Invertebrate Zoology Lecture 10: Phylum Platyhelminthes Part 2

Lecture outline  Phylum Platyhelminthes  Asexual reproduction/regeneration (Class Turbellaria)  Sexual reproduction (Class Turbellaria  Life cycles: Class Monogenea  Life cycles: Class Trematoda  Life cycles: Class Cestoda  Life cycles: Wrap-up discussion!

Asexual reproduction/ regeneration: Class Turbellaria  Transverse fission  Note eye development in each section  Develop  separate

Asexual reproduction/ regeneration: Class Turbellaria  Capacity for regeneration  Transverse cuts  new individuals  Capacity to regenerate is greatest at head (Planaria)  Polarity maintained  Undifferentiated mesenchyme cells  particular cell types  Capacity depends on body position

Asexual reproduction/ regeneration: Class Turbellaria  Capacity for regeneration  Longitudinal cuts  Multiple heads  Multiple tails

Sexual reproduction Class Turbellaria (Planaria focus)  Reproductive systems  Hermaphroditic  Multiple testes and ovaries  Many variations  Male/female openings  Common vs. one for each  Planaria: common opening  Sperm have two tails!

Sexual reproduction Class Turbellaria (Planaria focus)  Mating  Penis everts during mating  Stored retracted in “male atrium”  Opening sometimes in mouth: Orogenital pore  Internal fertilization  Sperm travel via oviducts to ovary

Sexual reproduction Class Turbellaria (Planaria focus)  Mating (cont.)  If no female opening  hypodermic impregnation (a.k.a. penis fencing)  Eggs travel through body to ovary.

Sexual reproduction Class Turbellaria (Planaria focus)  Fertilization  In ovary  Return trip via oviducts  Yolk deposited  Eggs encapsulated & released or brooded in “uterus”  Summer vs. resting eggs  Difference?  Direct development or larvae

Life cycles: Class Monogenea  Single host  Sample life cycle 1: Dactylogyrus vastator, a gill parasite of carp  Adult: on gills  Fertilized eggs  encased zygotes  Drop to substrate, hatch into…  Oncomiracidium  Swimming form  Host attachment  Maturation

Dactylogyrus vastator, a gill parasite of carp

Life cycles: Class Monogenea  Sample life cycle 2: Polystoma integerrimum, a parasite of frogs  Basic stages similar to Dactylogyrus  Key: Two variations tied to host life cycle  Young tadpoles as host  Larvae attach to external gill  Rapid life cycle  Older tadpole as host  Larvae attach to external gill of tadpole  Migrate to bladder after frog metamorphosis  Mature after ~2-3 years in response to frog sex hormones

Polystoma integerrimum

Life cycles: Class Trematoda  Example: Chinese liver fluke, Clonorchis sinensis  Overview  Definitive hosts: Humans and other carnivorous mammals  Causes liver disease & associated symptoms  Two intermediate hosts  Intermediate host 1 = snail  Intermediate host 2 = fish or crustacean  How does the infection occur?

Life cycles: Class Trematoda  Clonorchis sinensis: life stages  Egg  Miracidium  Sporocyst  Redia  Cercaria  Metacercaria  Adult From Pearse & Buchsbaum, Living Invertebrates

Life cycles: Class Trematoda  Clonorchis sinensis: life cycle (in hosts) From Pearse & Buchsbaum, Living Invertebrates

Life cycles: Class Cestoda  Anatomy of a proglottid  Hermaphrodites  Most mature/functional systems in posterior proglottids  Fertilized eggs stored in posterior proglottids  Eggs released or proglottids break off  feces.

Life cycles: Class Cestoda  Example: Beef tapeworm, Taenia saginata  Proglottid with zygotes  Onchospheres  Cysticercus  Maturation  Reproduction

Parasite life cycle wrap-up  Advantages of multiple hosts?  Think carefully about life cycles presented!  Disadvantages of multiple hosts?  Think carefully about life cycles presented!  How are life cycles “broken”?  Mechanisms to insure transfer  Example 1: Killfish infected by trematodes  Example 2: Particular snails infected by trematodes