Protostome Flashcards

Protostomes

Overview

• Molecular phylogenies support the hypothesis that protostomes are a monophyletic group, divided into two major subgroups: Lophotrochozoa and Ecdysozoa.
• Many protostome phyla have limbless, wormlike bodies and live in marine sediments.
• The most diverse and species-rich lineages—Mollusca and Arthropoda—have body plans with a series of distinctive, complex features.
• Key events triggered the diversification of protostomes:
  • Several lineages making the water-to-land transition.
  • Diversification in appendages and mouth parts.
  • The evolution of metamorphosis in both marine and terrestrial forms.
• Protostomes include some of the most familiar and abundant animals on Earth, such as arthropods (including insects, spiders, and crustaceans) and molluscs (including snails, clams, octopuses, and squids).
• Most animals are protostomes.

Protostome vs. Deuterostome Development

• Cleavage:
  • Protostomes: Spiral cleavage.
  • Deuterostomes: Radial cleavage.
• Gastrulation:
  • Protostomes: Pore becomes the mouth.
  • Deuterostomes: Pore becomes the anus.
• Coelom Formation:
  • Protostomes: Block of solid mesoderm splits to form the coelom.
  • Deuterostomes: Mesoderm pockets pinch off of the gut to form the coelom.

Major Protostome Phyla

• Arthropoda (insects, spiders, crustaceans)
• Mollusca (clams, snails, octopuses)
• Annelida (segmented worms)
• Platyhelminthes (flatworms)
• Nematoda
• Ectoprocta

Non-Protostome Phyla

• Porifera
• Cnidaria
• Echinodermata
• Chordata

Protostome Evolution

• Protostomes are bilaterally symmetric, triploblastic, coelomate animals.
• Protostomes are a monophyletic group comprising two major lineages: Lophotrochozoa and Ecdysozoa.
• There are 22 phyla of protostomes, but the eight major phyla account for about 99.5% of the known species.

Lophotrochozoa vs. Ecdysozoa

• Protostomes split into the Lophotrochozoa and Ecdysozoa lineages after the protostome pattern of development evolved.

Coelom Variations

• Coelom.
• Reduced coelom.
• Pseudocoelom.
• No coelom.

Table 32.1: Overview of Major Animal Phyla

Protostomes (basal group)

• Chaetognatha (arrow worms): ~100 species

Protostomes: Lophotrochozoa

• Rotifera (rotifers): ~1,800 species
• Platyhelminthes (flatworms): ~20,000 species
• Nemertea (ribbon worms): ~900 species
• Gastrotricha (gastrotrichs): ~450 species
• Acanthocephala (acanthocephalans): ~1,100 species
• Entoprocta (entroprocts): ~150 species
• Gnathostomulida (gnathostomulids): ~80 species
• Sipuncula (peanut worms): ~320 species
• Echiura (spoon worms): ~135 species
• Annelida (segmented worms): ~16,500 species
• Mollusca (molluscs): ~94,000 species
• Phoronida (horseshoe worms): ~20 species
• Ectoprocta (ectoprocts): ~4,500 species
• Brachiopoda (brachiopods; lamp shells): ~335 species

Protostomes: Ecdysozoa

• Nematoda (roundworms): ~25,000 species
• Kinorhyncha (kinorhynchs): ~150 species
• Nematomorpha (hair worms): ~320 species
• Priapula (priapulans): ~16 species
• Onychophora (velvet worms): ~110 species
• Tardigrada (water bears): ~800 species
• Arthropoda (arthropods): ~1,100,000 species

Lophotrochozoa

• The 14 phyla of lophotrochozoans include molluscs, annelids, and flatworms.
• The name lophotrochozoan was inspired by the presence of a feeding structure called a lophophore and a type of larva called a trochophore; however, only some phyla have these morphological traits.
  • A lophophore is a specialized structure that rings the mouth of these animals and functions in suspension feeding.
  • Trocophores are a type of larva common to several phyla of lophotrochozoa.

Lophophores

• Function in suspension feeding in adults.

Trochophore Larvae

• Swim and feed.
• Cilia are used in locomotion and feeding.

Ecdysozoa

• The primary contrast between lophotrochozoans and ecdysozoans involves their methods of growth.
• Ecdysozoans grow by molting—shedding of the soft cuticle or hard exoskeleton.
• The cuticle and exoskeleton serve to protect these animals from predators.
• The most prominent of the seven ecdysozoan phyla are the roundworms (Nematoda) and the arthropods (Arthropoda).

Diversification of Protostomes

• Protostomes have diverged into 22 different phyla that are recognized by distinctive body plans or specialized mouthparts.
• This diversification was triggered by evolutionary innovations in body plan, feeding, moving, and reproducing.

Basic Body Plan

• Most protostome phyla have wormlike bodies with a basic tube-within-a-tube design.
  • Body wall derived from ectoderm.
  • Muscles and organs derived from mesoderm.
  • Gut derived from endoderm.

Coelom Formation

• Radical changes occurred in coelom formation as protostomes diversified.
• For example, platyhelminthes (flatworms) are acoelomate, lacking a body cavity.

Pseudocoelom

• The pseudocoelom, which forms from an opening that originates between the ectoderm and mesoderm layers, arose independently in rotifers and ecdysozoans.
• In the Arthropoda and Mollusca, the coelom is drastically reduced.
• A fully functioning coelom has two roles:
  • Providing space for fluids to circulate among organs.
  • Providing a hydrostatic skeleton for movement.
• In arthropods and molluscs, other structures fulfill these functions.

The Arthropod Body Plan

• Arthropods have segmented bodies that are organized into prominent regions, usually called the head, thorax, and abdomen.
• Arthropods are distinguished by their jointed limbs and an exoskeleton made of chitin.
• Arthropod locomotion is based on muscles that apply force against the exoskeleton to move legs or wings.
• Arthropods have a spacious body cavity called the hemocoel that provides space for internal organs and the circulation of fluids.

The Molluscan Body Plan

• The molluscan body plan is based on three major components:
  • The foot: a large muscle located at the base of the animal and usually used in movement.
  • The visceral mass: the region containing most of the internal organs and external gill.
  • The mantle: a tissue layer that covers the visceral mass and secretes a shell in some species.
• In molluscs, the coelom's functions are replaced by the visceral mass and the muscular foot.

The Water-to-Land Transition

• The ability to live in terrestrial environments evolved independently in arthropods, molluscs, roundworms, and annelids.
• The protostome groups that made the water-to-land transition already had hydrostatic skeletons, exoskeletons, appendages, or other adaptations for support and locomotion that happened to work on land as well as in water, facilitating the transition.
• To make the transition to land, new adaptations allowed protostomes to exchange gases and avoid drying out.

Adaptations for Moving

• In protostomes, variation in movement depends on variation in the presence or absence of limbs and the type of skeleton present.
• Protostome movement includes walking, running, jumping, flying, gliding, crawling, and jet propulsion.

Reproduction

• Protostomes can reproduce asexually or sexually, although sexual reproduction is the predominant mode in most groups.
• Asexual reproduction is common in many wormlike phyla. Many crustacean and insect species can reproduce asexually by parthenogenesis, in which unfertilized eggs develop into offspring.
• Sexual reproduction starts with external fertilization in sessile forms, and internal fertilization in groups that can move.
• Two unique reproductive innovations occurred during protostome diversification: the evolution of metamorphosis and an egg that would not dry out on land.

Key Lophotrochozoan Groups

• The Lophotrochozoa are a monophyletic group that are highly diverse in morphology.

Major Lophotrochozoans

• Rotifera
• Platyhelminthes
• Annelida
• Mollusca

Rotifera (Rotifers)

• The 1800 rotifer species identified thus far live in damp soils as well as marine and freshwater environments.
• Rotifers have a cluster of cilia at their anterior end called a corona that is used for suspension feeding.
• Most rotifers swim by the beating of the cilia in the corona.

Platyhelminthes (Flatworms)

• The 20,000 species in this group belong to one of three major subgroups:
  • Turbellarians: Free-living flatworms.
  • Cestodes: Endoparasitic tapeworms.
  • Trematodes: Endo- or ectoparasitic flukes.
• Flatworms have a broad, flattened body shape with a large surface area for gas exchange.
• Flatworms lack a lophophore and have a digestive tract with only one opening for ingestion of food and elimination of wastes.

Annelida (Segmented Worms)

• Annelids have a segmented body plan and a coelom that functions as a hydrostatic skeleton.
• The 16,500 species that have been described so far can be divided into two major lineages:
  • Polychaeta
  • Clitellata

Polychaeta

• Named for their numerous, bristle-like extensions called chaetae.
• They are mostly marine, and can range from 1 mm to 3.5 m in size.

Clitellata

• Composed of the oligochaetes and leeches.
• Oligochaetes include the earthworms, which burrow in moist soils.
• Leeches live in freshwater as well as marine habitats.

Mollusca (Molluscs)

• Molluscs are by far the most species-rich (93,000 to date) and morphologically diverse group in the Lophotrochozoa.
• Most molluscs live in marine environments, although there are some terrestrial and freshwater forms.
• The four most important lineages are:
  • Bivalves
  • Gastropods
  • Chitons
  • Cephalopods

Bivalvia (Clams, Mussels, Scallops, Oysters)

• The bivalves have two separate shells, made of calcium carbonate, that are hinged.
• They are suspension feeders.
• Most bivalves live in the ocean, although there are freshwater forms.
• Only external sexual reproduction occurs in bivalves.

Gastropodia (Snails, Slugs, Nudibranchs)

• About 70,000 species are known; there are both terrestrial and marine gastropods.
• These organisms have a large muscular foot on their ventral side, and many lack shells. During gastropod development, a process called torsion rotates the visceral mass.
• Gastropods and other molluscs have a unique structure in their mouths called a radula to scrape away food.
• Most gastropod reproduction is sexual.

Polyplacophora (Chitons)

• Chitons have eight calcium carbonate plates along their dorsal side that form a protective shell.
• The approximately 1000 species of chitons are marine. They are usually found in the intertidal zone.
• Reproduction is sexual and external; many chiton species have trochophore larvae.

Cephalopoda (Nautilus, Cuttlefish, Squid, Octopuses)

• The cephalopods have a well-developed head and a foot that is modified to form long, muscular tentacles.
• Except for the nautilus, cephalopods have either highly reduced shells or none at all.
• They also have large brains and eyes with sophisticated lenses. They are highly intelligent predators that hunt by sight and use their tentacles to capture prey.
• Cephalopods have a radula and a beak that can exert powerful biting forces.
• Reproduction is sexual and internal via a spermatophore packet.

Key Ecdysozoan Groups

• Ecdysozoans are a monophyletic group with seven currently recognized phyla. The focus is on the four major ones.

Major Ecdysozoans

• Onychophora
• Tardigrada
• Nematoda
• Arthropoda

Onychophora and Tardigrada

• The onychophorans and the tardigrades are similar to arthropods in having a segmented body and limbs. However, their limbs are not jointed and they do not have an exoskeleton.
• The onychophorans, or velvet worms, are small, caterpillar-like organisms that live in moist leaf litter and prey on small invertebrates.
• The tardigrades, or water bears, are microscopic animals that live in bottom habitats of marine or freshwater environments. Most feed by sucking fluids from plants or animals.

Nematoda (Roundworms)

• Of the 25,000 species of nematodes, most are free-living, although a few are parasitic and cause disease in humans. They are found in virtually every habitat, often in great abundance.
• Nematodes are unsegmented worms with a pseudocoelom, a tube-within-a-tube body plan, and no appendages.
• The nematode Caenorhabditis elegans is one of the thoroughly studied model organisms.

Arthropoda (Arthropods)

• In terms of duration, species diversity, and abundance, arthropods are the most successful lineage of eukaryotes.
• Their phylogeny is still being worked out.
• They are distinguished by segmented bodies and complex, jointed exoskeletons. They are organized into head and trunk regions; at least some trunk segments produce paired, jointed appendages.
• Metamorphosis is common in arthropods. The larval and adult forms grow by molting.

Arthropoda: Myriapods (Millipedes, Centipedes)

• The 11,600 species of myriapods inhabit terrestrial environments all over the world.
• Myriapods have relatively simple bodies with a series of short segments, each with one or two pairs of legs.
• Myriapod sexes are separate, and fertilization is internal; females lay eggs.

Chelicerata (Spiders, Ticks, Mites)

• Most of the 70,000 species of chelicerates are terrestrial, although the horseshoe crabs and sea spiders are marine.
• The most prominent chelicerate lineage is the arachnids (spiders, scorpions, mites, and ticks).
• The chelicerates are named for appendages called chelicerae found near the mouth that are used in feeding, defense, copulation, movement, or sensory reception.

Insecta (Insects)

• About 925,000 species of insects have been named thus far, and many more are certain to exist. In terms of species diversity and abundance, insects dominate terrestrial environments.
• Insects are distinguished by having head, thorax, and abdomen body regions, with three pairs of walking legs on the ventral side of the thorax. In most species, two pairs of wings are mounted on the dorsal side of the thorax.
• Typically the head contains a total of four sets of mouthparts, a pair of antennae, and a pair of compound eyes.

Prominent Orders of Insects

• Coleoptera (Beetles):
  • ~350,000 known species.
  • Key traits: Hardened forewings (elytra) protect membranous hindwings.
  • Complete metamorphosis.
• Lepidoptera (Butterflies, Moths):
  • ~180,000 known species.
  • Key traits: Wings covered in scales.
  • Complete metamorphosis.
• Diptera (Flies):
  • ~120,000 known species.
  • Key traits: Reduced hindwings (halteres) for flight stabilization.
  • Complete metamorphosis.
• Hymenoptera (Ants, Bees, Wasps):
  • ~115,000 known species.
  • Key traits: Membranous wings locked together during flight; often have a "wasp-waist."
  • Complete metamorphosis. Males are haploid, females are diploid. Some are eusocial.
• Hemiptera (Bugs):
  • ~85,000 known species.
  • Key traits: Thickened forewing with membranous tip; piercing/sucking mouthparts.
  • Incomplete metamorphosis.
• Orthoptera (Grasshoppers, Crickets):
  • ~20,000 known species.
  • Key traits: Large hind legs for jumping.
  • Incomplete metamorphosis.
• Trichoptera (Caddisflies):
  • ~12,000 known species.
  • Key traits: Hairy wings.
  • Complete metamorphosis. Larvae are aquatic and build protective cases.
• Odonata (Dragonflies, Damselflies):
  • ~6,500 known species.
  • Key traits: Four membranous wings; long, slender abdomens.
  • Incomplete metamorphosis. Predatory larvae and adults.

Crustaceans (Shrimp, Lobster, Crabs)

• The 67,000 species of crustaceans identified to date live primarily in marine and freshwater environments.
• Crustaceans have a segmented body divided into the cephalothorax (head and thorax) and the abdomen.
• Many crustaceans have a carapace, a platelike section of their exoskeleton that covers and protects the cephalothorax.