Ctenophores

Ctenophore Grade of Construction

Having tissues with two distinct layers (endoderm and ectoderm), and a middle mesoglea derived from the ectoderm; not homologus to triploblast mesoderm.

Colloblasts

Adhesive cells found on the two tentacles of a ctenophore, used to attach to prey.

Ctenes (Comb Plates)

Eight longitudinal bands of cilia on a ctenophore's surface that are used for locomotion by beating for propulsion.

Apical Sense Organ

A structure at the aboral end of a ctenophore that includes a statocyst for sensing orientation and balance.

Ctenophore Gut

A complete gastrovascular cavity with a mouth, two small anal pores, and interconnected subcanals.

Asexual Reproduction in Ctenophores

The ability to regenerate any part of their body and reproduce through budding or fission.

Sexual Reproduction in Ctenophores

They are simultaneous hermaphrodites that release gametes through their mouth. Their eggs are centrolecithal and develop into free-swimming cydippid larvae after fertilization.

Colloblasts

Single cells on the tentacles covered in adhesive granules that burst upon contact with prey, discharging a 'marine glue' to capture it.

Feeding Process in Cydippid Ctenophores

Capture: Most planktonic adults use two long, contractile tentacles that emanate from a tentacle pouch. The tentacles are covered in colloblasts. When in contact with prey, the colloblasts burst and discharge marine glue to attach to the prey.

Ingestion: Once the ctenophore has lassoed the prey, the tentacles are periodically wiped across the mouth.

Digestion: Begins in the muscular pharynx, which produces digestive juices and circulates food with ciliary action. Mostly digested food then passes from the infundibulum to a series of radiating canals. These branched gastrovascular canals efficiently distribute nutrients and serve as a means for waste removal.

Waste Removal: They use their two small anal pores and diffusion across body surfaces for excretion.

Gastrovascular Canals

A system of branched canals that distributes nutrients and removes waste, compensating for the lack of a circulatory system.

Ctenophore Phylogenetic Placement

Evidence suggests that they should be considered the outgroups to all other animals.

Genome of Mnemiopsis leidyi

The sequencing of this ctenophore's genome elucidated more information, suggesting their placement as an animal outgroup.

Ctenophore Mesoderm

It is considered analogous, as the genes responsible for it do not appear to be homologous to those of bilaterians.

Evolution of Simplicity in Ctenophores

Fossils from 500 mya suggest that ctenophores used to have more complex nervous systems.

Ctenophore Ancestry

Fossils from 520 mya suggest that ctenophores evolved from a polyp type form, indicating a possible link with cnidarians.

Ctenophore Symmetry

A body plan that displays something like biradial symmetry but is better explained as rotational.

Ciliary Beat Control

The balancers control this action; two comb rows innervated by a single balancer beat synchronously, with the power stroke normally directed toward the aboral pole.

Ctenophore Development

Cleavage is neither spiral nor radial, and development is more determinant than in cnidarians. Fertilized eggs become free-swimming cydippid larvae that undergo gradual development into adults.

Colloblast Function

When in contact with prey, these cells burst and discharge an adhesive material, or 'marine glue', from a spiral filament that straightens and adheres to the prey.

Nutrient Distribution in Ctenophores

Lacking a circulatory system, they use a network of branched gastrovascular canals to efficiently distribute nutrients to distant cells in the mesoglea.

Cell Rosettes

Structures that surround small pores leading into the mesenchyme, likely functioning to regulate the flow between the gastrovascular canals and the mesenchyme.

Ctenophore Support

The body shape is maintained in the water column by looped and radial muscles.

Analogous Mesoderm in Ctenophores

The idea that the genes responsible for the ctenophore mesoderm are not homologous to those of bilaterians.

Genetic Consequence of Ctenophore Evolution

The idea that an evolutionary transition, like the one proposed for ctenophores, would result in the loss of many genes.

Tentacle Pouch

A structure from which the long, contractile tentacles of a ctenophore emanate and can be retracted into.

Tenillae

The lateral branches found on the tentacles of a ctenophore.

Ctenophore Excretion

The process of waste removal which occurs via diffusion across the body surfaces.

Ctenophore Nerve Net

A unique, diffuse nervous system with no ganglia, where neurons are concentrated in structures beneath the comb rows and around the mouth.

Gonad Location

In ctenophores, the gonads are located on the walls of the meridional canals.

Polyspermy

A characteristic of ctenophore sexual reproduction where an egg can be fertilized by multiple sperm.

Colloblast Structure

A single cell with a spiral filament wrapped around a straight filament, which is a modified nucleus of the cell.

Cydippid Life Cycle:

Ctenophores are simultaneous hermaphrodites (both male and female gonads) with indirect development.

Gamete Release: Gonads are located on the walls of the meridional canals, and gametes (sperm and egg) are released via the mouth; self-fertilization may occur.

Development: Fertilized eggs develop into a free-swimming cydippid larvae.

Growth: The larva undergoes gradual development into the adult form.

Ctenophores external structures

Comb Plates (Ctenes): Eight longitudinal bands of cilia. The beating of these plates is used for locomotion and allows them to move up and down in the water column ("comb jellies").

Mouth: Located at the oral pole (the direction the animal usually swims).

Tentacles: Typically two, bearing lateral branches called tenillae. Used to capture prey and can be retracted into a tentacle pouch.

Anal Pores: Two small pores found at the aboral end

Internal structures

Gastrovascular Cavity: A complete gut. It is composed of a system of interconnected

subcanals.

Mesoglea: The middle layer, possibly derived from the ectoderm (diploblastic?). Contains muscles for contractile function and maintaining body shape.

Nerve Net: A unique, diffuse nervous system with no ganglia. Neurons are concentrated beneath the comb rows and around the mouth.

Apical Sense Organ: Located at the aboral end. It includes a statocyst for sensing orientation and balance.

Explain the challenge in determining the relationship of ctenophores to other animals groups and the major evidence for placing it within eumetazoa and that for placing it as an animal outgroup.

outgroups to all other animals (meaning they branched off first).

Unique Genes: Genome sequencing of Mnemiopsis leidyi provided new information. The genes responsible for their middle mesoglea layer do

not appear to be homologous to the mesoderm found in bilaterians. This suggests an independent evolution of complexity.

Evidence Linking Ctenophores to Eumetazoa (or an Evolutionary Transition):

Fossil Records: Fossils from 520 million years ago suggest that ctenophores may have evolved from a polyp-type form, hinting at a possible link with cnidarians.

Evolution of Simplicity/Gene Loss: Older fossils (500 mya) suggest ctenophores used to have more complex nervous systems. This indicates that their current simple structure and nervous system might be a result of

evolutionary simplicity (gene loss) rather than being primitively simple