Ch. 7

Question 1

main features of ctenophores are:

Answer 1

1. Fused Cilia in Rows (Comb Plates):
   Ctenophores have plates of fused cilia arranged in rows, which they use for locomotion.
2. Colloblasts: They possess adhesive cells called colloblasts, which are used to capture prey.
3. Predatory Nature: Nearly all ctenophores are predators, feeding on small zooplankton and fish larvae.
4. Mostly Transparent: Most ctenophores are nearly transparent, helping them avoid predators and blend in with the environment.
5. Marine and Planktonic: Ctenophores are exclusively marine and primarily planktonic, drifting with ocean currents, though some can swim weakly.
6. Body Structure: Their bodies resemble those of cnidarian medusae, with a
   gelatinous middle layer (mesoglea), an outer epidermis, and an inner gastrodermis. They have radial symmetry, though more specifically biradial symmetry.
7. Gastrovascular System: The digestive system is similar to cnidarians, with a pharynx leading to a stomach and then to gastrovascular canals for intracellular digestion. 8. Lack of Specialized Excretory and Respiratory Systems: There are no specialized organs for excretion or respiration.
8. 1
   Nervous System: Ctenophores have a subepidermal nerve network, similar to cnidarians.
9. Musculature: Their muscles develop from amoeboid cells within the mesoglea, with muscle fibers residing in this gelatinous layer.
10. Embryological Development: Ctenophores may be diploblastic (like cnidarians) or potentially triploblastic, depending on the development of their musculature. They may have a planula larval stage in their life cycle.
11. Nematocysts (In One Species): Some ctenophores, like Mnemiopsis leidyi, can ingest cnidarians (jellyfish) and use their nematocysts for prey capture, though they don’t produce these cells themselves.

Question 2

supports that ctenophores are triploblastic

Answer 2

1. Musculature: The musculature of ctenophores is made up of smooth muscle tissue, distinct from the myoepithelial cells found in cnidarians. Ctenophores lack the myoepithelial cells typical of cnidarians ano instead possess true smooth muscle fibers.
   These muscle fibers are located within the mesoglea (the middle layer of their body).
   The presence of smooth muscle tissue and the absence of myoepithelial cells in ctenophores suggest a more complex, triploblastic organization (where the three germ layers-ectoderm, mesoderm, and endoderm-are sent) compared to cnidarians, which are typically diploblastic (only having ectoderm and endoderm).
2. Muscle Fibers: The isolation of giant smooth muscle fibers from ctenophores such as Mnemiopsis leidyi and Beroë sp. further reinforces the idea that ctenophores have true, developed musculature. These muscle fibers are an important feature that differentiates them from cnidarians, whose musculature is much less specialized.
3. Ciliary Locomotion: While the musculature in ctenophores plays a minor role in locomotion, the ctenes (fused cilia) are the main organs of movement. The coordination of these ciliary movements is controlled by a single apical se organ. This differs from cnidarians, where movement is typically achieved via jet propulsion. The complex coordination of ciliary movement, combined with smooth muscle fibers and a central
   nervous system coordinating these movements, suggests a higher level of organization and supports the argument for ctenophores being triploblastic.
4. Balance and Orientation: Ctenophores also use a statolith and balancer system to control their orientation in the water. This
   sophisticated sensory mechanism involves nerve impulse conduction from the apical sense organ to the comb rows (ctenes), showing that ctenophores have a more developed nervous system compared to cnidarians, which further supports their triploblastic nature.

_{In contrast to cnidarians, whose musculature is mostly confined to the epidermis and gastrodermis, ctenophores exhibit a more complex, mesodermal musculature and an advanced nervous system, indicating their triploblastic status.}

Question 3

several important insights into ctenophores, supporting the complexity of their biological features.

Answer 3

1. Nervous Coordination and Epidermal
   Nerve Net: The epidermal nerve net in ctenophores coordinates the activity of the comb rows (ctenes). This network plays a role in responses such as the reversal of ciliary beats when mechanical stimulation is applied, even when the apical sense organ is removed. This suggests a relatively sophisticated nervous coordination, even though the nervous system is not as centralized as in higher organisms.
2. Tentacles and Prey Capture: Unlike cnidarians, cter hores use colloblasts (sticky cells) instead of nematocysts (stinging cells) for prey capture. These colloblasts have a bulbous, sticky head and a long, contractile filament, which trap prey that is then retracted into pits or sheaths. This method is unique to ctenophores and highlights their distinct approach to feeding
3. Locomotion and Ciliary Movement:
   Ctenophores rely on ctenes (fused cilia) for swimming, with eight rows of ciliary bands that beat in a coordinated fashion to propel the animal through the water. This ciliary-based locomotion contrasts with the jet propulsion seen in cnidarian medusae. The apical sense organ coordinates the ciliary movements, ensuring proper orientation and swimming behavior.
4. Digestive System: Unlike cnidarians, which have a single opening for both ingestion and egestion, ctenophores have a gastrovascular system with four canals leading from the stomach to the aboral surface, with two canals opening to the outside for waste expulsion. This more complex digestive structure further supports their classification as a more evolved organism compared to cnidarians.
5. Reproduction: Most ctenophores are simultaneous hermaphrodites, possessing both male and female gonads. They generally release gametes into the digestive tract, which are then expelled through the mouth. Some species fertilize externally, but a few platyctene species fertilize internally. This reproductiva strategy adds another layer of comple…y compared to cnidarians.
6. Development: Ctenophores exhibit determinate cleavage, where cell fates are fixed early in development, which is different from cnidarians, where cell fates are determined later. Ctenophores also have a unique mechanism of gastrulation (formation of germ layers), either by epiboly (a sheet of micromeres spreading over macromeres) or invagination (cells pushing into the blastocoel), both of which are different from the processes seen in cnidarians. Ctenophore embryos also typically develop directly into a cydippid (a miniature adult form), rather than a ciliated planula larva as in cnidarians.
7. Bioluminescence: Nearly all ctenophores are bioluminescent, meaning they can produce light through a chemical reaction. This bioluminescence is distinct from the iridescence created by diffraction and serves various ecological purposes, including camouflage and communication.

Question 4

Bioluminescence:

Answer 4

Bioluminescence: Ctenophores exhibit bioluminescence, where light is produced through a chemical reaction instead of heat.

This is a characteristic feature of many animal phyla, not just ctenophores.

The precise role of bioluminescence in ctenophores is not well understood, but it could serve various functions such as mate location, prey attraction, predator deterrence, or camouflage.

For example, ctenophores may produce light to blend into the surrounding environment and avoid visual predators, a strategy similar to counterillumination used by other bioluminescent organisms.

Despite its beauty, mate recognition via bioluminescence in ctenophores seems
unlikely due to the absence of photoreceptors and the limitations of their nervous system to process such complex visual signals.

Question 5

Iridescence and Graceful Movement:

Answer 5

In addition to bioluminescence, ctenophores are known for their iridescent comb rows (ctenes), which reflect light in a mesmerizing way.

These features, combined with their delicate bodies and graceful swimming, contribute to their reputation as some of the most beautiful marine organisms to observe.

Question 6

Iridescence and Graceful Movement:

Answer 6

In addition to bioluminescence, ctenophores are known for their iridescent comb rows (ctenes), which reflect light in a mesmerizing way.

These features, combined with their delicate bodies and graceful swimming, contribute to their reputation as some of the most beautiful marine organisms to observe.

Question 7

Diversity and Discovery:

Answer 7

Most ctenophore species live in the open ocean, away from coastlines, making them difficult to study.

Traditional collection methods, such as towing nets, were often too harsh for these fragile animals.

However, with the advent of more gentle collection techniques, such as diving and submersible exploration, scientists have discovered many new species that were previously unknown.

In a recent two-week expedition, up to a dozen new species were identified.

Question 8

Classification:

Answer 8

Ctenophores are currently divided into two main classes based on the presence of tentacles in adults and/or cydippid stages.

These classes help categorize species based on their developmental / morphological characteristics, including the presence of tentacles in adults and/or cydippid stages.

including the presence of tentacles and the structure of their body.

Question 9

Class Tentaculata

Answer 9

1. Cydippida (Cydippid Form):
   These species resemble the cydippid larvae described earlier but are fully developed adults with functional gonads. They have long, retractable tentacles, which are used exclusively for capturing prey. These ctenophores are typically more spherical or ellipsoidal, and their tentacles are well-developed, reflecting the ancestral or more primitive form of the class.
2. Lobata (Lobate Ctenophores):
   In these species the body is laterally compressed, and they have only four fully developed comb rows. The tentacles are much reduced in length. Instead, the primary food collection surfaces are the oral lobes, which are covered with mucus and colloblasts (sticky cells). In some species, muscular activity of the oral lobes helps with locomotion, and two pairs of auricles (tentacle-like structures) assist in prey capture. The increase in body surface area and the redistribution of colloblasts reflect an adaptation for more efficient food collection.
3. Cestida (Ribbon-like Ctenophores):
   These ctenophores have a highly compressed, rihhon-like body with the mouth and apicar sense organ at opposite sides. Four comb rows are fully developed, and they rely on a combination of ciliary activity and muscular, sinuous movements for locomotion. Numerous short tentacles line the oral edge of the ctenophore and are used for capturing prey, despite the large surface area of the body itself. This
   adaptation indicates a further specialization of the body structure for prey capture.
4. Platyctenida (Flat Ctenophores):
   The body of these ctenophores is flattened, with the oral and aboral surfaces pressed together to form a plate-like structure. The pharynx is permanently everted and forms much of the underside of the plate. Some species are non-lanktonic, meaning they creep slowly over solid surfaces, while others float in the water. Locomotion is a mix of cilial action on the pharynx and muscular contractions. The tentacles in adults are typically long and assist in food capture, but many species lack comb rows altogether.

Question 10

Evolutionary Trends:

Answer 10

• Lateral body compression is a recurring theme, especially in the Lobata and Cestida, where the body shape has evolved to optimize food collection. For Lobata, this involves increased surface area for the distribution of colloblasts, while for Cestida, it involves a redistribution of tentacles along the body.

• Tentacle modifications are evident across these groups: in the Cydippida, long tentacles are present; in Lobata, tentacles are reduced, and food collection is aided by oral lobes; in Cestida, short tentacles are distributed along the oral edge; and in Platyctenida, tentacles are still long, but the animals also use the body’s flatness and pharyngeal cilia for movement.

These adaptations reflect how the tentaculate ctenophores have evolved to maximize food collection and movement in diverse environments.

While the Cydippida represents a more primitive form, the other orders show increasing specialization in both body shape and function for effective predation.

Question 11

Class Nuda

Answer 11

The Class Nuda is a distinct group of ctenophores, represented by the single order Beroida.

This class is characterized by the absence of tentacles and oral lobes, which sets it apart from other ctenophores in the class Tentaculata.

The key features of Nuda are:

1. No Tentacles or Oral Lobes:
   Unlike most ctenophores, members of the Nuda class do not possess tentacles at any stage of development, including the cydippid stage. Additionally, they lack the oral lobes seen in some other groups (like the Lobata).
2. Well-developed Comb Rows:
   All eight comb rows are fully developed and are used for locomotion, just like other ctenophores. These comb rows are crucial for their movement through the water.
3. Muscular Lips for Prey Capture: Instead of tentacles, Nuda species use muscular lips surrounding their mouths to
   capture prey. These lips can widen significantly to ingest prey that may be much larger than the predator itself. This adaptation is key for feeding on larger organisms.
4. Macrocilia: Inside the mouth, macrocilia (specialized cilia) are present. These are large cilia that consist of thousands of axonemes (the structural cores of cilia) and are used as teeth to chop and break down prey. This feature is particularly useful for consuming prey that is larger or more difficult to swallow whole. The macrocilia allow the ctenophore to process prey to bite-sized pieces.