The Natural History of Our Ancestors the Jellyfish Quiz

Answer: The Cambrian Period (>500 million years ago)

The Quaternary Period is the comparative present day where man evolved from apes and learned to walk on his own two feet (literally).
The Cretaceous and Jurassic Periods were the ages of dinosaurs, the end of the Cretaceous Period marking the extinction of the dinosaurs.
There is fossil evidence that jellyfish were alive and well in the Cambrian period and some authors have suggested that jellyfish have been inhabiting the earth for over 680 million years which pre-dates even the Cambrian period. This makes the jellyfish among the oldest multi-organ animals in the world.

Answer: No

Jellyfish is a misnomer: jellyfish are not fish. They are not even vertebrates.
Jellyfish belong to the phylum Cnidaria, which is a group of marine animals that contain cnidariacytes, specialized cells for trapping food. This group includes two subgroups: sea anemones and corals (still or sessile); and jellyfish (swimming). The jellyfish subphylum is called Medusozoa, and contain Scyphozoa,(true jellyfish), Cubozoa (box jellyfish), Staurozoa (stalked jellyfish) and Hydrazoa (hydra-like animals and some parasites).

True jellyfish have two body forms: an immature polyp with mouth pointing upwards, or a medusa which is the bell-shaped form with a mouth pointing downwards. Both forms are radially symmetrical (tube and wheel respectively). This type of symmetry belies the animals' primitivity with respect to evolution: the animals have no left or right sides. This development took a few more hundreds of million years to evolve in animals, though they do have a top and bottom.

Because of the misnomer of the word fish in their name, jellyfish are increasingly being called "Jellies".

Answer: Jet propulsion

In the true jellyfish, the medusa is the umbrella shaped body of the mature jellyfish. This comprises three layers: an outer thin layer, epidermis (we as humans have this layer on our skin); the middle layer called the mesoglea and an inner layer called the gastrodermis which acts like a stomach. The mesoglea is a non-living(!) jelly-like substance that is mostly water with a few fibrous proteins. The mesoglea maintains rigidity in the jellyfish (a primitive form of skeleton, if you will) but if you take a jellyfish out of water, (not recommended), it loses the water and hence its shape. The jellyfish moves along by filling its mesoglea with water and then contracting, forcing the water out. This propels the jellyfish forward like a jet engine albeit in a pulsating motion rather than with a constant thrust.

The jellyfish can move but not that strongly. It cannot fight currents and tends to drift with them. It tends to move vertically, up and down, rather than horizontally against or with the current.
With the remaining options: flagellation is a popular motion mechanism in bacteria and I hope you had a wry smile about the possibility that a jellyfish that could do backstroke without a back.

Answer: Neither brain nor heart

The jellyfish has no brain, only a rudimentary "nerve net". This is a series of nerve like fibres spread through the epidermis. For one of the first multi-organ animals, this is quite an elaborate evolutionary development. This network allows the jellyfish to perceive the senses of touch, smell and a few other rudimentary senses.
Jellyfish have no heart because they have no need for blood.

Answer: No

Jellyfish have no need for a respiratory organ as the epidermis is so thin, oxygen diffuses though this membrane very easily. As oxygen reaches all tissues in this manner there is no need to have blood circulating to carry the oxygen and as there is no need for blood, there is no need for a heart.

It would be fairly safe to assume that the process is inefficient compared with, say mammalian systems otherwise why would the latter mechanisms have evolved?

Answer: It stings any type of food it touches, immobilising it then ingests the food through its mouth

Jellyfish have specialised cells called cnidocytes on its tentacles which contain organelles called nematocysts. These intricate structures harbor tiny harpoons that contain venom. When an animal (food) touches the tentacle, the harpoon shoots out and immobilises it as the venom is impregnated into the flesh of the victim.

This is quick - it takes 700 nanoseconds from touch to injection of venom. It is this physiological reaction in two species, box jellyfish (Chironex fleckeri) and Irukandji jellyfish (Carukia barnesi) that are so dangerous to humans: their venom is very potent to humans and can even cause death.

Answer: Secondary locomotion mechanism

The mouth is a large structure located on the underside of the medusa and opens out into the gastrodermis, the jellyfish's rudimentary multi-organ. The mouth points downwards and is in close proximity to the tentacles containing the nematocysts facilitating the ingestion of food. Jellyfish have an incomplete digestive system which means the same orifice (mouth) is used for both ingestion and excretion of food.

The reproduction structures are located in the gastrodermis. Sperm are released through the male mouth and eggs are released by the female mouth. The mouth has no role in locomotion.

Answer: Both sexual and asexual reproduction

Jellyfish have complex reproductive behavior: they take on two morphological forms; and they reproduce both sexually and asexually.
The adult form of jellyfish are the medusa. They spawn by males releasing sperm and females releasing ova through their mouths into the ocean. Fertilisation takes place either in the ocean or sometimes the sperm swim into the female's mouth and fertiization occurs there.
A fertilised egg then turns into a planula (plural - planulae) which are ciliated free-swimming larvae. After a few day's development, planulae attach to a hard surface and transform into a polyp which has a mouth and tentacles for feeding on zooplankton. A polyp reproduces asexually by budding. An immature medusa is produced and is called an ephyra which in turn, develops into an adult medusa.

Answer: Yes

The ability to detect light is based on opsins, light sensitive protein-based receptors. These can be traced back 700 million years to a primitive organism called a placozoan. This organism has an opsin-like protein but it has a crucial single amino acid in its structure that prevents it from detecting light. That amino acid changed in the next wave of evolution when jellyfish evolved containing an opsin that could detect light. What is surprising is that all models of light detection or vision in all animals can be traced back to the opsin in the jellyfish.

This is the single most compelling argument that all animals, including humans, have a single common ancestor and that ancestor is jellyfish!

Answer: Pack

Collective nouns used to describe jellyfish include, bloom, swarm and smack. Scientists tend not to use smack as a descriptor as it implies a small quantity or a taste or a smattering which tends not to be the case. As jellyfish, are to some degree, at the mercy of ocean currents, if you find a jellyfish, you tend to find a lot in the same area. For that reason bloom and swarm are much better descriptors for groups of jellyfish.

A bloom implies a number in a set area or a number more than expected. With their dependence on tidal current and because of their sporing behavior you tend to see more jellyfish than you expect in any given area.

A swarm implies that a group has some mechanism of staying together and some jellyfish species such as moon jellyfish (Aurelia species) exhibit signs of this behavior.