Ch. 9

Question 1

Key features of the Class Orthonectida

Answer 1

1. Parasitism: Orthonectids parasitize various marine invertebrates, including mollusks, annelids, echinoderms, and flatworms.
2. Size and Sexuality: They are tiny, rarely exceeding 300 um, and exhibit gonochoristic sexuality, meaning individuals have either male or female gonads, but not both.
3. Juvenile Stage: Orthonectid juveniles, known as plasmodia, are multinucleate, asymmetrical, amoeba-like organisms that reside in the host’s gonad and often cause castration.
4. Asexual Reproduction: Plasmodia do not
   “mature” traditionally but produce ciliated sexual forms within themselves, which are functionally “adults” that can reproduce.
5. Sexual Reproduction: These adults, typically less than 150 um, leave the plasmodium and host to swim freely in the sea. Mating occurs when individuals of opposite sexes meet,
   releasing sperm and fertilizing eggs. The fertilized eggs develop into ciliated larvae.
6. Larvae and New Hosts: Larvae seek new hosts by entering their genital duct, where they lose their ciliated cells and transform into plasmodia through mitotic division, continuing the life cycle.
7. Species Count: Around 20 species of orthonectids have been described.

Question 2

Key features of the Class Rhombozoa

Answer 2

1. Parasitism: Rhombozoans parasitize the nephridia (kidneys) of cephalopod mollusks, especially cuttlefish and octopuses.
2. Life Cycle: They begin as primary nematogens, which are worm-shaped, ciliated organisms with bodies composed of around 30 cells. These nematogens reproduce asexually within the host, forming more nematogens over generations.
3. Reproduction: After several generations of asexual reproduction, the nematogens mature into rhombogens. Rhombogens develop sexual forms called infusorigens, which are hermaphroditic and self-fertilize, producing zygotes.
4. Larvae: The fertilized zygotes develop into ciliated larvae that are expelled into the sea, though it is unclear how they find new hosts.
5. Nutrition: Rhombozoans feed on the host urine, living in an anaerobic environment with low oxygen, and they are more resilient in the presence of cyanide than oxygen.
6. Minimal Host Impact: Despite their extensive proliferation in the host’s kidneys, rhombozoans cause little harm to the host.

The rhombozoan life cycle is complex and highly specialized, differing significantly from other metazoans.

Question 3

The Mesozoans:

Possible Flatworm Relatives

Answer 3

1. Unclear Phylogeny: The relationship between mesozoans and other multicellular animals is not yet fully understood.
2. Unique Organization: Mesozoans are multicellular but small (10-40 cells) and have only two tissue layers, corresponding to ectoderm and mesoderm, without endoderm.
   They lack collagenous connective tissue and basal lamina.
3. Parasitic Lifestyle: Mesozoans are parasitic, similar to cestodes and trematodes, and lack a mouth, digestive system, and specialized circulatory, respiratory, sensory, or nervous systems.
4. Intracellular Development: Mesozoans develop their reproductive cells and embryos inside other cells, a unique feature among animals, suggesting they may represent an early branch of multicellular evolution.
5. Simple Morphology or Degeneration: Their simple structure may reflect either early multicellular evolution or degeneration from more complex ancestors as an adaptation to parasitism.
6. Possible Genetic Relationships: Some studies suggest mesozoans, like dicyemid rhombozoans, could be related to flatworms and other lophotrochozoans, while others propose they are multicellular protozoans that gained genes via lateral gene transfer.
7. Unclear Group Relationships: The relationship between the two mesozoan groups, orthonectids and rhombozoans, is unclear, with some studies suggesting they form a monophyletic group despite significant differences.