Ctenophora: General Characteristics and Classification
General Characteristics of Ctenophora
1. Body Structure
Symmetry and Body Plan:
Ctenophores exhibit radial symmetry or biradial symmetry, where their body can be divided into similar halves through more than one plane. The body is usually elongated and oval or spherical.
Tissue Organization:
They are diploblastic meaning their body is composed of two primary cell layers: the ectoderm(outer layer) and the endoderm (inner layer). These layers are separated by a thick, jelly-like substance called mesoglea.
Mesoglea:
The mesoglea is non-cellular and provides the organism with structural support and buoyancy. It contains scattered cells that contribute to the jelly-like consistency of the body.
Bioluminescence:
Many ctenophores are bioluminescent, meaning they can produce light. This is primarily used for communication or to ward off predators.
2. Locomotion
Ctenes or Comb Rows:
Ctenophores are named for their unique locomotory structures known as ctenes. Ctenes are rows of cilia fused together into plates that look like combs. These comb rows are arranged in eight longitudinal rows along the body.
The coordinated beating of the cilia in these comb rows propels the ctenophore through the water, usually in a slow and graceful manner. This movement also creates the characteristic shimmering effect often seen in ctenophores.
3. Feeding and Digestion
Feeding Mechanism:
Ctenophores are primarily carnivorous, feeding on small planktonic organisms, including other small ctenophores, larvae, and fish eggs.
They possess specialized sticky cells called colloblasts on their tentacles (in species that have them) which help them capture prey by sticking to it.
Digestive System:
Ctenophores have a simple digestive system with a mouth, pharynx, stomach, and a series of branching canals that distribute nutrients throughout the body. Waste is expelled through two small anal pores.
4. Nervous System
Simple Nerve Net:
Ctenophores lack a central nervous system. Instead, they have a diffuse nerve net that helps coordinate their movements and responses to the environment.
Apical Organ:
They possess a sensory structure known as the apical organ located at one end of the body. This organ contains statoliths (small calcareous particles) that help the organism maintain balance and orientation in the water.
5. Reproduction
Hermaphroditism:
Most ctenophores are hermaphroditic, meaning that each individual possesses both male and female reproductive organs.
Fertilization:
Fertilization typically occurs externally, where the ctenophores release eggs and sperm into the water. In some species, self-fertilization is possible.
Development:
After fertilization, the development of the larvae occurs rapidly. The larval form, known as a cydippid larva, resembles a small adult and undergoes direct development, meaning it gradually transforms into an adult without a distinct metamorphic stage.
6. Ecological Role
Predators and Prey:
Ctenophores play a crucial role in marine ecosystems as both predators and prey. They help regulate plankton populations and are a food source for larger marine animals.
Invasive Species:
Some ctenophore species, like Mnemiopsis leidyi, have become invasive in certain ecosystems (e.g., the Black Sea) and have caused significant ecological disruptions by outcompeting native species and altering food webs.
Classification of Ctenophora
Kingdom: Animalia
Phylum:Ctenophora
The phylum Ctenophora is divided into two main classes based on morphological and anatomical differences:
1. Class Tentaculata
Characteristics:
Members of this class possess tentacles, usually two, which can be retracted. These tentacles are used for capturing prey and are equipped with colloblasts, the adhesive cells unique to ctenophores.
Examples:
Pleurobrachia (commonly known as the sea gooseberry) is a well-known example. These species have a spherical body with long, retractable tentacles.
Habitats: Species in this class are found in a wide range of marine environments, from coastal waters to deep seas.
2. Class Nuda
Characteristics:
Members of this class lack tentacles altogether. They have a more streamlined and simplified body structure compared to Tentaculata.
Feeding:
Ctenophores in this class are often predatory and feed primarily on other ctenophores. They engulf prey whole using their large mouth.
Examples:
Beroe is a prominent genus within this class. These species are known for their absence of tentacles and their voracious appetite for other ctenophores.
Summary
Ctenophora, commonly known as comb jellies, are unique marine invertebrates characterized by their radially symmetrical body, bioluminescence, and the presence of ciliary combs used for locomotion. The phylum is divided into two main classes: Tentaculata, which possesses tentacles, and Nuda, which lacks tentacles. Ctenophores play a significant role in marine ecosystems and offer valuable insights into early animal evolution.
