Zoology: Introduction to Protostomes, Deuterostomes, and Arthropods #4
Developmental Biology: Protostomes and Deuterostomes
Protostomes
The term Proto means "first" and stome means "opening."
Developmentally, this indicates that the first opening (the blastopore) in the embryo becomes the mouth.
The anus is a new evolutionary development in this group of organisms.
Key groups categorized as protostomes include both the Phylum Mollusca (mollusks) and the Phylum Arthropoda (arthropods).
Deuterostomes
The term Deutera means "second"; thus, the mouth is the second opening to form during development.
Developmentally, the first opening formed in the embryo becomes the anus, and the mouth is the new evolutionary development.
This category includes the Phylum Echinodermata (echinoderms) and the Phylum Chordata (chordates).
Most vertebrates, including humans, are members of the chordate group and are therefore deuterostomes.
Phylum Annelida: Diversity and Predatory Behavior
Feeding Diversity
Annelids exhibit a significant range of eating styles, from sedentary filter feeders to active sit-and-wait predators.
Fan worms serve as examples of sedentary (immobile) annelids. They possess large sprays that function as both gills and food collectors to gather organic debris from the ocean floor.
The Bobbit Worm
The Bobbit worm is a highly predatory marine polychaete annelid known for its speed and aggression.
It is not a shallow-water species typically found at beaches like Savannah or Tybee; it resides in deeper marine environments.
Physiology: It lacks eyes but utilizes sophisticated chemoreceptors to sense prey.
Hunting Strategy: It is a sit-and-wait predator that buries itself in the sand with only its mouth parts exposed. These mouth parts features serrated, teeth-like structures.
Prey Capture: When it detects prey, such as a live fish, it strikes with extreme speed, grabbing the victim and pulling it instantly beneath the sand.
Velvet Worms: Ancient Terrestrial Hunters
Characteristics and Habitat
Velvet worms belong to a side phylum (Onychophora) distinct from the "big eight" animal phyla.
They are found in wet, humid forests, such as those in Costa Rica, and typically hunt at night.
Fossil records indicate that velvet worms have remained largely unchanged for approximately years.
Appearance: They have soft, slumpy legs, sensitive feelers (antennae) on the front end, and a non-segmented external body (though they show signs of internal segmentation).
Predatory Mechanics and "Glue"
Velvet worms move in total silence to approach prey like crickets using a master-of-stealth approach.
They possess two nozzles located beneath their feelers.
Attack Method: They squirt twin streams of a sticky, mucus-based "glue" to entangle prey. This substance is highly effective; even human fingers can be stuck together by it, behaving much like super glue.
The more the prey struggles, the more entangled it becomes in the thick mucus.
Phylum Echinodermata: Morphology and Lifestyle
Diversity of Echinoderms
This group includes starfish (sea stars), sand dollars, sea cucumbers, sea urchins, and sea biscuits.
Sea stars commonly have five arms, but some species possess to arms.
Nervous System and Regeneration
They possess a centralized circular brain located in the middle of the body with nerves extending down each tentacle (arm).
Regeneration: Echinoderms can grow a new body from a severed leg, provided that a portion of the central disc (containing the brain) remains attached to the leg. A leg fragment lacking the central disc will not regenerate a new individual.
Feeding Styles: Predators vs. Detritivores
Sea Star (Predatory):
Sea stars use tube feet, which are sucker-like structures, to exert constant pressure on the shells of prey like muscles.
Once a gap is breached in the muscle's shell, the sea star everts its translucent stomach, pushing it inside the shell.
The stomach releases digestive juices (chemical weapons) that dissolve the prey's flesh into a nutrient-rich soup (broth), which the sea star then absorbs.
Sea Cucumber (Detritivore):
Sea cucumbers are detritivores, meaning they consume detritus—decaying organic material in the process of breaking down.
This includes microorganisms, algae, leaf litter, and carcasses raining down onto the ocean floor.
They process sand by gobbling it up, scraping off organic material, and depositing clean sand out the other end.
Reproduction
Echinoderms typically utilize external fertilization, where sperm and eggs are released into the water column simultaneously.
Phylum Chordata: Non-Vertebrate Groups
Evolutionary Context
While humans are vertebrate chordates, there are subphyla of chordates that do not possess vertebrae.
All chordates share key characteristics: a notochord and a dorsal nerve cord (unlike arthropods, which often have ventral nerve cords along the bottom of the body).
Sea Squirts (Tunicates)
Subphylum Tunicata: These are colorful organisms that sit on the ocean floor.
They possess openings at the top to take in water, process organic material, and squirt the water out as a defensive mechanism.
They retain a notochord with a minimal nerve cord but lack vertebrae.
Salps
Salps are free-floating (planktonic), barrel-shaped transparent tunicates.
Locomotion: They move via jet propulsion, taking water in at one end and expelling it at the other.
Feeding: Known as the "ocean's vacuum cleaners," they use an internal mucus net to filter large and small plankton from the water as it is pumped through their gelatinous bodies.
Life Cycle and Colonies: They can reach in length. They have a complex cycle including solitary phases (oozoids) and colonial chains (blastozooids). These chains can reproduce asexually to produce hundreds of individuals. Individuals in older chains can develop into males, while younger chains are female.
Phylum Arthropoda: Morphology and Structure
Etymology and Definition
Arthro means "joint" and pod means "feet" or "appendage."
Arthropods are defined by their jointed appendages and a hard exoskeleton.
The Exoskeleton
The exoskeleton is an external skeleton that provides structural support, protection of vital organs (similar to how the human skull protects the brain), and movement.
Movement is achieved because muscles cross the joints between the rigid pieces of the skeleton.
The exoskeleton is highly water-impermeable, preventing desiccation in terrestrial environments.
Arthropod Subphyla and Classification
Trilobites: An extinct marine group. A related extinct group, the Eurypterids, represented the largest arthropods to ever exist.
Chelicerata: Defined by complex biting mouthparts called chelicerae. Includes spiders, ticks, chiggers, scorpions, sea spiders (Pycnogonids), and horseshoe crabs.
Myriapoda: Includes centipedes and millipedes, as well as lesser-known groups like pauropods and symphylans.
Crustacea: Includes shrimp, lobsters, crabs, and crawdads.
Hexapoda: Defined by having six legs. This includes all insects and three basal groups.
Pancrustacea: Modern 2019 taxonomic findings (based on molecular, behavioral, and morphological data) show that hexapods are actually nested within the crustacean branch, forming a larger group called Pancrustacea.
Biological Highlights of Specific Arthropods
Horseshoe Crabs
These have remained virtually unchanged for nearly years.
Spawning: Females require specific sand conditions—lubricated with enough water to dig, but dry enough to create a cavity for eggs. A single female can lay approximately eggs in one tide (about to per cluster).
Mating: A male clips onto a female to come ashore, but other "satellite" males often cluster around trying to achieve paternity through external sperm competition.
Spider Diversity
Darwin’s Bark Spider: Capable of producing the toughest natural fiber— stronger than Kevlar at the molecular level. It can spray silk (over ) across rivers to form bridge lines for orb webs up to wide.
Peacock Spiders (Jumping Spiders): These show intense predator avoidance and mating rituals. Males perform elaborate dances involving leg waving and abdomen displays. This dance is a "dance for his life," as females often engage in sexual cannibalism, eating males who do not meet expectations.
Sexual Cannibalism Evolution: In some species, such as those described in figure of the text, a male flips into the female's jaws during mating. This provides her with nutrients for his offspring and physically blocks other males from mating while he is being eaten, ensuring his paternity.
Ticks and Their Strategies
Questing: Ticks engage in questing, where they climb to the ends of grass stalks and wait with their front legs extended to grab a passing mammalian or avian host.
Tick Bombs: A mass of hundreds of tiny tick larvae waiting on a single stalk of grass can explode onto a host upon contact.
The Vinegaroon (Uropygid)
These arachnids have large, triple-jointed, serrated pincers and a long, hair-like sensory tail (whip) instead of a stinger.
Defense: They spray a chemical composed of acetic acid, which smells like strong vinegar.
Scorpions and Ultraviolet Light
Scorpions possess fluorescent chemicals in their bodies that cause them to glow under ultraviolet (UV) light.
While the exact reason is debated, one theory suggests it may attract insects like moths, which can see into the UV spectrum.
Scorpions detect prey using hypersensitive hairs on their claws that pick up vibrations in the air or on the ground.
Questions & Discussion
Question: When is the guest lecturer coming?
Response: He is scheduled to arrive the Monday after next, on the .
Question: What does detritus mean?
Response: It refers to dead organic matter or organic material that is in the process of breaking down, such as microorganisms, algae, or leaf litter.
Question: What makes a chordate a chordate?
Response: A notochord and a dorsal nerve cord are the primary defining features, along with three other characteristics to be discussed later in the course.
Question: Concerning the vinegaroon, were you sprayed when you were handling it?
Response: Yes, and it smelled like vinegar, leading the handler to smell like ketchup all day.
Question: Were they saying that moths can see UV light?
Response: Yes, moths are able to see into the UV spectrum, though there is very little UV light available at night.
Question: Does the hexapod/crustacean branching pattern from 2019 have any further revisions or shuffling?
Response: There has been fine-tuning within smaller species groups, but the higher-level branching pattern (Pancrustacea) is currently considered very stable and solid.
Question: The sea spider feeds on sea anemones; why is that interesting?
Response: Because anemones have strong defenses like stinging cells (nematocysts). Other animals like nudibranchs also eat them and actually incorporate the stinging cells into their own skin.
Question: If you are an upper-level student who is new to this and needs help studying, what should you do?
Response: Check in with the instructors via email or in person to discuss preparation for more advanced exams.