unlikely to ever come across a Venus Girdle, unless we dive in submersibles. They are deep-water animals that are unknown from shallow water. They look nothing like the other ctenophores being long and ribbon-like, with very reduced tentacles and frequently without eight clear ctene rows. Two species have been described from a single family.