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Protostome Animals. 33. Key Concepts. Molecular phylogenies support the hypothesis that protostomes are a monophyletic group divided into two major subgroups: the Lophotrochozoa and the Ecdysozoa.
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Key Concepts • Molecular phylogenies support the hypothesis that protostomes are a monophyletic group divided into two major subgroups: the Lophotrochozoa and the Ecdysozoa. • Although the members of many protostome phyla have limbless, wormlike bodies and live in marine sediments, the most diverse and species-rich lineages—Mollusca and Arthropoda—have bodies with distinctive, complex features. Mollusks and arthropods inhabit a wide range of environments.
Key Concepts • Key events triggered the diversification of protostomes, including several lineages making the water-to-land transition, a diversification in appendages and mouthparts, and the evolution of metamorphosis in both marine and terrestrial forms.
Introduction • Protostomes include some of the most familiar and abundant animals on Earth, such as arthropods (including insects, spiders, and crustaceans) and mollusks (including snails, clams, octopuses, and squids). Most animals are protostomes. • Protostomes also include some of the most important model organisms in all of biological science: the fruit fly Drosophila melanogaster and the roundworm Caenorhabditis elegans.
An Overview of Protostome Evolution • There are two major groups of bilaterally symmetric, triploblastic, coelomate animals: the protostomes and deuterostomes. • Phylogenetic studies have long supported the hypothesis that protostomes are a monophyletic group, meaning the protostome developmental sequence arose just once. • The two monophyletic groups of protostomes are called the Lophotrochozoa and Ecdysozoa.
What Is a Lophotrochozoan? • The 13 phyla of lophotrochozoans include mollusks, 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. 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. • Trochophores are a type of larva common to several phyla of lophotrochozoa.
What Is an Ecdysozoan? • 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).
Themes in the 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.
How Do Body Plans Vary among Phyla? • All protostomes are triploblastic bilaterians and undergo similar embryonic development. • Most protostome phyla have wormlike bodies with a basic tube-within-a-tube design. • The outer tube is the ectoderm-derived skin. • The inner tube is the endoderm-derived gut. • Between the two tubes are mesoderm-derived muscles and organs.
How Do Body Plans Vary among Phyla? • In the wormlike phyla, the coelom is well developed and functions as a hydrostatic skeleton that is the basis of movement. But the coelom is absent in flatworms, and in the most species-rich and morphologically complex protostome phyla—the Arthropoda and the Mollusca (snails, clams, squid)—it is drastically reduced. • A fully functioning coelom: • Provides space for fluids to circulate among organs. • Provides a hydrostatic skeleton for movement. • Other structures fulfill these functions in arthropods and mollusks.
The Arthropod Body Plan • Arthropods have segmented bodies that are organized into prominent regions called tagmata. • The head, thorax, and abdomen are the most common tagmata. • Arthropods are distinguished by their jointed limbs and chitinous exoskeleton. • 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 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, which contains most of the internal organs. • The mantle, a tissue layer that covers the visceral mass and forms the mantle cavity. • In many species the mantle secretes one or more calcium carbonate shells.
Variation among Wormlike Phyla Body Plans • The wormlike protostomes are characterized by similar body plans but specialized mouthparts or feeding structures. For example: • Echiurans (a type of annelid, or segmented worm) deposit feed using a proboscis, an extended structure which forms a gutter leading to the mouth. • Priapulids have a toothed throat that can be turned inside out to grab prey and then retracted. • Nemerteans have a barb-tipped proboscis that extends and spears or entangles the prey and then retracts.
Variation among Wormlike Phyla Body Plans • Once a specific body plan had evolved, subsequent diversification was largely driven by adaptations that allowed protostomes to live on land or feed, move, or reproduce in novel ways. • Adaptations are traits that increase the fitness (reproductive success) of individuals relative to individuals without the trait.
Evidence for Multiple Water-to-Land Transitions • The ability to live in terrestrial environments evolved independently in arthropods, mollusks, roundworms, and annelids. • The evidence for multiple water-to-land transitions in protostomes is based on phylogenetic analyses, which support the hypothesis that the ancestors of the terrestrial lineages in each major subgroup of protostomes were aquatic.
Adaptations to Terrestrial Environments • 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 must allow protostomes to: • Exchange gases. • Avoid drying out. • Terrestrial protostomes have evolved many solutions to these challenges.
Adaptations to Terrestrial Environments • Roundworms and earthworms have a high surface-area-to-volume ratio, which increases the efficiency of gas exchange across their body surface in their moist environments. • Arthropods and many mollusks have gills or other respiratory structures located inside the body, minimizing water loss when moving onto land.
Adaptations to Terrestrial Environments • In mollusks, the mantle cavity that encloses the gills of aquatic snails evolved into the lung found in terrestrial snails. • Insects evolved a waxy layer to minimize water loss from the body surface, with openings to respiratory passages that can be closed if the environment dries. • Water-to-land transitions are important because they open up entirely new habitats and new types of resources to exploit.
Adaptations for Feeding • Protostomes include suspension, deposit, liquid, and mass feeders. • They feed, prey on, and parasitize plants, algae, or other animals. • While all arthropods have the same basic body plan, their mouthparts and food sources are very diverse, ranging from tubes to pincers that allow the various species to pierce, suck, grind, bite, mop, chew, engulf, cut, or mash.
Adaptations for Moving • In protostomes, variation in movement depends on variation in the presence or absence of limbs and the type of skeleton present. • Diversification in locomotion came about because of important evolutionary adaptations.
Adaptations for Moving • Jointed limbs in arthropods enabled rapid, precise running, walking, and jumping. • The insect wing is one of the most important adaptations in the history of life. • About two-thirds of the multicellular species living today are winged insects. • The muscular mollusk foot allows individuals to crawl along a surface. • Jet propulsion in cephalopods propels the animal forward when the mantle muscles force water from the siphon.
Adaptations in Reproduction • Protostomes can reproduce asexually or sexually, although sexual reproduction is the predominant mode in most groups. • Asexual reproduction by splitting the body sideways or fragmenting the body is common in many wormlike phyla. • Many crustacean and insect species can reproduce asexually by parthenogenesis, in which unfertilized eggs develop into offspring.
Adaptations in Reproduction • 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. • An egg that would not dry out on land.
Key Lineages: Lophotrochozoans • The Lophotrochozoa are a monophyletic group that are highly diverse in morphology.
