Chapter 33 - Protostome Animals

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1 Chapter 33 - Protostome Animals Learning Objectives: Students should be able to... Describe the major characteristics that differentiate the Ecdysozoa and the Lophotrochozoa. List and describe the basic traits of four major phyla of lophotrochozoans (including subgroups of mollusks) and two major phyla of ecdysozoans (including subgroups of arthropods). Describe the major evolutionary innovations that triggered the diversification of the protostomes, especially relating to the water-toland transition, appendages and mouthparts, and metamorphosis. Lecture Outline I. An Overview of Protostome Evolution A. Protostomes are the most abundant animals on Earth. (Fig. 33.1) B. Protostomes can be divided into Lophotrochozoa and Ecdysozoa. (Fig. 33.2) C. What is a lophotrochozoan? 1. The Lophotrochozoa comprise 13 phyla and include the mollusks, annelids, and flatworms. 2. The Lophotrochozoa lack a distinct synapomorphy but many have: a. A feeding structure called a lophophore (in three phyla) (Fig. 33.3a) b. A type of ciliated larva called a trochophore (in several marine phyla) (Fig. 33.3b) c. Growth by incremental additions to the body rather than by molting (Fig. 33.4a) D. What is an ecdysozoan? 1. The Ecdysozoa comprise seven phyla and include nematodes and arthropods. 2. All grow by molting shedding an exoskeleton or covering. (Fig. 33.4b) II. Themes in the Diversification of Protostomes A. How do body plans vary among phyla? 1. All 22 protostome phyla are triploblastic and bilaterally symmetric and undergo embryonic development in a similar way. 2. The coelom was reduced or eliminated in some groups. a. Wormlike protostomes have a well-developed coelom that serves as a hydrostatic skeleton or for circulation. b. In some other phyla, including flatworms, arthropods, and mollusks, the coelom was no longer needed for these functions and was reduced or eliminated. 3. The arthropod body plan (Fig. 33.5a) a. Arthropods have segmented bodies divided into sections called tagmata (singular: tagma).

2 b. The exoskeleton is made of chitin and hardened by calcium carbonate. c. Movement occurs by contraction of muscles that pull against the exoskeleton to move jointed limbs. d. A fluid-filled hemocoel holds internal organs and circulates body fluid, and functions as a hydrostatic skeleton in some larvae. e. Students should be able to describe the major features of the arthropod body. 4. The molluscan body plan (Fig. 33.5b) a. A muscular foot is used for movement. b. A visceral mass contains the internal organs. c. A mantle covers the visceral mass and may secrete a shell. 5. Variation among body plans of the wormlike phyla a. These phyla all have similar body plans but are distinguished by specialized mouthparts or feeding structures. b. Echiurans (a group of annelids) have a proboscis with which they capture pieces of detritis in marine mud. (Fig. 33.6a) c. Priapulids have a tooth-lined throat that they can turn inside out to grab their prey. (Fig. 33.6b) d. Nemerteans are active predators that move around the ocean floor and spear their prey with a proboscis. (Fig. 33.6c) B. The water-to-land transition 1. Evidence for multiple transitions a. Phylogenetic analysis indicates that the ability to live in terrestrial environments evolved independently in arthropods (at least twice), mollusks, roundworms, and annelids. (Fig. 33.7) 2. Adaptations to terrestrial environments a. The protostomes that made the water-to-land transition already had adaptations for support and locomotion (hydroskeletons, limbs, etc.). b. New adaptations were required to accomplish gas exchange without drying out. (1) Roundworms and earthworms exchange gases across their entire body surface, but they must stay in moist environments. (2) Arthropods and mollusks have gills that are located inside the body where they are protected from desiccation. (3) Insects evolved a waxy layer to minimize water loss, with respiratory passages that can be closed when necessary. 3. Students should be able to name three problems that arise during a water-to-land transition in animals.

3 C. Adaptations for feeding 1. Protostomes have extremely diverse mouthparts, which allow them to eat a wide diversity of feeds with a wide variety of methods. (Fig. 33.8) 2. Larvae and adults often exploit different food sources. D. Adaptations for moving 1. Protostomes that lack limbs often move via a hydrostatic skeleton. 2. Arthropods evolved jointed limbs that permit rapid, precise movement. (Fig. 33.9a) 3. The evolution of the insect wing allowed flight. (Fig. 33.9b) a. About two-thirds of the multicellular species living today are winged insects. 4. Mollusks can glide with waves of muscle contractions of the foot. (Fig. 33.9c) 5. Cephalopods can move rapidly using jet propulsion. (Fig ) 6. Students should be able to explain the advantage of a jointed limb versus an unjointed limb. E. Adaptations in reproduction 1. Many wormlike phyla can perform asexual reproduction via splitting or fragmenting of the body. 2. Some crustaceans and insects reproduce asexually via parthenogenesis, when an unfertilized egg develops into an offspring. 3. Sexual reproduction with external fertilization occurs in clams, bryozoans, brachiopods, and other groups. 4. Sexual reproduction occurs with internal fertilization in crustaceans, snails, and insects probably because individuals in these groups can move (i.e., can meet each other). 5. A few groups of snails and insects are ovoviviparous, retaining eggs until they hatch. 6. Metamorphosis occurs in many protostomes. a. In marine species, this allows larvae to disperse to new habitats. b. In insects, it reduces competition between larvae and adults. 7. Terrestrial lineages evolved desiccation-resistant eggs several times. III. Key Lineages: Lophotrochozoans A. Rotifera (rotifers) (Fig ) 1. Habitat: Habitats are damp or aquatic. Rotifers are important components of plankton. 2. Morphology: Most are less than 1 mm long. They have a coelom and a distinctive cluster of cilia at the anterior end called a corona. 3. Feeding: Many rotifers suspension feed using the cilia of the corona. 4. Movement: Most swim via the beating of cilia in the corona.

4 5. Reproduction: Some are parthenogenetic, with direct development (no metamorphosis). B. Platyhelm inthes (flatworms) (Fig ) 1. Habitat: This large and diverse phylum includes free-living species (Turbellaria), endoparasitic tapeworms (Cestoda), and endoparasitic or ectoparasitic flukes (Trematoda). 2. Morphology: The body is broad and flat, unsegmented, and has no coelom. Flatworms have no gas exchange structures; instead, they perform gas exchange across the body wall (and are thus restricted to moist environments). 3. Feeding: There is no lophophore. The digestive tract is blind, with only one opening. 4. Movement: Most do not move much. 5. Reproduction: a. All three groups can reproduce sexually by cross-fertilization and self-fertilization. b. Turbellarians reproduce asexually by splitting in half or by fragmentation. c. Flukes and tapeworms have complex life cycles. (1) Sexual reproduction occurs in a definitive host. (2) Asexual reproduction occurs in one or more intermediate hosts. C. Annelida (segmented worms) (Fig ) 1. Habitat: There is wide variety of habitats. 2. Morphology: The body is usually segmented, with a coelom that functions as a hydroskeleton. 3. Findings of recent phylogenetic analyses a. The common ancestor of annelids had bristle-like extensions called chaetae, extending from appendages called parapodia. b. Polychaeta is a paraphyletic group consisting of several separate lineages of marine annelids that retained the ancestral trait of chaetae. c. Several other lineages independently lost the chaetae, including Clitellata (earthworms, leeches) and two other groups now known to be annelids: Sipunculida (peanut worms) and Echiura (spoon worms). (1) Sipunculida and Echiura also lost segmentation. 4. Feeding: Feeding methods include suspension feeding, active predation, deposit feeding (earthworms), and blood-feeding ectoparasitism (about half of leeches). 5. Movement: Annelids crawl, burrow, or swim. 6. Reproduction a. Many can reproduce asexually via transverse fission or fragmentation. b. Sexual reproduction in polychaetes involves separate sexes with eggs released into the water.

5 c. Sexual reproduction in oligochaetes and leeches involves hermaphroditic individuals that cross-fertilize and generate mucus-rich eggs. D. Mollusca (mollusks) 1. Bivalvia (clams, mussels, scallops, oysters) (Fig ) a. Habitat: Most are marine. Many live buried in the substrate and have produced an excellent fossil record. b. Morphology: They are named for their two separate shells. c. Feeding: Most bivalves suspension feed using their gills; no radula. d. Movement: Many are sessile, although some can burrow with their foot. Scallops swim actively by clapping their shells together. e. Reproduction: Sexual reproduction produces trochophore larvae that metamorphose into a distinct form called a veliger. 2. Gastropoda (snails, slugs, nudibranchs) (Fig ) a. Body plan: They are named for their large, muscular foot. b. Feeding: Gastropods feed with a rasping structure called a radula. Some graze on algae and other plants; others are predators. c. Movement: Most glide via wavelike foot contractions. d. Reproduction: Reproduction is mainly sexual. Most gastropods produce a veliger larva; other species stay within the egg during metamorphosis and hatch as mini-adults. 3. Polyplacophora (chitons) (Fig ) a. Habitat: All are marine, mostly in intertidal zone. b. Morphology: Chitons have eight calcium carbonate plates on their dorsal side. c. Feeding: They scrape algae off rocks using a radula. d. Movement: They move by gliding on their muscular foot. e. Reproduction: Reproduction is sexual, with external or internal fertilization; most species have trochophore larvae. 4. Cephalopoda (nautilus, cuttlefish, squid, octopuses) (Fig ) a. Morphology: They have a well-developed head and a foot that is modified to form tentacles. The shell is highly reduced or absent. Cephalopods have large brains and image-forming eyes. b. Feeding: These intelligent predators hunt by sight, using their tentacles and beak. c. Movement: They swim with jet propulsion (squid) or crawl (octopi). d. Reproduction: Reproduction is sexual, with separate sexes, internal fertilization, and elaborate courtship rituals. IV. Key Lineages: Ecdysozoans A. Onychophorans and the tardigrades (Fig )

6 1. Both these groups have segmented bodies and limbs, but the limbs are not jointed and they do not have an exoskeleton like arthropods. 2. Onychophorans (velvet worms) are small, caterpillar-like organisms that live in moist leaf litter and prey on small invertebrates. 3. Tardigrades (water bears) are microscopic animals that live in benthic (bottom) habitats of marine or freshwater environments. B. Nematoda (roundworms) (Fig ) 1. Habitat: Most are free-living. Nematodes are incredibly abundant and found in every possible habitat. Some are parasitic (pinworms, elephantiasis, trichinosis). 2. Morphology: These unsegmented worms have a coelom and longitudinal muscles that can shorten or lengthen the body. There are no respiratory or circulatory organs. 3. Feeding: They feed on a wide variety of foods using diverse mouthparts. 4. Movement: They wriggle via their hydrostatic skeleton. 5. Reproduction: Reproduction is sexual, with separate sexes, internal fertilization, and direct development. C. Arthropoda (arthropods) 1. Traits of all arthropods a. Segmented bodies with a reduced coelom and a large hemocoel b. Exoskeleton of chitin c. A well-defined head and trunk. The trunk is sometimes further organized into abdomen and thorax. d. Paired, segmented appendages that function in gas exchange, feeding, and movement e. Sophisticated sense organs including compound eyes and antennae f. Metamorphosis is common. 2. Myriapods (millipedes, centipedes) (Fig ) a. Habitat: They are found in most terrestrial environments. b. Morphology: They have a head region and a long segmented trunk, with each segment bearing one or two pairs of legs. c. Feeding (1) Millipedes live in rotting plant materials and are detritivores. (2) Centipedes hunt insects and have poison-containing fangs. d. Movement: They walk or run on their many legs. e. Reproduction: Reproduction is sexual, with separate sexes and internal fertilization. 3. Insecta (insects) (prominent orders of insects, Table 33.1) a. Habitat: Insects dominate terrestrial environments. Larvae are also common in many freshwater habitats.

7 b. Morphology: They have three tagmata head, thorax, and abdomen with wings and three pairs of legs. c. Feeding: Most insects have four sets of mouthparts that have diversified to allow them to feed in every conceivable way. d. Movement: They walk, run, swim, and fly. e. Reproduction: Reproduction is sexual, with separate sexes and internal fertilization. Some are parthenogenetic. Most undergo complete metamorphosis. 4. Chelicerata (spiders, ticks, mites, horseshoe crabs, daddy longlegs, scorpions) (Fig ) a. Habitat: Most are terrestrial, but a few are marine. b. Morphology: The body is divided into anterior and posterior regions. The anterior region has no antennae but usually has eyes. They are named for the chelicerae, a unique pair of appendages found near the mouth. c. Feeding (1) Spiders, scorpions, and daddy longlegs are predators. Spiders often build sticky webs to trap their prey. (2) Mites and ticks are ectoparasites that feed on the body fluids of animals. (3) Horseshoe crabs are omnivores. d. Movement: They walk or crawl on four pairs of jointed legs. e. Reproduction: Reproduction is sexual, with internal fertilization and direct development. Spiders often have elaborate courtship rituals. 5. Crustaceans (shrimp, lobster, crabs, barnacles, isopods, copepods) (Fig ) a. Habitat: Most are aquatic (freshwater or marine). b. Morphology: The body is divided into the cephalothorax and abdomen. Many have a carapace. They have two pairs of antennae and sophisticated compound eyes. c. Feeding (1) Barnacles and shrimp are suspension feeders that use feathery appendages to capture their prey. (Fig ) (2) Crabs and lobsters are active hunters, herbivores, and scavengers that use mouthparts called mandibles to bite or chew. d. Movement: Crustaceans swim, walk, or run using highly diverse limbs. (1) Barnacles are the only sessile crustaceans. e. Reproduction: They use sexual reproduction with internal fertilization. Many species have a distinct larval stage called a nauplius.

8 Chapter Vocabulary To emphasize the functional meanings of these terms, the list is organized by topic rather than by first occurrence in the chapter. It includes terms that may have been introduced in earlier chapters but are important to the current chapter as well. It also includes terms other than those highlighted in bold type in the chapter text protostome gastrulation coelom lophophore trochophore larva cuticle exoskeleton molting triploblastic bilateral symmetry tube-within-a-tube design hydrostatic skeleton tagma (plural: tagmata) head thorax abdomen jointed limb chitin hemocoel foot visceral mass mantle mantle cavity proboscis adaptation siphon jet propulsion parthenogenesis internal fertilization external fertilization ovoviviparity Lophotrochozoa/lop hotrochozoans Rotifera (rotifers) corona direct development Platyhelminthes (flatworms) Turbellaria Cestoda (tapeworms) Trematoda (flukes) definitive host intermediate host Annelida (segmented worms) Polychaeta polychaetes Clitellata chaeta (plural: chaetae) parapodium (plural: parapodia) Oligochaeta oligochaetes Hirudinea (leeches) Sipunculida (peanut worms) Echiura (spoon worms) Mollusca/mollusks Bivalvia/bivalves gill siphon veliger Gastropoda/gastrop ods radula Polyplacophora (chitons) Cephalopoda/cephal opods tentacle beak spermatophore Ecdysozoa ecdysozoans Onychophora (velvet worms) Tardigrada (water bears) detritivore compound eye simple eye antenna (plural: antennae) Nematoda (roundworms) Arthropoda/arthropods Myriapoda/myriapods Insecta/insects Chelicerata chelicerates chelicera (plural: chelicerae) Crustacea/crustaceans carapace mandible nauplius

Chapter 7. Marine Animals Without a Backbone

Chapter 7. Marine Animals Without a Backbone Chapter 7 Marine Animals Without a Backbone Molluscs Characteristics of Phylum: More than 200,000 species Name means "soft body" Basic body plan head, muscular foot and visceral mass in most species Mantle-