Jellyfish and Comb Jellies

• Jellyfish and comb jellies are gelatinous animals found drifting throughout the ocean
• Jellyfish belong to the phylum Cnidaria
• Comb jellies belong to the phylum Ctenophora
• Both have roamed the seas for at least 500 million years
• Increasing seawater temperatures, removal of jelly predators by fishing, built structures in seawater, and nutrient flow into the ocean contribute to the growth and survival of jellyfish and comb jellies.
• Jelly blooms, or huge increases in jelly populations, can disrupt fisheries, be unpleasant for swimming, and obstruct power plants using seawater for cooling
• Invasive jellies have caused ecological problems in some areas

Anatomy

• Both jellyfish and comb jellies share similar basic anatomy.
• Both have two major cell layers, the external epidermis and the internal gastrodermis
• Ctenophores have musculature in their mesoderm
• The gastrodermis lines the gut and an opening for food and reproductive cells
• Nutrients and oxygen pass through the gastrodermis or outer cells
• The epidermis has a nerve net, the most basic nervous system in multicellular animals
• Mesoglea, a gelatinous material mostly made of water, forms the bulk of their bodies and functions as an internal skeleton
• Jellyfish and comb jellies are 95% water

Comb Jellies' Unique Features

• Comb jellies have plates of fused cilia, called combs, arranged in eight rows that propel them through the water
• The comb rows create a rainbow effect by scattering light
• Comb jellies release indigestible particles through pores on their rear end
• Many comb jellies use a pair of branched tentacles armed with sticky cells (colloblasts) to capture prey
• Comb jelly tentacles do not sting

Jellyfishes' Unique Features

• Jellyfish alternate between medusa (bell-shaped with tentacles) and polyp body forms
• Jellyfish possess nematocysts, stinging cells unique to them and their close relatives.

Size

• Sizes vary greatly
• Lion’s Mane Jellyfish (Cyanea capillata) can be almost 6 feet wide with tentacles over 49 feet long
• Venus’s girdle (Cestum veneris) can be 40 inches long

The Nervous System

• Jellies do not have brains, but rather a network of neurons that allows them to sense their environment
• Statocysts are balance sensors
• Ocelli are light-sensing organs
• Some jellyfish have sensory structures called rhopalia, which contain receptors to detect light, chemicals, and movement
• Cubozoan jellyfish have complex eyes with lenses, corneas, and retinas in their rhopalia
• A 2017 study of Cassiopea found that a brain is not required to experience sleep

Diversity

• Jellies inhabit oceans worldwide and even some freshwater environments
• All jellyfish are Cnidaria
• There are over 10,000 species of Cnidaria
• Less than 4,000 of these are Medusazoa—animals considered jellyfish

Types of Jellyfish

• Divided into four groups
  • Scyphozoa
    • The most familiar jellyfish
    • Spend most of their lives in the medusa body form
    • At least 200 species exist
  • Hydrozoa
    • Jellyfish look-alikes
    • Swimming medusa stages are small and inconspicuous
    • Bottom-dwelling polyps usually take the form of large colonies
    • Siphonophores are colonial
    • Colonial siphonophores consist of specialized individuals called zooids that are genetically identical
    • Around 3,700 species exist
  • Cubozoa
    • Box jellyfish
    • Some produce potent venom
    • Have complex eyes
    • Some engage in courtship behavior
    • At least 36 species
  • Staurozoa
    • Stalked jellyfishes
    • Live attached to rocks or seaweed
    • Trumpet-shaped
    • Mostly live in cold water
    • Around 50 species

Types of Comb Jellies

• Far fewer species of ctenophores than jellyfish
• 100-150 species have been found
• Coastal comb jellies can be divided into three groups
  • Cydippids
    • Have rounded bodies with branched tentacles that can be withdrawn into sheaths
  • Lobates
    • Have two flattened lobes extending from their bodies below their mouths
    • Have short tentacles
    • Tend to grow larger than cydippids
  • Beroirds
    • Sack-shaped
    • Have no tentacles
    • Have a very large mouth

Evolution

• Jellyfish and comb jellies are in different phyla, but may have a close relationship
• Cnidaria and Ctenophora were once described as Coelenterata
• Some researchers believe they are sister groups
• Others believe they are not closely related
• Understanding their place in the tree of life helps people understand how all other animals—including people—evolved
• Comb jellies and jellyfish (and other Cnidarians) are the earliest known animals to have organized tissues and a nervous system
• They're also the first animals known to swim using muscles instead of drifting with the waves
• The oldest ancestors of modern-day jellies lived at least 500 million years ago, and maybe as long as 700 million years ago
• Because jellies have no bones, jellyfish fossils are rare

In the Food Web

• Jellyfish and ctenophores are carnivorous
• Most jellies primarily eat plankton, but larger ones may also eat crustaceans, fish, and other jellyfish and comb jellies
• Some jellyfish have symbiotic algae (zooxanthellae) in their tissues for photosynthesis
• Over 150 animal species are known to eat jellies
• Jellies are a favorite food of the ocean sunfish (Mola mola) and endangered leatherback turtle (Dermochelys coriacea)
• Humans also eat jellyfish

Feeding Adaptations

• Jellyfish use nematocysts to capture prey
• Comb jellies use colloblasts to capture prey
• Jellyfishes' nematocysts are organelles within special cells (cnidocytes) that contain venom-bearing harpoons
• The cell is activated upon touch or chemical cue
• Chironex fleckeri and Carukia barnesi are very toxic to humans
• Many comb jellies have colloblasts lining their tentacles, which release glue instead of venom
• Haeckelia rubra recycles nematocysts from hydrozoan jellyfish and uses these to stun and kill prey

Many Ctenophores, Many Ways to Feed

• Cydippids use branched tentacles lined with colloblasts to trap food
• Lobate ctenophores use special cilia between their lobes to generate a current that pulls planktonic food into their mouths; they also use colloblast-lined tentacles
• Beroids swallow their prey whole and clamp their mouths shut
• Inside their mouths they have small cilia that act as teeth

Defense Adaptations

• Many jellyfish and comb jellies are able to produce light, known as bioluminescence
• Jellies have adapted their body color to camouflage in the darkness
• Most are nearly colorless and transparent
• Some deep-sea jellyfish and comb jellies are a bright red or orange color

Reproduction & Lifecycle

• Jellyfish have a complex lifecycle: a single jellyfish reproduces both sexually and asexually during its lifetime, and takes on two different body forms
• An adult jellyfish is called a medusa, which reproduces sexually by spawning
• Fertilized eggs develop into planulae, which attach to a firm surface and transform into flower-like polyps
• Polyps reproduce asexually by budding
• Hydrozoan polyps bud medusae from their sides; cubozoan polyps each transform into a medusa
• In schyphozoans, strobilation takes place
• During strobilation, a polyp splits into 10-15 plate-like segments stacked atop one another in a tower called a strobila
• After a segment separates from the strobila, it is called an ephyra, a juvenile jellyfish
• Ephyrae mature into the medusa form
• Most jellyfish are short lived
• Turritopsis nutricula can revert back to the polyp stage after reaching adult medusa stage through a process called transdifferentiation

Comb Jellies

• Most species are hermaphroditic and able to release both eggs and sperm into the water
• Most species have both male and female gametes, it's thought that they can self-fertilize
• The embryo develops into a larva that looks just like a small adult ctenophore
• Mertensia ovum can reproduce even when it is still larva

Jellyfish Blooms

• Vast aggregations of jellyfish and comb jellies seem to be more common
• These aggregations are known as "jellyfish blooms" or "jellyfish outbreaks"
• The spread of jellies is human-caused
• Overfishing
  • Jellies have less competition for food
• Nutrients
  • Many jellies can better tolerate low-oxygen environments than other animals
• Climate change
  • The ocean is warming
  • Warmer water could help jelly embryos and larvae develop more quickly
• Submarine Sprawl
  • Industries build docks, oil platforms and other structures in the water
  • Jellyfish need solid surfaces to settle upon to undergo their polyp stage

Invasive Species & Fisheries

• Jellies are very good at surviving and thriving in new ecosystems once they arrive
• In the 1980s, the sea walnut (Mnemiopsis leidyi), a type of comb jelly, was brought to the Black Sea in ship ballast water
• It reproduced and spread quickly, gobbling up zooplankton and leaving little behind for the larvae of commercial fish species