Ch. 20 Flashcards
(69 cards)
1
Q
Phylum Echinodermata
Defining characteristics
A
1) A complex series of fluid-filled canals (the water vascular system) derived from a pair of colomic compartments and which service numerous flexible feeding and locomotory appendages (tube feet);
2) 5-pointed (pentamerous) radial symmetry in adults;
3) calcareous ossicles derived from mesodermal tissue forming an endoskeleton; (The
skeleton is made of calcium carbonate and is derived from mesodermal tissue.)
4) connective tissue is mutable: Its stiffness and fluidity can be rapidly and dramatically altered by the nervous system
2
Q
Phylum Echinodermata
General Features of Echinoderms
A
• Includes sea lilies, feather stars, brittle stars, sea stars, sand dollars, sea urchins, sea biscuits, and sea cucumbers.
• Echinoderms are deuterostomes, like vertebrates, and share a close evolutionary relationship with hemichordates.
• The purple sea urchin genome (Strongylocentrotus purpuratus) was the first sequenced genome of a nonchordate deuterostome, revealing genetic similarities with vertebrates.
• Most of the 6,500 living species are marine; none inhabit freshwater.
• The fossil record contains around 13,000 additional species, many belonging to extinct classes.
3
Q
Phylum Echinodermata
Skeletal and Structural Adaptations
A
• The endoskeleton contains calcium carbonate (up to 95%), magnesium carbonate (up to 15%), trace metals, and organic material.
• Unlike mollusks, whose shells form from mineral deposits in an extracellular protein matrix, echinoderm skeletons are formed within specialized mesodermal cells.
4
Q
Phylum Echinodermata
Water Vascular System (WVS) and
Tube Feet
A
• The WVS is a system of canals originating from the hydrocoel in embryos.
• The podia (tube feet) extend through the body wall and skeleton in areas called ambulacral zones or ambulacral grooves.
• The WVS connects to seawater through a sieve plate (madreporite).
- Stone Canal: Fluid flows down a reinforced stone canal, connected to a ring canal that encircles the esophagus in most echinoderms.
- Polian Vesicles & Tiedemann’s Bodies: These are accessory fluid-storage structures that help maintain body turgor and filter fluid from the WVS into the perivisceral coelom.
- Radial Canals: Five or more radial canals extend from the ring canal, with bulb-shaped ampullae connected to each radial canal.
- Tube Feet Function: Tube feet extend hydraulically when fluid is pumped into them from the ampullae. A one-way valve ensures fluid flows only into the tube foot during contraction, and retracts when longitudinal muscles contract.
- Coordination and Locomotion: Echinoderms can have over 2,000 tube feet that work in a coordinated manner for locomotion, though the mechanisms behind this coordination
remain poorly understood. - Adhesion Mechanism: Tube feet attach to surfaces through ionic interactions, suction, and a duo-gland adhesive system (one gland secretes adhesive, while another releases a chemical to break the bond).
5
Q
Phylum Echinodermata
Tube Foot Functions
A
- Gas Exchange and Circulation: The inner surface of the tube feet is ciliated, facilitating fluid circulation and functioning in gas exchange.
- Excretion & Chemoreception: Tube feet also serve as excretory organs by simple diffusion, and possibly function in chemoreception and food collection.
- Vision: Some tube feet have genes related to vision, indicating a role in light perception and possibly vision.
6
Q
Phylum Echinodermata
Excretory and Circulatory Systems
A
- Excretion: Adult echinoderms lack specialized excretory organs, though larvae possess a cilia-driven nephridial system.
- Absence of Heart: Echinoderms do not have a true heart.
- Hemal System: The hemal system consists of a spongy axial organ, which lies next to the stone canal, and two hemal rings (oral and aboral).
- Hemal System Function: Although its exact function is unclear, it is believed to transport nutrients from the coelomic fluid to the gonads. The hemal system contains a high concentration of nutrients, suggesting it plays a role in nutrient distribution.
7
Q
Phylum Echinodermata
Axial Organ and Coelomocytes
A
- Axial Organ: The axial organ in asteroids (sea stars) and echinoids (sea urchins) may have an excretory function, though this is not yet experimentally confirmed.
- Coelomocytes: These specialized cells are found in most echinoderm tissues and fluids, including coelomic fluid. Their roles include:
• Recognizing and phagocytosing foreign material, such as bacteria.
• Synthesizing pigments and collagen for connective tissue.
• Transporting oxygen and nutrients (some contain hemoglobin).
• Digesting food particles.
• Playing a role in wound repair
8
Q
Phylum Echinodermata
Regeneration and Mutable Connective
Tissue
A
- Regenerative Capabilities: Echinoderms have remarkable regenerative abilities, capable of regrowing lost body parts or organs.
- Mutable Connective Tissue (Catch Tissue):
• A unique characteristic of echinoderms, catch tissue can rapidly change its stiffness and fluidity.
Nerve impulses can cause the tissue to go from rock-hard to almost liquid in a fraction of a second, and back to stiff again.
• This change is essential for feeding, locomotion, autotomy (shedding limbs or viscera as a defense mechanism against predators)
9
Q
Phylum Echinodermata
Toxicity and Venom
A
• Toxic or Venomous Species: At least 85 echinoderm species are known to be toxic or venomous, though few are deadly to humans.
10
Q
Phylum Echinodermata
Classification and Evolution
A
- Major Groups: There are five main groups of echinoderms, but their exact evolutionary relationships are still debated.
- Crinoidea: This class (sea lilies and feather stars) is the oldest among the extant echinoderm groups.
11
Q
Class Crinoidea
Defining characteristics
A
The main part of the body is supported above the substrate either by a long stalk or by a series of grasping claws (cirri)
12
Q
Class Crinoidea
Evolutionary Significance
A
- Fossil Record: Crinoids have a fossil record dating back nearly 600 million years, showing many “primitive” characteristics (not implying simplicity, but rather limited evolutionary change).
- Ancient Success: Crinoids were more abundant in the past and dominate the fossilized echinoderm record.
- Modern Diversity: The most diverse crinoid
species are found on the Great Barrier Reef, where over 50 species coexist in some areas.
13
Q
Class Crinoidea
Types of Crinoids
A
- Stalked Crinoids (Sea Lilies): About 100 species exist today, mostly in deep waters, and they remain permanently attached to the substrate via a stalk.
- Non-Stalked, Motile Crinoids (Feather Stars): Around 600 species are living, and they are free-moving.
14
Q
Class Crinoidea
Anatomical Features
A
- Stalk Structure: The stalk is flexible and composed of stacked calcareous discs (columnals) held together by connective tissue.
- Feeding and Reproductive Area: The top of the stalk contains the calyx, a cup-shaped structure that holds the digestive system. The tegmen, a lid-like membrane, covers the calyx and bears the mouth.
- Oral Surface: Unique to crinoids, the oral surface is on the upper half of the body, an adaptation for suspension feeding.
- Arms and Tube Feet:
• Arms: Crinoids have 5 to 200 arms that extend from the calyx. These arms are jointed with calcareous ossicles, allowing movement and flexibility.
• Pinnules: Two rows of tubular pinnules extend from each arm and bear tube feet.
• Tube Feet: These are used for food collection (not locomotion), gas
exchange, and possibly waste elimination by diffusion.
15
Q
Class Crinoidea
Feeding Mechanism
A
- Suspension Feeding: Crinoids collect food by extending arms, pinnules, and tube feet into the water current.
- Food Capture: Mucus-secreting glands on the tube feet help entangle food particles, which are then moved into the ambulacral grooves bv cilia and transported to the mouth.
This combination of structure and function allows crinoids to efficiently filter food from water.
16
Q
Class Crinoidea
Reorientation and Autotomy in Sea
Lilies and Crinoids
A
- Reorientation: Sea lilies can quickly reorient their bodies for efficient food capture by altering the stiffness of the connective tissue that holds the stalk’s columnal discs together.
This reorientation happens rapidly, enabling adaptation to changes in current speed and direction. - Autotomy (Arm Loss): Crinoids can autotomize (shed) one or more arms when attacked by predators. This is achieved by liquefying the connective tissue between the arms, and the transition between solid and liquid states in the tissue happens in less than one second, under nervous control.
17
Q
Class Crinoidea
Water Vascular System (WVS) and
Tube Feet
A
- Absence of Ampullae: Crinoids do not have ampullae associated with their tube feet, which distinguishes them from most other echinoderms. Instead, they protract their podia (tube feet) by contracting muscles in the radial canals.
- No Madreporite: Crinoids lack a madreporite, although numerous stone canals open into the coelom. The WVS opens to the outside through many ciliated tubes that penetrate the tegmen (the covering membrane).
18
Q
Class Crinoidea
Feather Stars (Comatulids)
A
- Body Structure: Feather stars, or comatulids, resemble sea lilies from the calyx upwards but have cirri (jointed, flexible appendages) near the base instead of a long stalk. These cirri are used to grasp solid substrates during resting and feeding.
- Feeding: Comatulids extend their feeding appendages into faster-moving water to increase food capture, similar to sea lilies.
Food collection occurs in the same way as in stalked crinoids, using arms, pinnules, and tube feet to trap particles.
19
Q
Class Crinoidea
Locomotion in Feather Stars
A
- Cirri Movement: Feather stars move by “snowshoeing” atop soft sediments with their cirri or by swimming short distances using forceful downward movements of the arms.
- Swimming Mechanism: Swimming involves a coordinated series of arm movements, where
one group of arms beats downward while another group recovers. This enables the comatulids to escape or avoid predators, a mobility advantage over stalked crinoids.
These features highlight the adaptive capabilities and unique characteristics of crinoids and feather stars in response to their environment and feeding strategies.
20
Q
Class Stelleroidea
Defining characteristics
A
Arms (generally 5, or a multiple of 5) extend from a central disc
21
Q
Class Stelleroidea
General Characteristics
A
- Arms: Stelleroidea generally have 5 arms (or multiples of 5) that radiate from the central body.
- Lack of Stalks: Unlike crinoids (which have stalks), members of Stelleroidea, including brittle stars and sea stars, lack stalks and have their arms arranged star-like around a flattened body.
22
Q
Class Stelleroidea
Taxonomic Classification
A
- Brittle Stars and Sea Stars: The class
Stelleroidea includes brittle stars (ophiuroids) and sea stars (asteroids). - Evolutionary Relationship: Despite differences in morphology, fossil evidence indicates a close evolutionary relationship between brittle stars and sea stars, justifying their grouping into the same class.
- Mitochondrial DNA Evidence: Both classes share a peculiar arrangement of mitochondrial DNA, including a notable multigene inversion, further supporting their evolutionary link.
Subclasses
• Brittle stars and sea stars are placed into separate subclasses within the class Stelleroidea.
This text highlights the shared evolutionary traits and classification structure of brittle stars and
sea stars within the broader context of echinoderms.
23
Q
Subclass Ophiuroidea (Brittle Stars):
Defining characteristics
A
1) Well-developed ossicles in the arms form a linear series of articulating
, “vertebrae,” joined together by connective tissue and muscles;
2) the oral surface bears five pairs of invaginations (bursal slits), which may serve for gas exchange and as brood chambers for developing embryos
24
Q
Subclass Ophiuroidea
General Characteristics
A
- Mobility: Ophiuroids are motile, unlike stalked crinoids. They are capable of moving with their flexible, jointed arms. Tube feet may also aid in movement, particularly in young or small species and those that burrow.
- Arm Structure: Typically, ophiuroids have five arms radiating symmetrically from a central disc. Some species, like basket stars, have arms that branch multiple times, resulting in a much larger span.
- Autotomy: A distinctive feature of ophiuroids is the ability to detach their arms when provoked (autotomy), often regenerating them over several months. This is mediated by mutable connective tissue.
25
Subclass Ophiuroidea
Body Surface Features
1. Protective Plates: The oral surface of the body disc is often covered by a thin layer of calcareous scales, while the aboral surface usually has protective calcareous plates (shields).
2. Light Sensitivity: In some species, the shields have a microscopic array of hills that act as lenses to focus light precisely on underlying nerve fibers, suggesting light sensitivity.
26
Subclass Ophiuroidea
Digestive System
1. Centralized Digestion: The digestive system is confined to the central disc, with a single opening for ingestion. Unlike other echinoderms, ophiuroids lack an anus.
27
Subclass Ophiuroidea
Water Vascular System (WVS)
1. No Ampullae: Similar to crinoids, ampullae are absent, and the tube feet function by muscular action in the radial canals.
2. Multiple Madreporites: Unlike most echinoderms, ophiuroids may have numerous madreporites, all opening on the oral surface.
28
Subclass Ophiuroidea
Bursal Slits (Bursae)
1. Function: The bursae are slit-shaped infoldings along the arm margins, adjacent to the arm shields, and extend into the coelomic space. They serve various roles:
• Gas exchange: Seawater circulates through bursae, likely for gas exchange.
• Reproduction: Bursae act as brood chambers for developing embryos in
many species
• Waste elimination: May aid in excreting waste, especially in species with muscular contractions driving water flow through the bursae.
29
Subclass Ophiuroidea
Feeding Behavior
1. Feeding Modes:
• Deposit Feeders: Many ophiuroids feed by ingesting sediment and assimilating the organic fraction.
• Capture Small Animals: Some ophiuroid species also capture and ingest small animals found within the sediment.
• Suspension Feeders: Other species filter food particles from the water.
• Carnivores/Scavengers: Some ophiuroids function as carnivores or scavengers.
2. Use of Mutable Connective Tissue:
• Ophiuroids can use their mutable connective tissue to stiffen their arms, allowing them to maintain a filtering posture for extended periods.
30
Subclass Ophiuroidea
Behavior and Habitat
1. Nocturnal Feeding:
• Ophiuroids typically hide under rocks or in crevices during the day, emerging to feed only at night.
2. Feeding Aggregations:
• Large feeding aggregations of ophiuroids, sometimes numbering in the thousands per square meter, have been reported in some shallow-water habitats.
3. Associations with Other Invertebrates:
Many species live in association with other invertebrates, especially sponges and sessile cnidarians.
31
Subclass Ophiuroidea
Coloration Changes
• A few tropical ophiuroid species exhibit diurnal alterations in body coloration, which are regulated by light-sensitive chromatophores (cells that change color).
32
Subclass Asteroidea (sea stars)
Defining characteristics
The gonads and portions of the digestive tract extend into each arm
33
Subclass Asteroidea
General Description
• Species Count: About 1,900 species of sea stars exist, making Asteroidea the second-largest group in the Echinodermata (after the Ophiuroidea).
• Size: Most sea stars are 15-25 cm in diameter, though some species can be much larger. Few are smaller than several centimeters in diameter.
• Arm and Body Structure: The arms of sea stars are not distinct from the central body disc, unlike brittle stars (ophiuroids). The arms are involved in locomotion, but the central disc plays a more significant role.
34
Subclass Asteroidea
Locomotion
• Slow Movement: Sea stars move slowly through coordinated movements of the tube feet.
• Tube Feet: Found along the oral surface of each arm in distinct ambulacral grooves.
• Each tube foot is operated by an ampulla and typically ends in a small suction cup.
• Tube feet extend through the contraction of the ampulla and move by the contraction of longitudinal muscles in the podium.
• Suction-cup locomotion is suited for movement on firm substrates; species
that move over soft substrates have tube feet that lack suction cups.
35
Subclass Asteroidea
Digestive System
• Mouth and Stomach: The mouth is directed downward and opens into a short esophagus, leading to a lower stomach (cardiac stomach), which is primarily responsible for digestion.
• Pyloric Stomach and Caeca:
• Above the cardiac stomach is the upper (pyloric) stomach, with branches
extending into the arms as pyloric caeca.
• Pyloric caeca are involved in the secretion of digestive enzymes, absorption of nutrients, and storage of assimilated food.
• The great surface area of the pyloric caeca aids in these functions.
36
Subclass Asteroidea
Feeding Behavior
• Prey: Asteroids typically prey on large invertebrates like sponges, gastropods,
polychaetes, bivalves, and other echinoderms. Some species also feed on small fish and coral polyps.
• Feeding Method:
• For large prey, the cardiac stomach is protruded out of the body through the mouth and placed in contact with the prey's soft tissues. This allows for external digestion, with the nutrient-rich fluid transferred to the pyloric stomach via ciliated channels.
• If the prey is small, the stomach remains retracted, and digestion proceeds internally.
• Small prey may also be ingested directly through the mouth, without the need to protrude the stomach.
37
Subclass Asteroidea
Autotomy (Arm Loss)
• Defense Mechanism: Many sea stars will autotomize or sever some of their arms when disturbed by predators. This serves as a form of escape, as it leaves the predator with a
"nutritious souvenir" while the sea star escapes.
• The response is chemically mediated, involving coelomic fluid that induces autotomy in other sea stars.
• This process is controlled by mutable connective tissue that liquifies rapidly.
• Lost arms are eventually regenerated.
38
Subclass Asteroidea
Calcareous Skeleton
• Structure: The sea star's skeleton consists of discrete ossicles (rods, crosses, and plates) embedded in connective tissue.
• Spines: Calcareous spines protrude from the ossicles, which can be moved by muscles.
• Cuticle: The ossicles and spines are covered by a thick cuticle secreted by the underlying ciliated epidermis.
39
Subclass Asteroidea
Additional Appendages
• Papulae: These are thin, non-calcified outfoldings of the body wall that serve a respiratory function. They protrude between ossicles and are connected to the main coelomic cavity.
• Pedicellariae: These appendages consist of two or three calcium carbonate ossicles that can move together or apart by muscles. They have multiple functions:
• Debris Removal: Pedicellariae remove unwanted organisms and debris from the surface of the sea star.
• Prey Capture: Some species use pedicellariae to capture small prey, including small fish.
• Occurrence: Pedicellariae are also found in Echinoidea (sea urchins and sand dollars).
40
The Concentricycloids
Defining characteristics
1) The water vascular system includes what appear to be 2 concentric water vascular rings;
2) the tube feet are arranged in a circular patter along the animal's periphery which distinguishes them from other echinoderms that have tube feet radiating from a central ring.
41
The Concentricycloids
Morphology
• Body Shape: The body is circular, flat, and lacks arms, givina it a resemblance to a jellyfish or a flower rather than typical echinoderms.
• Symmetry: Unlike most echinoderms, the sea daisies do not exhibit pentamerous radial symmetry.
• Size: These animals are small, less than 1 cm in diameter.
• Calcareous Endoskeleton: The body is supported by overlapping skeletal plates (ossicles) arranged in concentric rings, and their "petals" are calcareous spines.
42
The Concentricycloids
Reproductive and Biological Features
• Sperm Structure: The sperm have long, thin threads, and the nucleus and mitochondrion are drawn out with the acrosome divided into distinct segments, which is unique compared to other echinoderms.
• Feeding and Digestion:
• Some species lack a gut entirely and are presumed to feed on dissolved organic matter.
• Another species has a large mouth, a well-defined stomach, which may be eversible, resembling the digestive system of sea stars (asteroids), but it lacks an anus.
43
The Concentricycloids
Water Vascular System
• Double-ringed System: The water vascular system has two concentric rings connected by short radial canals, which is distinct from the single-ringed system seen in most other echinoderms.
44
The Concentricycloids
Classification and Evolution
• Initially classified under a separate class, Concentricycloidea, but later analyses suggest they may be a highly modified form of sea stars (Asteroidea, possibly reflecting the retention of a juvenile body plan into adulthood.
• Their evolutionary relationships to other echinoderms, particularly sea stars, are still unclear.
45
The Concentricycloids
Additional Characteristics
• The oral surface of one species is covered by a thin sheet of tissue called the velum, further enhancing the similarity to hydrozoan medusae (jellyfish-like appearance).
• Gas Exchange: No specialized gas exchange organs have been identified in these species.
46
The Concentricycloids
Habitat
• Found at great depths (over 1,000 meters), typically associated with submerged wood.
47
The Concentricycloids
Summary
• The Concentricycloids (sea daisies) are a small, unique group of echinoderms with a double-ringed water vascular system and tube feet arranged in a circular pattern.
• They show exceptional features such as lack of a gut in some species, distinct sperm morphology, and unique skeletal structure.
• Despite their differences, they are still classified as echinoderms, likely closely related to sea stars.
48
Class Echinoidea
Defining characteristics
1) Ossicles are joined to form a rigid test;
2) podia pores pass through the am-bulacral plates; which are distributed symmetrically around the body.
3) adults generally possess a complex system of ossicles and muscles (Aristotle's lantern) that can be partially protruded from the mouth for grazing and chewing
49
Class Echinoidea
Physical Features
• Spines: The body is covered by rigid calcium carbonate spines, which serve for protection, defense, and sometimes locomotion. The spines may be thin and sharp or thick and blunt depending on the species.
• Spines are attached to the skeleton via ball-and-socket joints and can be moved in various directions by specialized muscle fibers.
• Some species' spines can extrude toxins, offering chemical defense.
• Spines are replaced or repaired within a month or two if damaged.
• Test (Skeleton): The skeleton is made up of flat ossicles joined by collagenous ligaments, creating a solid, inflexible test.
• The test grows as the animal increases in size, with new material added at the edges of ossicles and ocular plates near the anus.
Body Shape and Spines:
• Sand dollars have short spines and a flattened test, forming a thin disc. This shape is an adaptation for burrowing.
• Heart urchins and regular urchins have a convex aboral surface.
• The shorter spines in some species of urchins are another adaptation to burrowing.
50
Class Echinoidea
Tube Feet and Locomotion
• Tube Feet: The tube feet are distributed over the surface and protrude through double rows of pores in the ambulacral plates.
• These feet have suction-cup ends and are mainly used for locomotion, similar to other echinoderms.
• Tube feet may also function in light reception in some species, essentially acting as a multifaceted eye.
51
Class Echinoidea
Pedicellariae and Defense
• Pedicellariae: Echinoids have pedicellariae with three jaws (as opposed to the two found in asteroids).
• Pedicellariae are often equipped with calcareous support rods and discharge defensive poisons.
• Gills: Some species possess small appendages called gills, which are likely involved in gas exchange.
52
Class Echinoidea
Symmetry and Classification
• Regular Echinoids: These species exhibit near-perfect spherical symmetry (e.g., most sea urchins).
• Irregular Echinoids: These species exhibit bilateral symmetry to varying degrees and are typically adapted to burrowing behaviors (e.g., heart urchins, sand dollars).
• Irregular echinoids have ambulacral areas restricted to the oral and aboral surfaces, unlike regular echinoids, which have a continuous line from oral to aboral surfaces.
• Heart urchins have distinct anterior and posterior ends, with the mouth located anteriorly and the anus posteriorly.
53
Class Echinoidea
Habitat and Behavior
• Burrowing: Many irregular urchins (such as heart urchins) are adapted for burrowing through sand, mud, or gravel. Their tube feet lack terminal suckers in these species to aid in movement through these substrates.
54
Class Echinoidea
Feeding and Digestion
1. Aristotles lantern:
• A complex system of ossicles and muscles, called Aristotle's lantern, surrounds the esophagus in regular echinoids and some irregular echinoids.
• Aristotle's lantern is used to scrape food like algae from solid substrates or to eat seaweed. Some deep-sea urchins also use it to scoop up mud for feeding.
• In species lacking Aristotle's lantern, feeding is typically done by modified tube feet, spines, or external ciliary tracts to collect small organic debris.
2. Stomach and Digestion:
Echinoids do not have a true stomach.
Instead, the esophagus leads to a long, convoluted intestine, where digestion and absorption occur.
• The anus is located on the aboral surface and is surrounded by a series of plates called the periproct.
55
Class Echinoidea
Coelomic Fluid and Circulation
Coelomic Fluid:
• The coelomic cavity in echinoids is large, particularly in regular urchins.
• Coelomic fluid functions as the primary circulatory fluid, transporting food and wastes.
• The mesodermal lining of the coelomic cavity is ciliated, ensuring constant movement of the circulatory medium.
56
Class Echinoidea
Water Vascular System (WVS)
The water vascular system (WVS)
follows the archetypical pattern seen in other echinoderms, with a single madreporite opening on the aboral surface.
57
Class Echinoidea
Human Consumption and Economic
Importance
Gonads as Delicacies:
• Echinoid gonads are considered a delicacy in Japan and the Mediterranean, where they are highly prized and sold for over $100 per pound.
• Due to overfishing and pollution, local populations of urchins in Japan can no
longer meet the demand, leading to the importation of canned gonads from the United States.
58
Class Holothuroidea
Defining characteristics
1) The body is worm-shaped, being greatly elongated along the oral/aboral axis;
2) the calcareous ossicles are reduced in size and embedded individually in the body wall;
3) highly branched, muscular respiratory structures (the respiratory trees)-generally one pair-extend from the cloaca into the coelomic cavity
59
Class Holothuroidea
Physical Features
1. Body Shape and Structure:
• Worm-shaped body, greatly elongated along the oral/aboral axis.
• Bilaterally symmetric, with distinct anterior and posterior ends.
• Typically soft-bodied, with a stretchable body wall due to the reduced size and detachment of calcareous ossicles, which are embedded individually in the body wall.
• The body wall is composed largely of connective tissue.
2. Ossicles:
• The calcareous ossicles in holothurians are microscopic and embedded in the body wall, in contrast to other echinoderms where ossicles form a rigid skeleton.
• In some species, ossicles can compose up to 80% of the total dry weight of the body wall, but some species lack ossicles entirely.
• The outer body wall can appear warty and dark-colored.
3. Size:
• Adults range in size from several centimeters to over 1 meter in length.
4. Cephalization:
• Cephalization (development of a head region) is not pronounced, despite the presence of a distinct anterior end.
60
Class Holothuroidea
Feeding and Digestion
Oral Tentacles:
• The oral tube feet are modified into large, feathery tentacles at the anterior end.
• These tentacles can be protracted from the mouth and are used for capturing food. Some species coat their tentacles with sticky mucus to trap food particles from suspension.
• In some species, a single large ampulla controls each tentacle.
Feeding Habits:
• Most species are deposit feeders, consuming sediment and extracting the organic component.
• Some species are ectocommensal, attaching to fish.
• Sea cucumbers thrive on fine mud ooze on the deep-sea floor and can process over 130 kg of substrate per year.
• They make up over 90% of the biomass in certain abyssal habitats.
61
Class Holothuroidea
General Ecology
Ecological Role:
• Sea cucumbers are a dominant presence in the deep-sea floor and are often found in large numbers in soft, deep-sea sediments, along with other deposit-feeding species like brittle stars (class Ophiuroidea).
62
Class Holothuroidea
Physical Features & Locomotion
1. Ambulacral Tube Feet:
• The tube feet of surface-living sea cucumbers have suckers and are used for locomotion and attachment, similar to echinoids (sea urchins).
2. Body Wall Musculature:
• Holothurians have well-developed circular and longitudinal muscles in their body wall, which aids in locomotion (similar to earthworm-style movement) and supports the use of a hydrostatic skeleton.
• The body wall is deformable, which allows for flexible movement.
3. Burrowing Species:
Burrowing species of holothurians use tentacles to push substrate away and primarily use muscular contraction waves (circular and longitudinal muscles)
for movement.
• Some burrowing species have reduced tube feet or may lack them entirely.
63
Class Holothuroidea
Digestive and Circulatory Systems
Digestive System:
• The digestive system of holothurians is elongated compared to echinoids, and their digestive system resembles that of echinoids.
• The digestive system is connected to the cloaca, which plays a role in water circulation for respiratory functions.
Water Vascular System (WVS):
• The WVS follows the typical echinoderm pattern with a ring canal encircling the esophagus.
• The madreporite lies free within the coelomic cavity, and the WVS is not connected directly to the outside, unlike in some other echinoderms.
• The coelomic fluid serves as the primary circulatory medium.
• Some holothurians possess a hemal system with pulsatile hearts.
64
Class Holothuroidea
Respiration
Respiratory Trees:
• Holothurians possess specialized internal respiratory structures called respiratory trees that are highly branched and muscular.
• These respiratory trees are connected to the cloaca, which pumps water into them.Water is expelled through the cloaca by contraction of the respiratory tree tubules.
65
Class Holothuroidea
Defense Mechanisms
Evisceration & Cuvierian Tubules:
• Some species can expel internal organs in response to environmental or physical threats, a process called evisceration.
• In some species, only Cuvierian tubules are expelled. These sticky and toxic structures are used for defense,
deterring predators by entangling them.
• Evisceration involves the expulsion of the digestive system, respiratory trees, and gonads, but the lost organs are regenerated within weeks.
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Reproduction & Asexual Reproduction
1. Asexual Reproduction:
• Asexual reproduction is common among many echinoderm species.
• Asteroids and ophiuroids can regenerate missing arms and organs after the central disc separates.
• Some asteroids, echinoids, and ophiuroids larvae replicate asexually, especially in response to predators.
• Holothurians (sea cucumbers) also exhibit asexual reproduction, where the adult body can split transversely, each half regenerating missing parts.
• Crinoids and echinoids do not exhibit asexual reproduction.
2. Sexual Reproduction:
• Most echinoderms reproduce sexually, with separate sexes in most species.
• Male and female individuals are often externally indistinguishable, except in a few cases like some ophiuroid species.
• Echinoderms (except holothurians and crinoids) typically have multiple gonads.
• For example, asteroids have gonads extending into each arm.
• Concentricycloids possess 10
gonads (5 pairs) in the coelomic cavity.
• Ophiuroids have gonads that empty into bursae.
• Echinoids usually have 5 gonads.
• Holothurians are unique in often having a single gonad.
• Gametogenesis (gamete formation) is regulated by steroid hormones, similar to vertebrates.
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Fertilization & Larval Development
3. Fertilization:
• In most echinoderms, fertilization is external, with gametes released into seawater.
• Concentricycloids may have internal fertilization, with ducts leading from the male testes possibly serving as copulatory organs.
4. Larval Development:
• Ciliated larval stages are typical for most echinoderm classes, with each class having distinct larval forms.
• Echinoderm metamorphosis to the adult form is often rapid and involves dramatic morphological reorganization.
• Some species exhibit reduced larval structures and accelerated adult development, leading to a more direct transition from embryo to adult.
• Parental brood care is observed in some species, such as asteroids, where embryos develop inside the parental gonad and emerge as small, fully-formed sea stars.
• These evolutionary changes are independent across different echinoderm classes, providing insight into the molecular mechanisms that modify developmental patterns.
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Research Potential
• Echinoderms are valuable for studying developmental modifications and the molecular mechanisms behind evolutionary changes in reproduction and development.
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Nervous System
1. Lack of Centralized Brain:
• Echinoderms do not have a centralized brain or distinct ganglia.
• The nervous system is instead composed of three diffuse nerve networks.
2. Ectoneural System:
• This system receives sensory input from the epidermis.
• It is highly developed in all echinoderms except crinoids.
• The ectoneural system consists of a ring around the esophagus with five associated radial nerves radiating
outward.
• In species with arms, the radial nerve cords extend down each arm to the tube feet, ampullae, and pedicellariae.
3. Hyponeural System:
• The hyponeural system is primarily involved in motor functions.
• It also has a circumoral nerve ring with five associated radial nerves, but it lies deeper within the tissues.
• This system is well-developed in ophiuroids (brittle stars) and somewhat less so in asteroids (sea stars).
4. Entoneural System (Crinoids):
• In crinoids, the major nerve network is an entoneural system, which is associated with the aboral end of the animal.
• Nerves radiate from a central mass in the calyx/tegmen complex, extending down the stalk to the cirri and up into each arm.
• The entoneural system is either inconspicuous or entirely absent in other echinoderm classes.
