Smaller Ecdysozoans Chapter 18

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.

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

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.

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.

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.

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

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

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

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

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.

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.

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.

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

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

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

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.

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.

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

Phylum Kinorhyncha Kinorhynchs are usually <1 mm long. Most live in mud, but some have been found in algae, sponges, and other invertebrates. Spines (scalids) function in locomotion, chemoreception, and mechanoreception. Retractile head has a circle of spines and a retractile proboscis (introvert).

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. Retractable introvert has papillae and rows of curved spines to sample and capture prey.

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.

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.

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.

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.

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. https://youtu.be/mrL2A7my1fc

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. Sense organs include pigment cup ocelli, taste spines, tactile papillae, and hygroscopic receptors that orient the animal toward water vapor.

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.

Phylum Tardigrada Tardigrades, or water bears, are less than a millimeter in length. Freshwater or marine Live in spaces between sand grains. http://www.youtube.com/watch?v=W9WWysGBAlU&feature=related http://www.youtube.com/watch?v=blX7Ef1gxcM&feature=related

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.

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

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.

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.

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.

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.

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.

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.

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

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.