Lecture 15 Vertebrates, Invertebrates, Bilateria, and Chordate Features 2

Course logistics (brief recap)

• Essay due on Thursday; grace period until Wednesday of next week. Lab on Friday for Tauranga. Tests handed back on Friday with printed copies available for discussion.
• Today’s focus: vertebrates and their invertebrate relatives; build from last week to more familiar organisms.

Learning outcomes (from the lecture)

• Summarize invertebrate diversity: how diverse are invertebrates and whether diversity is uniform across phyla or concentrated in certain lineages.
• Identify the general characteristics of major invertebrate groups (the lecture covers five/six phyla): Lophotrochozoa (low photrochozoans), Mollusca, Annelida, Ecdysozoa (nematodes and arthropods), and Echinodermata.
• Recognize four key features of chordates (vertebrates are a subset).

Quick context: animal diversity and early evolutionary history

• There are about $36$ extant animal phyla; about $15$ are shown in the referenced figure.
• All animals share a common ancestor (clad/monophyly concept) roughly $7.7 imes 10^8$ years ago (770 million years ago).
• Sponges (Porifera) are basal animals and lack true tissues; they are not eumetazoans, which do have true tissues. Eumatozoa are animals with true tissues and organized structures.
• Eumetazoans are typically bilaterians (bilateral symmetry at least during some life stage); some lineages are radial in the adult (e.g., cnidarians) but bilateral in development can occur in other groups.
• Not all animals are bilaterians; some cnidarians remain diploblastic (two germ layers: ectoderm and endoderm) rather than triploblastic (ectoderm, mesoderm, endoderm).
• Protostomes vs Deuterostomes (developmental patterns):
  • Protostomes: mouth first; spiral, determinate cleavage.
  • Deuterostomes: anus first; radial, indeterminate cleavage.
• Protostomes are not unanimously considered a monophyletic group in all analyses, whereas deuterostomes + protostomes together form the bilaterian clade; the embryology data contribute to ongoing debates about exact ancestry.

Major developmental and structural themes to track

• True tissues vs non-tissues (sponges lack true tissues; eumetazoans have tissues).
• Symmetry: radial (e.g., many cnidarians, echinoderm larvae) vs bilateral (most bilaterians).
• Germ layers: ectoderm, mesoderm, endoderm (three germ layers) vs diploblastic (two germ layers).
• The focus for today’s lecture is on bilaterians and, within them, the major invertebrate groups and the deuterostome/chordate lineage.

The three broad bilaterian groups discussed today (focus of the lecture)

• Deuterostomia (one major branch): includes echinoderms and chordates; embryos show radial cleavage and include the notochord/chordate features at later stages.
• Protostomes (the other major branch): includes two large clades—Lophotrochozoa and Ecdysozoa. Protostomes form the mouth first and have spiral cleavage (determinant cell fate).
• The two groups emphasized for diversity and body plans: Lophotrochozoa (includes flatworms, rotifers, mollusks, annelids, and lophophorates like brachiopods) and Ecdysozoa (nematodes and arthropods; the latter includes insects, crustaceans, arachnids). Insects are the most diverse group by far within arthropods, driving the numeric dominance of Ecdysozoa.

Lophotrochozoa: broad diversity and representative phyla

• Definition: a major clade identified through molecular data; includes a wide range of body plans with a variety of larval forms and sometimes shared feeding structures.
• Key idea: widest range of body shapes among animal phyla.
• Representative groups to know (from lecture): flatworms, rotifers, lamp shells (brachiopods; often grouped with lophophorates), mollusks, and annelids.

Flatworms (Platyhelminthes)

• Notable for being extremely flat; body plan has high surface area to volume ratio (SA:V), which supports efficient nutrient/gas exchange across the body surface.
• Many species are flat and only a few cells thick; this efficiency is advantageous in moist environments.
• Some flatworms are parasites (e.g., blood flukes).
• Blood fluke life cycle (illustrative):
  • Start: parasite resides in human intestinal wall; male and female pair mate and feed on nutrients.
  • Eggs are expelled in feces, reach water, and hatch into larvae that infect an intermediate host (snail).
  • In snails, larvae undergo asexual multiplication and development.
  • Larvae then re-infect humans (or the final host) and complete the cycle; humans are the definitive host for sexual reproduction.
• Moist environments are essential for their life cycles and moisture helps gas/nutrient exchange.

Rotifers

• Microscopic; appear unicellular at a glance but are metazoans with multicellular organization.
• Occupy freshwater and marine environments; notable for their compact body plans and often striking reproductive biology.

Lamp shells / Brachiopods (often grouped with Lophophorata)

• Referenced as “lampshadows” in the lecture; typical shell-bearing, sessile marine organisms with a lophophore feeding structure.

Mollusks (Mollusca)

• General body plan: three main parts — foot (muscular), visceral mass (internal organs), mantle (often secretive shell).
• Most mollusks have a calcareous shell produced by the mantle; some lineages (e.g., many slugs) have lost the shell.
• Diversity and common groups:
  • Gastropods: most diverse subgroup; about $3/4$ of living mollusks; many marine, but also freshwater and terrestrial.
  • Bivalves: two calcareous shells joined by a hinge; clams, mussels, oysters, etc.; rely on a muscular foot for movement and a siphon-based or gill-based feeding system.
  • Cephalopods: highly intelligent and behaviorally complex (octopus, squid, nautilus); large brains and sophisticated nervous systems; shells may be reduced or absent in many species.
• Key points:
  • Most mollusks are marine; mollusks are a major group with significant ecological services in marine and freshwater ecosystems.
  • Mollusks face anthropogenic pressures (pollution, sedimentation, fishing bycatch); mollusks account for a sizable share of recorded extinctions—about $40 ext{ extsuperscript{th}}$ of extinctions documented so far (note: this is a reflection of recorded data and ongoing monitoring is needed).
  • In cephalopods, notable features include advanced nervous systems and the ability to rapidly change color/patterns; body sizes range from tiny to very large (e.g., giant squid up to $13 ext{ m}$ in length).
• Ecological note: mollusks provide many ecosystem services (filter feeding, reef-building, bioturbation in some cases) but are pressured by human activity.

Annelids (Annelida)

• Segmented worms with repeated body units (annuli); body segmented along its length.
• Major lifestyle groups: errantia (mobile, often marine worms that move actively) and sedentaria (more sedentary, often with filtering behaviors or tube-dwelling lifestyles).
• Structure: a primitive brain (ganglion) and a complex internal organ system; highly developed digestive and circulatory systems in some taxa.
• Ecological roles: earthworms are key soil engineers and bioturbators, enhancing soil structure and nutrient cycling; marine annelids mix sediments and contribute to oxygenation of deeper layers.
• Notable anatomy notes: segmented with repeated structures; well-developed nervous system and circulatory networks enabling sophisticated locomotion and feeding strategies.

Ecdysozoa: molting and the most abundant animal group

• Definition: ecdysozoans are characterized by molting (ecdysis) in their life cycle, which provides a mechanism for growth by shedding the outer cuticle.
• Overall abundance and diversity: ecdysozoa is the most abundant and diverse animal group; nematodes and arthropods are its two major lineages.
• Key fact: ecdysozoans collectively surpass all other animal groups in sheer numbers of species and individuals.

Nematodes (Nematoda)

• Extremely diverse and abundant group; many species described, and many more likely undiscovered.
• Count note from lecture: about $2.5 imes 10^{4}$ described nematode species; the actual diversity is far greater.
• Ecology: occupy virtually every environment (soil, freshwater, marine), including extreme habitats; many are free-living, others are parasitic.
• Importance: nematodes are foundational for soil ecology and nutrient cycling; some are important parasites of plants and animals.

Arthropods (Arthropoda)

• Largest animal phylum by far; estimated to contain on the order of $10^{18}$ individuals at a global scale
  • The lecture notes describe this estimate as “a billion billion,” i.e., $10^{18}$ individuals, illustrating extraordinary abundance.
• Key features: segmented bodies; hard exoskeletons (often made of chitin); jointed appendages; extensive specialization and diversification across habitats.
• Insects (Hexapoda) dominate arthropod diversity:
  • Insects are hexapods (six legs) with three body segments: head, thorax, and abdomen.
  • Insects typically have a ventral nerve cord (in contrast to a dorsal nerve cord in vertebrates) and a primitive brain (cerebral ganglion).
  • Respiratory system in insects uses tracheal tubes and air-filled sacs rather than lungs; gas exchange occurs largely via diffusion aided by the tracheal system.
  • The thorax, head, and abdomen are distinct segments; Hox genes control body segment identity and leg placement (e.g., legs primarily on the thorax, not on the head).
• Why insects are so abundant:
  • They occupy nearly every habitat and have diversified into countless ecological roles (pollinators, decomposers, predators, prey, etc.).
• Notable anatomical features:
  • Three body regions: head, thorax, abdomen.
  • Ventral nerve cord running along the ventral side.
  • A cerebral ganglion (simple brain) and a segmented, largely externalized nervous system.
  • Gas exchange via tracheae and spiracles; no lungs in the typical sense.
• A broader evolutionary note: insects are more closely related to crustaceans than to spiders and other chelicerates; this reflects a surprising evolutionary relationship within arthropods.

Insects: a quick anatomy snapshot (in the context of the lecture)

• Three main body parts: head, thorax, abdomen.
• Hexapod designation: six legs; a defining characteristic of insects.
• Nerve system anchor: ventral nerve cord with a central brain (cerebral ganglion).
• Respiratory system: tracheal tubes and air sacs; gas exchange largely via diffusion.
• The “Hox gene” context (referenced from last week): these developmental genes help determine where legs and other appendages form along the body segments.
• A practical note: insect body plan has influenced many other animal groups and remains a fundamental example of segmented animal design and modular development.

Echinoderms (Echinodermata): deuterostome lineage with radial adults

• Echinoderms are deuterostomes (relatives of chordates in the deuterostome clade).
• Developmental pattern: larval stages are bilateral, but adults often show radial symmetry (fivefold, pentaradial symmetry) in many species.
• Key structural features: water vascular system and tube feet used for locomotion and feeding; unique calcareous endoskeleton.
• Representative groups and forms:
  • Sea stars (asteroids): typically predatory or scavenging; many species can regenerate lost arms (e.g., in New Zealand there are sea stars capable of regrowing arms).
  • Brittle stars (ophiuroids): central disc with long, flexible arms; often highly agile and capable of rapid movement.
  • Sea urchins and sand dollars: five-part articulation; rigid test composed of fused plates.
  • Sea lilies (crinoids): stalked, feather-like arms that collect food.
  • Sea cucumbers (holothurians): elongated bodies with a reduced skeleton and five-part symmetry; move using tube feet interspersed along the body.
• Ecological roles: echinoderms occupy a range of niches from predators to suspension feeders; their tube feet and water vascular system enable diverse feeding strategies.

Chordates: four key diagnostic features (the vertebrate lineage starts here)

• Four hallmark features shared by all chordates at some stage of development:
  • Notochord: a flexible, longitudinal support structure; in vertebrates, largely replaced by the vertebral column, but the notochord is essential in early development.
  • Dorsal hollow nerve cord: develops into the spinal cord and brain in vertebrates.
  • Pharyngeal slits or clefts: openings in the throat region; in some vertebrates they develop into gills, ears, or other structures in specialized contexts.
  • Post-anal tail: a tail that extends beyond the anus during some stage of development.
• Progressive vertebrate innovations (illustrative, not exhaustive):
  • Jaws evolved (calcareous jaws in early gnathostomes) enabling stronger predation and processing of food.
  • Lungs evolved for more efficient respiration and energy availability.
  • Limbs with digits evolved for improved locomotion and manipulation of the environment.
  • Amniotic eggs evolved to allow reproduction away from water, enabling colonization of dry habitats.
  • Mammary glands evolved for nurturing the young with high-energy nutrition.
• Note: While all chordates share the four key features, vertebrates are a subset; not all chordates retain all features into adulthood (e.g., some sea squirts retain larval features only during larval stages).

Four key takeaways about diversity and phylogeny (recap of the big picture)

• All animals (the Metazoa) share a common ancestor; sponges are basal and lack true tissues, while eumetazoans have true tissues and organized structures.
• Most animals are bilaterians; bilateral symmetry confers functional advantages for movement and predation; some groups retain radial symmetry as adults.
• The bilaterian lineage splits into two major developmental pathways: deuterostomes and protostomes, with protostomes further split into two large groups: Lophotrochozoa and Ecdysozoa.
• Diversity is especially high in invertebrates, with mollusks and arthropods among the most speciose and ecologically important groups; vertebrates are comparatively less diverse in terms of the number of phyla but are highly diverse in form and function within the chordate lineage.

Connections to earlier lectures and real-world relevance

• Developmental biology concepts revisited: radial vs spiral cleavage, determinate vs indeterminate cell fate, and how embryology informs evolutionary relationships.
• The idea of clades (monophyletic groups) vs competing hypotheses about ancestry (e.g., debates about whether protostomes are monophyletic).
• The ecological and practical importance of invertebrates: mollusks as ecosystem engineers and indicators of environmental health; annelids as soil bioturbators; ecdysozoans in every habitat; echinoderms as model systems for regeneration; insects driving ecological processes and food webs.
• Anthropogenic pressures: mollusks and other invertebrates face pollution, sedimentation, and overfishing; many species are under threat and require monitoring and conservation.

Quick study prompts (from the lecture content)

• How do protostomes differ from deuterostomes in embryo development and cleavage pattern?
• Why do flatworms benefit from a high surface area-to-volume ratio, and what are the implications for their physiology and habitats?
• What makes cephalopods particularly interesting in terms of nervous system complexity and behavior?
• How does the adult radial symmetry of echinoderms relate to their larval bilateral symmetry?
• List the four key chordate features and describe their significance in the development of vertebrates.
• Why is the estimate $10^{18}$ individuals for arthropods used to illustrate their abundance, and what implications does this have for ecology and evolution?
• Explain the life cycle of the blood fluke as described in the lecture and identify the roles of humans and snails in the parasite’s life cycle.

Closing reminder (from the lecturer)

• If there are questions about tests, essays, or lab activities, ask during the designated times; Friday is when printed test copies will be discussed.
• The next sessions will dive deeper into vertebrates and the chordate lineage, building on today’s overview of invertebrate diversity.