Lecture 6

Phylum Ctenophora

Definition and Classification: Phylum Ctenophora, commonly referred to as comb jellies or sea gooseberries.
Phonetic Note: The "C" in Ctenophora is silent. Correct pronunciation: Teenophora.
Features:
- Ctenophores are visually striking and can be large enough to be seen with the naked eye, often observed in marine environments.
- Part of the planktonic ecosystem, often encountered during snorkeling or scuba diving.

Morphology:
- Known as comb bearers due to their distinct comb rows comprised of cilia (referred to as tines or combs).
- Each ctenophore typically has eight rows of cilia (tines).
Bilateral Symmetry:
- Ctenophores exhibit both radial and bilateral symmetry, termed biradial symmetry.
- This symmetry means that they have a mouth (oral side) and an opposite side (aboral side) but lack clear anterior or posterior ends.
- They do not possess defined dorsal or ventral areas.
Unique Cells - Coloblasts:
- Ctenophores have specialized adhesive cells called coloblasts, used for capturing prey.
- Coloblasts contain adhesive granules that can extend and ensnare prey, referred to as lasso cells due to their shape and function.

Exclusively Marine: Ctenophores are found only in marine environments; none inhabit freshwater.
Species Count: Approximately 150 known species exist.

Absence of Nematocysts: Unlike cnidarians which possess nematocysts, ctenophores generally do not have them.
Kleptocnidism: Some ctenophores can acquire nematocysts from cnidarians they consume, incorporating them into their own tissues for defensive and feeding purposes.

Cilia Movement: Ctenophores swim using their cilia (tines) to push water around their bodies.
Directional Swimming: They swim with their mouths facing forward; contrasting with jellyfish that swim bell-first.
Feeding Mechanism: Tentacles are primarily used for feeding rather than for propulsion in swimming.

Coloration: Most ctenophores are transparent with colors resulting from light refraction off cilia.
Bioluminescence: Many ctenophores can exhibit bioluminescence, mainly in blue and green hues, facilitated by specific proteins in specialized cells called photocytes.
Difference between Bioluminescence and Fluorescence:
- Bioluminescence: Emission of light produced by living organisms without heat.
- Fluorescence: Absorption of light from an external source and re-emission.

Predation Strategy: Ctenophores are voracious predators; they rely on retractable tentacles to capture prey using the sticky coloblasts on their filaments.
Mouth Configuration: They utilize their tentacles to bring captured prey to their mouths, which is located at the aboral end of the body.
Genus Examples:
- Mnemiopsis: Lacks long tentacles and uses muscular mouth lobes to engulf prey.
- Beroe: Engulfs prey whole, often competing with planktonic fish larvae in their ecosystems.

Invasive Species: Mnemiopsis was introduced to the Black Sea through ballast water; its overpopulation depleted local fish stocks by preying on their larvae without natural predators.
Population Control: An invasion by another ctenophore, Beroe, eventually helped control the Mnemiopsis population, stabilizing the local ecology.

Tentaculate Forms: Most ctenophores are free-swimming with prominent tentacles, important for feeding.
Lobate Forms: Such as Mnemiopsis, which have discernible lobes rather than tentacles.
Thimble-Shaped Forms: Example includes Beroe, which resembles a thimble and can swim effectively.
Ribbon Forms: The Venus girdle, exceptionally long and notable for being the largest creature swim using only cilia.
Creeping Soul Forms: Resemble flatworms and are primarily benthic or sessile.

Nerve Net: Similar to cnidarians, ctenophores exhibit a nerve net that is concentrated below the comb plates.
Statocyst Function: Ctenophores possess statocysts situated aborally, which help maintain balance through a particle called a statolith.

Asexual Reproduction: Capable of remarkable regeneration; can fully regenerate body parts.
Sexual Reproduction: Predominantly monoecious (hermaphroditic), releasing sperm and eggs externally.
Larval Development: Free-swimming larvae resemble the adult tentaculate form; some species exhibit internal fertilization via brooding.

Tissue Level Organization: Both have tissue-level organization.
Symmetry: Ctenophores exhibit biradial symmetry, while some cnidarians maintain radial symmetry.
Body Cavities: Neither possesses true body cavities.
Development: Both exhibit diploblastic development.
Segmentation: Neither displays segmentation.

Introduction to Bilaterians: Following the discussion of ctenophores, attention is directed to bilaterians, a vast and diverse group.
Importance of Direction: Bilaterians demonstrate directional movement, advantageous for foraging, mating, and avoiding predation.
Cephalization: The evolutionary trend toward grouping sensory organs and developing a brain at the anterior end, furthering capabilities like communication and sensory perception.
Body Cavities: Presence of mesoderm permits development of true coelomic cavities that help organize organs and systems for efficient bodily function.
Types of Body Cavities:
- Acoelomate: No true body cavity, entirely filled with mesoderm.
- Pseudocoelomate: Body cavity partially lined with mesoderm.
- Coelomate: True body cavity lined with mesodermal tissue.

Basal Groups: Discussion includes Xenacoelomorpha and Platyhelminthes (flatworms).
Xenacoelomorpha Characteristics:
- Basic early bilaterians lacking complex structures like an anus or circulatory systems.
- Newly classified phylum derived from recent findings and analysis.
Examples: Includes the purple sock worm and mint sauce worms, exemplifying basic body plans and relationships with photosynthetic organisms.