8-Pseudocoelomate Phyla (9 phyla)

Introduction to Aquatic Invertebrates

  • Instructor: Emmanuel M. Vera Cruz

  • Course: AREM 2210 Aquatic Invertebrates

  • Institution: College of Fisheries, Central Luzon State University

Pseudocoelomate Animals

  • Definition: Animals with a pseudocoel, which is a body cavity derived from the embryonic blastocoel rather than the mesoderm.

  • Characteristics:

    • Not lined with peritoneum.

    • Located between gut and body wall structures.

  • Phyla:

    1. Rotifera

    2. Gastrotricha

    3. Kinorhyncha

    4. Nematoda

    5. Nematomorpha

    6. Loricifera

    7. Priapulida

    8. Acanthocephala

    9. Entoprocta

Phylum Rotifera (Rotifers)

  • General Features:

    • Possess a ciliated crown (corona) that resembles a rotating wheel when in motion.

    • Mostly found in freshwater, free-moving, and solitary.

    • Body divided into head, trunk, and foot.

  • Specialized organs:

    • Mastax: Internal grinding organ.

  • Locomotion:

    • Swimming and creeping motion using foot.

    • Foot has 1-4 toes and contains adhesive pedal glands.

  • Distribution: Cosmopolitan, found across various regions.

Types of Rotifers

  • Stephanoceros: Characterized by finger-like lobes with bristles used to trap prey.

  • Asplanchna: Pelagic, predatory type without a foot, considered the largest rotifer.

  • Squatinella: Features a non-retractable, transparent hood.

  • Machrochaetus: Notable for dorsoventral flattening.

Reproductive Strategies in Rotifers

  • Life Cycle:

    • Monogonont rotifers undergo amictic (asexual) and mictic (sexual) reproduction based on environmental stimuli.

    • Males are rare and smaller during breeding season.

    • Parthenogenesis is common in many species.

  • Survival Mechanisms:

    • Some species can endure desiccation and resume activity upon water return.

Nutrition in Rotifers

  • Feeding Strategies:

    • Suspension feeders: Collect particles using coronal cilia.

    • Raptorial feeders: Capture protozoa and small metazoans.

  • Digestive System: Complete; digestion aided by salivary and gastric glands.

Excretory and Nervous Systems of Rotifers

  • Excretory System:

    • Consists of protonephridial tubules and a common bladder allowing for osmoregulation.

  • Nervous System:

    • Bilobed brain sends nerves to sensory organs.

    • Includes paired eyespots, sensory bristles, and dorsal antennae.

Distribution of Rotifers in the Philippines

  • Common Species:

    • Brachionus caudatus: Cosmopolitan; shows cyclomorphosis.

    • Keratella cochlearis: Most commonly found in plankton.

    • Asplanchna: Largest carnivorous rotifer.

  • Rare Species observed in specific locations such as Lake Mainit.

Phylum Nematoda (Roundworms)

  • General Characteristics:

    • Approximately 12,000 known species.

    • Bilaterally symmetrical and unsegmented.

    • Mostly dioecious with males being smaller than females.

  • Parasitic Forms: Some require intermediate hosts for lifecycle completion.

Common Parasitic Nematodes in Humans

  • Hookworm:

    • Ancylostoma duodenale & Necator americanus; penetrate skin, causing anemia.

  • Pinworm:

    • Enterobius vermicularis; most common helminth in the US, eggs spread through scratching.

  • Trichinella spiralis:

    • Causes trichinellosis from undercooked pork.

Additional Nematodes

  • Infection Mechanisms: Diverse means of infecting hosts, including ingestion and cutaneous absorption.

  • Life Cycle Traits: Lifecycles typically include egg, larval, and adult stages.

Phylum Acanthocephala (Spiny-headed Worms)

  • Characteristics:

    • Cylindrical body with an invaginated proboscis covered in spines.

    • All are endoparasitic, typically living in vertebrate intestines.

  • Life Cycle:

    • Fertilized eggs undergo stages in various hosts before reaching maturity in definitive hosts.