Protochordate-and-the-Origin-of-Craniates.pptx

None

1. Protochordate and the Origin of Craniates Presentors : Artieda and Domingo

2. Objectives To discuss the characteristics, origin, reproduction, habitat, ecological role, and evolutionary significance of Basal Deuterostomes . To explain the origin, general characteristics, reproduction, ecological role, and evolutionary importance of Urochordates . To describe the reproduction, ecological role, and evolutionary relevance of Cephalochordates . To trace the origin and characteristics of Craniates , including their evolutionary innovations and representative examples . To examine the Ammocoete Larva , its general characteristics, habitat, development, ecological role, and evolutionary importance.

3. BASAL DEUTEROSTOMES Basal deuterostomes are primitive members of the superphylum Deuterostomia , a major group of animals that includes echinoderms, hemichordates, and chordates. The term deuterostome means "second mouth," referring to a developmental feature in which the anus forms first during embryonic development, while the mouth forms later. Basal deuterostomes are considered essential to understanding the early evolutionary steps that gave rise to more complex deuterostomes, particularly vertebrates.

4. THE ORIGIN OF DEUTEROSTOMES Deuterostomes first appeared during the Cambrian Explosion (around 520–540 million years ago). Fossil evidence suggests that hemichordates and echinoderms shared a common ancestor, while the lineage also gave rise to chordates. Basal deuterostomes represent some of the earliest branches of this group, making them important for reconstructing evolutionary relationships and body plan innovations.

5. GENERAL CHARACTERISTICS Bilateral symmetry at some stage of life . True coelom derived from the archenteron . Gill slits or pharyngeal openings (seen in hemichordates and chordates ). Internal skeleton and water vascular system in echinoderms . Presence of a tripartite body plan in hemichordates (proboscis, collar, trunk ). Nervous system ranging from simple nerve nets (echinoderms) to dorsal nerve cords (chordates).

6. MORPHOLOGY

7. REPRODUCTION AND DEVELOPMENT Reproduction : Most basal deuterostomes reproduce sexually , releasing eggs and sperm into the water for external fertilization . Some also show asexual reproduction, particularly through budding or fragmentation . Development : Their embryos exhibit typical deuterostome characteristics : Radial, indeterminate cleavage (cells retain the capacity to develop into complete organisms ). Blastopore becomes the anus , while the mouth develops secondarily . Formation of a coelom (body cavity) through enterocoely ( outpocketing of the gut).

8. HABITAT AND DISTRIBUTION Basal deuterostomes are exclusively marine organisms . They inhabit diverse environments such as shallow coastal waters, sandy and muddy seabeds , coral reefs, and deep-sea ecosystems . Hemichordates (e.g., acorn worms) are found burrowed in sediments, while echinoderms (e.g., sea stars, sea cucumbers) are distributed worldwide, from the intertidal zone to the abyssal depths.

9. ECOLOGICAL ROLE Sediment mixing : Hemichordates such as acorn worms burrow in sediments, recycling nutrients and oxygenating the seabed . Food source : Basal deuterostomes serve as prey for fishes, crustaceans, and other marine predators . Ecosystem engineers : Echinoderms influence marine ecosystems by grazing on algae, recycling detritus, and regulating population dynamics of other species.

10. EVOLUTIONARY SIGNIFICANCE Basal deuterostomes provide critical insights into the origin of vertebrates and the development of key body plan features such as : The notochord and dorsal nerve cord (precursors seen in hemichordates ). Pharyngeal gill slits, which later evolved into jaws and other structures in vertebrates . Evolution of radial symmetry in echinoderms from bilaterally symmetrical ancestors.

11. UROCHORDATES Urochordates , also called tunicates or sea squirts , are a group of marine invertebrates belonging to the subphylum Urochordata under the phylum Chordata . They are called urochordates because they possess a notochord only in the larval stage , located in the tail region. As adults, they lose most chordate features and take on a sac-like body enclosed in a tunic.

12. THE ORIGIN OF UROCHORDATES Urochordates first appeared in the Cambrian period . Fossil evidence, such as Shankouclava from China, shows that they are among the earliest chordates. Scientists believe they are closely related to vertebrates, and genetic studies suggest that vertebrates and urochordates share a common ancestor , making them important in evolutionary biology.

13. GENERAL CHARACTERISTICS Body enclosed in a tunic made of tunicin . Two siphons used for filter feeding and water flow . Sessile as adults, but free-swimming as larvae . Nervous system: well-developed in larvae, but reduced in adults . Adults lack a notochord and dorsal nerve cord, but larvae show all chordate features . Simple circulatory system, with a heart that can pump blood in both directions.

14. REPRODUCTION AND DEVELOPMENT REPRODUCTION: Most urochordates are hermaphrodites Fertilization can be external or internal Some species reproduce asexually through budding, especially in colonial tunicates . DEVELOPMENT: Their larvae resemble tadpoles and show typical chordate features: notochord, dorsal nerve cord, and tail . The larva undergoes metamorphosis (a major change in body form), losing the notochord and tail to become a sedentary (sessile) adult . Adults are sac-like and filter-feeding, no longer showing most chordate traits.

15. HABITAT AND DISTRIBUTION Urochordates are found only in marine environments . They are cosmopolitan , living in oceans worldwide from shallow coastal waters to the deep sea . They often attach to rocks, coral reefs, mangrove roots, docks, and ship hulls . Some, like salps and larvaceans , are free-floating in open water, forming large colonies.

16. ECOLOGICAL ROLE Filter feeders : They pump seawater through their bodies, removing plankton and organic particles, which helps keep ocean water clean . Food source : Many fishes, mollusks, and sea stars feed on them . Biofouling organisms : They often attach to ship hulls, docks, and underwater structures, influencing human activities . Ecosystem role : By filtering large amounts of water, they contribute to nutrient cycling in marine ecosystems.

17. EVOLUTIONARY SIGNIFICANCE Urochordates are important because their larvae show all the chordate features (notochord, nerve cord, gill slits, and post-anal tail), suggesting they are closely related to vertebrates . Genetic studies reveal that vertebrates share more DNA similarities with urochordates than with cephalochordates, making them key to understanding the origin of vertebrates . Their metamorphosis highlights how chordate features can be lost or modified during evolution.

18. CEPHALOCHORDATES Cephalochordates are exclusively marine organisms . They typically inhabit shallow, temperate, and tropical seas, often burrowing in sandy or muddy substrates . These benthic creatures live on or near the ocean floor, commonly in intertidal zones and continental shelves

19. REPRODUCTION AND DEVELOPMENT Cephalochordates reproduce sexually, with separate sexes and external fertilization . During the summer, adults swim up into the water column after sunset to release their gametes . The embryos and larvae are planktonic, developing through metamorphosis into the juvenile form.

20. ECOLOGICAL ROLE As filter feeders, cephalochordates play a crucial role in marine ecosystems . They feed by drawing water into their mouths and trapping food particles in mucus produced by the endostyle . This process helps maintain plankton populations and contributes to nutrient cycling . Cephalochordates also serve as a food source for various predators, including fish and invertebrates

21. EVOLUTIONARY SIGNIFICANCE Cephalochordates are vital for understanding the evolution of vertebrates . They possess the five key characteristics of chordates, offering insights into the transition from invertebrates to vertebrates . Their simple body plan and retention of the notochord throughout life make them a valuable model for studying the evolutionary origins of chordates . Recent studies suggest that cephalochordates are the most basal chordate group, providing critical information about the genome and developmental mechanisms of early chordates

22. THE ORIGIN OF CRANIATES C raniates were first found in the late 1900's Fossils were formed 530 million years ago during the Cambrian explosion and appeared to represent the transition of the early chordates into craniates .

23. BACKGROUND INFORMATION CHORDATES: WHAT CRANIATES EVOLVE FROM Chordates , or members of the phylum Chordata, are bilaterally symmetrical animals that belong to the clade Deuterostomia . All chordates share four key characters: a notochord; a dorsal, hollow nerve cord; pharyngeal slits; and a post-anal tail ( Figure 1 ). Notochord - This is a longitudinal, flexible rod located between the digestive tube and the nerve cord. It provides support throughout the chordate's body as well as a firm but flexible structure that muscles can work against when the chordate is swimming. In humans, the notochord is the gelatinous disks layered between vertebrae . Pharyngeal slit - In chordate embryos, this is one of the slits that form from the pharyngeal clefts and communicate to the outside of the body. They later develop into gill slits in many vertebrates.

24. Figure 1: Anatomical structures of all chordates

25. EARLY CHORDATES figure 2. Lancelet figure 3 tunicates

26. CRANIATES A craniates is a chordate with a head (the term craniates comes from the word cranium, meaning "skull"). This includes all animals that possess skulls, either cartilaginous or bony. -primary characteristic is the presence of a cranium, which serves as a protective enclosure for the brain. -possess paired sensory organs, such such as eyes for vision and ears for hearing, contributing to their sophisticated perception of the environment. -The origin of this head and its parts - brain at the anterior end of the dorsal nerve cord, sensory organs, and a skull - allowed chordates to coordinate more complex movement and feeding behaviors.

27. CRANIATES CHARACTERISTICS C raniates have heads (Figure 1 ) They have 2 clusters of Hox genes while the early chordates lancelets and tunicates only have one . O ther genes that produce signaling molecules and transcription factors are duplicated in craniates . This genetic complexity may have made it possible for craniates to develop more complex morphologies than those of tunicates and lancelets .

28. Craniates have a special neural crest as well (Figure 2). This is the collection of cells near the dorsal margins of the neural tube where it pinches off from the ectoderm. These cells disperse throughout the embryo and give rise to a variety of structures: teeth, some of the skull's bone and cartilage, the inner layer of skin (dermis) of the facial region, several types of neurons, and sensory capsules in which the eyes and other sense organs can develop.

29. In aquatic craniates , the pharyngeal clefts evolved into gill slits (Figure 3). While lancelets used their pharyngeal slits mainly for suspension feeding, craniates ' gill slits are associated with muscles and nerves that enable water to be pumped through the slits. This pumping aids in sucking in food and facilitates gas exchange.

30. Craniates are more active than tunicates and lancelets. They have a higher metabolism and a much more extensive muscular system. Muscles lining the craniates ' digestive tract aid digestion by moving food through the tract. All craniates , except most tetrapods , have a caudal fin strengthened by cartilaginous radials. They also possess an endoskeleton, which is primitively cartilaginous but becomes mineralized in many ways (bone, calcified cartilage) in vertebrates. Only certain craniates,such as the gnathostome and some fossil jawless vertebrates have a mineralized exoskeleton . Craniates have a circulatory system of arteries, capillaries and veins, and a chambered, muscular main heart. They have red blood cells with hemoglobin along with kidneys that remove waste from the blood. The circulatory system is entirely closed, and the heart has at least 2 chambers, the atrium and the ventricle.

31. They also have a complete digestive system . The digestive tract is differentiated into mouth and oral cavity, pharynx, esophagus, intestine, rectum, and anus. All craniates have a pancreas that produces digestive enzymes and hormones, such as insulin and glucagon, that regulate the blood sugar level. All craniates have a liver or hepatic organ that serves several functions, including food storage and production of bile.

32. EXAMPLES OF CRANIATES Figure 1: X-ray tetra fish Figure 2: Puppies Figure 3: Amphibians

33. EXAMPLES OF CRANIATES Figure 4: Birds Figure 5: Reptiles Figure 6: Mammals

34. The least derived craniate lineage that still survives today is Myxini , the hagfishes (Figure 7). They have skulls made of cartilage but lack jaws and vertebrae. Hagfish swim in a snakelike fashion like eels by using segmental muscles to exert force against their notochord, which becomes a strong flexible rod of cartilage in the fishes' adulthood. They each have a small brain, eyes, ears, and a nasal opening that connects with the pharynx, and their mouths have teeth-like formations made of keratin.

35. All of the 30 living species of hagfishes are marine . They can grow up to 60 centimeters in length and get food as bottom-dwelling scavengers, feeding on worms and sick or dead fish. Hagfish have slime glands that secrete a substance that absorbs water, forming a slime that may repulse other scavengers when the hagfish is feeding. It can produce several liters of slime in less than a single minute if it happens to be attacked by a predator. The slime coats the gills of the attacking fish, making it retreat or even suffocate. HAGFISHES

36. HAIKOUELLA Haikouella also possessed some characteristics of craniates . Well-formed brain, small eyes, and muscle segments along its body, similar to those of vertebrate fish,possessed respiratory gills in the pharynx. -did not have a skull or ear organs, hinting that these traits emerged with further innovations to the chordate nervous system.

37. Figure 1 A fossil of Haikouella Figure 2: Illustration of an early Haikouella

38. MYLLOKUNMINGIA Myllokunmingia -about the same size as Haikouella -had ear capsules and eye capsules, parts of the skull that surround the ears and eyes -paleontologists identify Myllokunmingia as a true craniate The least derived, undisputed craniate lineage that still survives is Myxini , the hagfishes.

39. Figure 3: Myllokunmingia

40. THE AMMOCOETE LARVA Ammocoete larva is the larval stage of lampreys, which are primitive jawless fish belonging to the class Petromyzontida , . Lampreys have a life cycle that includes two distinct phases: a larval phase and an adult phase, The ammocoete larva is significantly different from the adult lamprey and exhibits characteristics that provide insights into vertebrate evolution. It has many properties of ancient chordates and has some properties of the vertebrates like dorsal nerve chord, primitive form of nervous system, fill slits, notochord, brain and eyes.

41. GENERAL CHARACTERISTICS The ammocoete larva has an eel-like body but differs from the adult in several respects. Key characteristics include:  • A continuous, single median dorsal fin. • They are blind and toothless filter feeders, unlike the parasitic adults. • Possession of a semi-circular upper lip or oral hood around the mouth, similar to that of Branchiostoma . • A transverse lower lip is present, and they lack a suctorial buccal funnel.

42. HABITAT AND DISTRIBUTION Ammocoete larvae inhabit freshwater streams, where they burrow in the sediments . They construct and live in U-shaped tunnels in mud and sand in quiet waters, . These larvae emerge from their burrows at night to feed on organic matter, unicellular algae, and bacteria.

43. REPRODUCTION AND DEVELOPMENT Lampreys lay eggs that hatch into ammocoetes in about three weeks. The larval period lasts for 3 to 7 years, during which they grow and eventually undergo metamorphosis into the adult form. Metamorphosis involves significant changes, including the development of a buccal funnel with horny teeth, the transformation of the endostyle into the thyroid gland, and the development of fully formed paired eyes

44. ECOLOGICAL ROLE Ammocoetes are filter feeders, consuming detritus and microorganisms in freshwater sediments . They play a role in the stream ecosystem by processing organic matter and serving as a food source for other organisms. Their presence and habitat quality can be indicators of stream health, with streamflow diversions potentially impacting their habitat.

45. EVOLUTIONARY SIGNIFICANCE The ammocoete larva is of significant evolutionary importance because it represents a primitive and generalized vertebrate form, intermediate between cephalochordates and vertebrates, . It exhibits characteristics of ancient chordates and early vertebrates, such as a dorsal nerve cord, notochord, and gill slits, . The similarities between ammocoetes and amphioxus, an invertebrate chordate, suggest an evolutionary link between them,,, . Some researchers propose that the filter-feeding structures in ammocoetes are similar to those in invertebrate chordates like amphioxus and tunicates, supporting the traditional view of vertebrate evolution.

46. END Presentors : Artieda and Domingo