Smaller Ecdysozoans

1. Smaller Ecdysozoans Chapter 18

2. Ecdysozoans • Many protostomes possess a cuticle. • Non-living outer layer secreted by epidermis. • Cuticle restricts growth and must be molted via ecdysis. • Regulation of molting achieved by the hormone ecdysone. • Members of Ecdysozoa molt cuticle as they grow.

3. Ecdysozoan Phyla • Phylum Nematoda • Phylum Nematomorpha • Phylum Kinorhyncha • Phylum Loricifera • Phylum Priapulida • All have a nonliving cuticle that is shed as the organism grows.

4. Diversity • Ecdysozoans do not share the same body plan. • Members of Nematoda, Nematomorpha, and Kinorhyncha are pseudocoelomate. • Members of Priapulida are presumed to be pseudocoelomate, but have not been carefully studied yet. • Loricifera species may be pseudocoelomate or acoelomate.

5. Phylum Nematoda: Roundworms • About 25,000 species of nematodes are described, but as many as half a million may exist. • Many prefer the name Nemata for this phylum. • Found in virtually all habitats in all biomes. • Topsoil may contain billions per acre.

6. Phylum Nematoda: Roundworms • Free-living nematodes feed on bacteria, yeasts, fungal hyphae, and algae. • Predatory nematodes eat rotifers, tardigrades, small annelids, and other nematodes. • Important as food for mites, insects, larvae, and fungi.

7. Phylum Nematoda: Roundworms • Caenorhabditis elegans is an important model for studies of genomics and cell development and differentiation.

8. Phylum Nematoda • Some species of nematodes are important parasites of plants and animals. • Nematode parasites exist in nearly all animal and plant species.

9. Phylum Nematoda • Triploblastic – three embryonic germ layers. • Pseudocoel – used as hydrostatic skeleton. • Cylindrical shape • Lack of motile cilia or flagella • Nonsegmented • Nonliving cuticle

10. Phylum Nematoda • The cylindrical bodies of nematodes are covered by a tough coat called a cuticle.

11. Phylum Nematoda • Complete digestive system. • Circulatory and respiratory systems are lacking. • Ring of nerve tissue and ganglia around the pharynx lead to dorsal and ventral nerve cords.

12. Phylum Nematoda • Nematodes have separate sexes. • Dioecious, females are larger. • Fertilization is internal. • Eggs are stored in uterus until deposited. • Cuticle is shed between each of four juvenile stages.

13. Phylum Nematoda: Roundworms • Sensory papillae at head and tail. • Amphids, pair of sensory organs on head, lead into a deep cuticular pit with modified cilia.

14. Phylum Nematoda • Some larvae are free-living, some require and intermediate host. • Parasitic nematodes of humans: • Intestinal roundworm (Ascaris) • Hookworm • Pinworm • Trichina worm • Whipworm

15. Phylum Nematomorpha • Phylum Nematomorpha, the horsehair worms, are free living as adults, and parasites in arthropods as juveniles. • Currently placed as the sister taxon to nematodes • About 320 species are known. • Occur worldwide • Pseudocoelomate • Triploblastic

16. Phylum Nematomorpha • Horsehair worms leaving the body of a katydid found dead near a river in Illinois. Photos by Bryan Suson

17. Phylum Nematomorpha • Adults utilize stored nutrients. • Recent studies reveal that they can absorb organic molecules through vestigial gut and body wall. • Circulatory, respiratory and excretory systems are absent. • Nematomorphs are dioecious. • Females discharge eggs into water in long strings.

18. Phylum Nematomorpha • Juveniles may encyst on vegetation and are eaten by an arthropod such as a grasshopper. • Larval stages have hooks that may be used to bore into a host. • May also cause infection via drinking water. • Larvae encyst within host. • Marine nematomorphs infect hermit crabs and other crabs.

19. Phylum Nematomorpha • After months in an arthropod host, mature worm emerges into nearby water or during rainfall. • Parasite stimulates terrestrial insects to seek water.

20. Phylum Kinorhyncha • Kinorhynchs are usually under 1 mm long. • About 179 species are known. • Found worldwide, from intertidal areas to 6000 m deep. • Most live in mud, but some have been found in algae, sponges, and other invertebrates.

21. Phylum Kinorhyncha • Spines (scalids) function in locomotion, chemoreception, and mechanoreception. • Retractile head has a circle of spines and a retractile proboscis (introvert). • Body wall made of a cuticle and syncytial epidermis.

22. Phylum Priapulida • The phylum Priapulida contains 16 species of marine worms that occur in colder waters. • Found from intertidal zones to deep ocean floors, several thousand meters deep. • Some are tube dwellers and feed on detritus.

23. Phylum Priapulida • Cylindrical bodies under 15 cm long. • Burrow by body contractions and orient mouth at the surface. • Retractable introvert has papillae and rows of curved spines to sample and capture prey. • Chitinous cuticle covers body and is molted regularly. • Sexes are separate.

24. Phylum Loricifera • Nine circlets of scalids on the introvert, similar to those of kinorhynchs. • Entire forepart can be retracted into the circular lorica. • Diet is unknown, possibly feed on bacteria. • Brain fills the head and nerves innervate scalids. • Dioecious with dimorphic males and females. • Body cavity is a pseudocoel.

25. Clade Panarthropoda • Clade Panarthropoda contains Arthropoda and two allied phyla, Onychophora and Tardigrada. • In onychophorans and arthropods coelom develops by schizocoely, but enterocoelic in tardigrades. • A new cavity, hemocoel, forms from fusion of the main coelomic cavity with the blastocoel. • Blood from open circulatory system enters the hemocoel and surrounds the internal organs. • Contains a muscular heart but limited muscular blood vessels.

26. Phylum Onychophora • About 70 living species of “velvet worms” in the phylum Onychophora. • 1.4 to 15 cm in length. • Limited to moist, leafy rain forest habitat in tropical and subtropical regions. • Changed little over 500 million years. • Fossil Aysheaia similar to modern forms. • Share traits with annelids and arthropods and were considered a “missing link”. • Most are predaceous, some live in termite nests.

27. Phylum Onychophora • No external segmentation except for paired appendages. • Skin is soft - cuticle contains protein and chitin but does not harden as in arthropods. • Body covered with tiny tubercles bearing sensory bristles. • Minute scales on tubercles impart an iridescent and velvety appearance. • Head has one pair of large antennae and an annelid-like eye at base.

28. Phylum Onychophora • 14 to 43 pairs of unjointed legs. • Legs move by waves of body contractions. • Slime glands on each side of body cavity open on oral papillae. • Mouth leads to a straight digestive tract. • Each segment contains a pair of nephridia, each containing a vesicle, ciliated funnel, and duct.

29. Phylum Onychophora • Tracheal system provides respiration to all parts of body. • Cannot close spiracles, so they are restricted to moist habitats. • Important differences suggest the tracheal system evolved independently from that of arthropods.

30. Phylum Onychophora • Open circulatory system. • Nervous system organized much like a ladder. • Sense organs include pigment cup ocelli, taste spines, tactile papillae, and hygroscopic receptors that orient the animal toward water vapor.

31. Phylum Onychophora • With exception of one parthenogenetic species, all are dioecious with paired reproductive organs. • Males deposit spermatophores on back of female. • White blood cells dissolve the skin and sperm migrate to ovaries. • May be oviparous, ovoviviparous, or viviparous.

32. Phylum Tardigrada • Tardigrades, or water bears, are less than a millimeter in length. • Freshwater or marine • Live in spaces between sand grains.

33. Tardigrade Video http://www.youtube.com/watch?v=W9WWysGBAlU&feature=related http://www.youtube.com/watch?v=blX7Ef1gxcM&feature=related

34. Phylum Tardigrada • Trunk bears four pairs of short, stubby, unjointed legs, each with four to eight claws. • Body covered by non-chitinous cuticle that is molted four or more times during lifetime. • Mouth leads to a muscular pharynx adapted for sucking. • Two stylets protrude to pierce integument of nematodes or walls of plant cells and allow them to suck juices.

35. Phylum Tardigrada • They share many characteristics with arthropods. • But legs are unjointed. • Non-chitinous cuticle that is molted.

36. Phylum Tardigrada • No circulatory or respiratory systems. • Gas exchange occurs at body surface. • Muscles are all longitudinal. • Uses hydrostatic pressure as a skeleton. • Brain is relatively large.

37. Phylum Tardigrada • Sexes are separate. • In parthenogenetic freshwater and moss-dwelling species, males are unknown. • Egg-laying, like defecation, occurs at molting. • Eggs may be highly ornate.

38. Phylum Tardigrada • Tardigrades can enter a state called cryptobiosis, where metabolism is imperceptible. • Tardigrades can dehydrate from 85% water to only 3% water. • In this state they can resist extreme temperatures, ionizing radiation, oxygen deficiency, etc. for years. • When water is available, they become metabolically active again.

39. Phylogeny • Evolutionary relationships among ecdysozoans are not well-understood. • Members of this clade do not share a common cleavage pattern: • Nematodes and nematomorphs - cleavage is unique, not spiral or radial. • Cleavage in kinorhynchs, lorificiferans, and tardigrades has yet to be studied. • Priapulid cleavage is nearly radial.

40. Phylogeny • Recent phylogenies place Nematoda and Nematomorpha as sister taxa since they share a collagenous cuticle. • Phylum Kinorhyncha and Priapulida are considered sister taxa because they share a two-layered pharynx. • Velvet worms, phylum Onychophora, are the sister taxon to tardigrades.

41. Phylogeny • Onychophorans share a number of characteristics with annelids: • Metamerically arranged nephridia, muscular body wall, pigment cup ocelli, and ciliated reproductive ducts. • Onychophorans also share features with the arthropods: • Including a cuticle, tubular heart, and hemocoel with open circulatory system, presence of tracheae (possibly not homologous), and large brain.

42. Phylogeny • Sequence analysis supports placement of Onychophora in clade Panarthropoda. • Tardigrades and arthropods also share arthropod-type setae and muscles inserted on the cuticle.

43. Adaptive Diversification • Nematodes show impressive adaptation – they are found in almost every habitat available to animals. • Body structure is plastic enough to allow adaptation. • Life cycle ranges from simple to complex. • Have been known to survive in suboptimal conditions.