Animals 2 - Protostomes

Overview of Protostomes

Protostomes are a monophyletic group, divided into Lophotrochozoa and Ecdysozoa.

Many protostomes are limbless, wormlike, and live in marine sediments, but Mollusca and Arthropoda are the most diverse.

Protostome diversification is driven by the water-to-land transition, the evolution of appendages, mouth parts, and metamorphosis.

Includes familiar and abundant animals like arthropods (insects, spiders, crustaceans) and molluscs (snails, clams, octopuses, squids).

Most animals on Earth are protostomes.

Protostome Evolution

Protostomes are bilaterally symmetric, triploblastic, and coelomate animals.

Protostomes form a monophyletic group with two major lineages: Lophotrochozoa and Ecdysozoa.

Lophotrochozoans

The 14 phyla of lophotrochozoans include molluscs, annelids, and flatworms.

The name "lophotrochozoan" is inspired by two features:

• Lophophore: A specialized structure around the mouth used for suspension feeding.

• Trochophore: A type of larva found in some lophotrochozoan phyla.

Ecdysozoans

Growth method: Ecdysozoans grow by molting—shedding of their soft cuticle or hard exoskeleton.

Protection: The cuticle and exoskeleton serve to protect them from predators.

Prominent Phyla:

• Nematoda (roundworms)

• Arthropoda (arthropods, including insects, spiders, crustaceans).

Basic Body Plan of Protostomes

Coelom Formation: Radical changes occurred in coelom formation as protostomes diversified.

• Example: Platyhelminthes (flatworms) are acoelomates, meaning they lack a body cavity.

Variation in Body Plans among Protostome Phyla

Pseudocoelom: Forms independently in rotifers and ecdysozoans, originating between the ectoderm and mesoderm layers.

Arthropoda and Mollusca: Coelom is drastically reduced.

Coelom Functions:

• Circulates fluids among organs.

• Acts as a hydrostatic skeleton for movement.

In arthropods and molluscs, other structures fulfill these functions instead of a fully functional coelom.

Arthropod Body Plan

• Segmentation: Body organized into head, thorax, and abdomen regions.

• Jointed Limbs & Exoskeleton: Made of chitin.

• Locomotion: Muscles apply force against exoskeleton to move legs or wings.

• Hemocoel: Spacious body cavity for internal organs and circulation of fluids.

Molluscan Body Plan

• Foot: Large muscle at the base, used for movement.

• Visceral Mass: Contains most internal organs and external gills.

• Mantle: Tissue layer that covers the visceral mass and secretes a shell in some species.

• Coelom Function: Replaced by the visceral mass and muscular foot.

The Water-to-Land Transition

• Independent Evolution: Arthropods, molluscs, roundworms, and annelids evolved the ability to live on land.

• Pre-existing Adaptations: Hydrostatic skeletons, exoskeletons, appendages, or other support and locomotion structures that worked both in water and on land.

• New Adaptations: Evolved for gas exchange and preventing desiccation.

Adaptations for Moving

• Movement Variation: Depends on limbs and skeleton type.

• Types of Movement: Walking, running, jumping, flying, gliding, crawling, and jet propulsion.

Reproduction in Protostomes

• Reproduction: Asexual (common in worms) and sexual (predominant in most groups).

• Parthenogenesis: Unfertilized eggs develop into offspring (common in some crustaceans and insects).

• Fertilization: External (in sessile forms) and internal (in mobile forms).

• Innovations: Metamorphosis and eggs that do not dry out on land.

Lophotrochozoans

are a monophyletic group that are highly diverse in morphology.

Rotifera (Rotifers)

• Habitat: 1,800 species found in damp soils, marine, and freshwater environments.

• Unique Feature: They have a corona, a cluster of cilia at the anterior end, used for suspension feeding.

• Movement: Most rotifers swim by beating the cilia in the corona.

Platyhelminthes (Flatworms)

Species Count: 20,000 species.

Subgroups:

• Turbellarians: Free-living flatworms.

• Cestodes: Endoparasitic tapeworms.

• Trematodes: Endo- or ectoparasitic flukes.

Platyhelminthes (Flatworms) Body Characteristics

• Body Shape: Broad, flattened body for large surface area.

• Gas Exchange: Large surface area facilitates gas exchange.

• Digestive System: One opening for both ingestion and elimination (no lophophore).

Annelida (Segmented Worms)

Body Plan: Segmented body and coelom acting as a hydrostatic skeleton.

Major Lineages:

• Polychaeta: Marine, bristle-like extensions called chaetae, range from 1 mm to 3.5 m in size.

• Clitellata: Includes oligochaetes (earthworms, burrow in soils) and leeches (found in freshwater and marine habitats).

Mollusca (Molluscs)

Species-rich & Diverse: 93,000 species, highly diverse in morphology.

Habitat: Mostly marine, with some terrestrial and freshwater forms.

Key Lineages:

• Bivalves

• Gastropods

• Chitons

• Cephalopods

Bivalvia (Clams, Mussels, Scallops, Oysters)

• Shells: Two hinged shells made of calcium carbonate.

• Feeding: Suspension feeders.

• Habitat: Mostly oceanic, with some freshwater species.

• Reproduction: Only external sexual reproduction occurs.

Gastropodia (Snails, Slugs, Nudibranchs)

• Species: About 70,000 species, both terrestrial and marine.

• Body: Large muscular foot on the ventral side, many lack shells.

• Development: Torsion rotates the visceral mass during development.

• Feeding: Radula (mouth structure) used to scrape food.

• Reproduction: Mostly sexual.

Polyplacophora (Chitons)

• Shell: Eight calcium carbonate plates along the dorsal side for protection.

• Species: Approximately 1000 species, marine, typically found in the intertidal zone.

• Reproduction: Sexual and external; many species have trochophore larvae.

Cephalopoda (Nautilus, Cuttlefish, Squid, Octopuses)

• Body Features: Well-developed head, muscular tentacles (modified foot), and large brains.

• Shell: Reduced or absent (except for the nautilus).

• Behavior: Intelligent predators, use sight to hunt and tentacles to capture prey.

• Eyes: Sophisticated lenses for advanced vision.

Cephalopoda (Nautilus, Cuttlefish, Squid, Octopuses)

• Body Structure: Well-developed head; foot modified into long, muscular tentacles.

• Shell: Reduced or absent (except in nautilus).

• Nervous System: Large brain; complex eyes with sophisticated lenses.

• Feeding: Radula and powerful beak; active predators that hunt using vision.

• Reproduction: Sexual and internal via spermatophore (packet of sperm).

Key Ecdysozoan groups - Onychophora & Tardigrada

Similarities to Arthropods: Segmented bodies & limbs, but no jointed limbs or exoskeleton.

Onychophora (Velvet Worms):

• Caterpillar-like

• Moist leaf litter

• Predators of small invertebrates

Tardigrada (Water Bears):

• Microscopic

• Marine/freshwater bottom dwellers

• Feed by sucking fluids from plants or animals

Key Ecdysozoan groups - Nematoda (Roundworms)

• ~25,000 species, mostly free-living, some parasitic

• Ubiquitous – found in nearly every habitat

• Unsegmented, pseudocoelomate, no appendages

• Body plan: tube-within-a-tube

• Model organism: Caenorhabditis elegans

Key Ecdysozoan groups - Arthropoda (Arthropods)

• Most successful eukaryotic lineage

• Segmented bodies, jointed exoskeleton

• Body: head + trunk; trunk may have paired, jointed appendages

• Metamorphosis common; grow by molting

Arthropoda: Myriapods (Millipedes, Centipedes)

• ~11,600 species; all terrestrial

• Body: short segments, each with 1 or 2 pairs of legs

• Separate sexes, internal fertilization, eggs laid by female

Chelicerata (Spiders, Ticks, Mites)

• ~70,000 described species, mostly terrestrial

• Includes horseshoe crabs and sea spiders (marine)

• Most prominent group: arachnids (spiders, scorpions, mites, ticks)

Characterized by chelicerae: specialized appendages near the mouth used for feeding, defense, reproduction, movement, or sensing

Insecta (Insects)

~925,000 species named, many more undiscovered

Dominant group in terrestrial environments by species diversity and abundance

Body regions: head, thorax, and abdomen

Three pairs of walking legs on the ventral side of the thorax

Two pairs of wings on the dorsal side of the thorax (in most species)

Head typically contains:

• Four sets of mouthparts

• One pair of antennae

• One pair of compound eyes

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 (which combines the head and thorax) and the abdomen.

Many crustaceans have a carapace, a platelike section of their exoskeleton that covers and protects the cephalothorax.