Lecture 17: Annelids II (Polychaetes, Earthworms)

Annelid Reproduction and Development

• regeneration and asexual reproduction

• sexual reproduction

Annelid Asexual Reproduction

• nearly all capable of regenerating lost appendages

  • not fragmentation

• most can regenerate posterior body segments, some anteriorly

• some bud/clone from parent parapodia

• see; epioky

Annelid Sexual Reproduction

• majority of annelids are gonochoristics

  • species has 2 distinct sexes

• hermaphroditism mostly in Clitellata clade

Clitellum

• thick “sleeve” that encircles body over several segments (# consistent in species) formed by secretory cells of epidermis of these segments

• used in simultaneous copulation

Epitoky

• specialized reproductive process in many marine polychaete worms where a sexually immature benthic worm (atoke) transforms into a pelagic, sexually mature form (epitoke)

  • transformation process known as asexual schizogamy

Annelid Larval Development

• do not have a lophophore feeding apparatus, instead they are united with the lophotrochozoa by their trochofore larvae

Prostomium

• “pro”= before, “stome”= mouth

• not a true segment

• may be a remnant of a larval body

Peristomium

• “peri”= around

• 1st true segment

Pygidium

• very end of body

  • appears to be last segment

• NOT a true segment

• may be remnant of larval body

Traditional Annelid Groups

• Polychaetes

• Oligochaetes

• Hirudinians

Modern Annelid Groups

• repositions Polychaetes as a major clade within the annelids which includes many of the Oligochaete and Hirudinean species, and which splits into 2 clades:

  • Errantia

  • Sedentaria

Errantia

• “errant” active polychaetes

  • motile via parapodia which are supported with chaetae, which aid in locomotion as well as respiration

• mostly epibenthic marine

Errantia Nervous System

• well developed sense organs and cephalization

  • head with 2-3 antennae

  • eyes/eyespots

  • sensory palps

  • enlarged cirri

  • 2+ chitinous jaws

  • eversible pharynx

Errantia Reproduction/Development

• external fertilization

• epitoky via epigamy or schiozogamy

• trochophore larvae

Sedentaria Characteristics

• reduced

  • parapodia

  • chaetae

  • sense organs

• fixed pharynx

• modified heads

• strong tagmatatization in some

Tagmatization

• in segmented animals when groups of segments fuse or specialize into distinct functional body regions; tagmata

Clitellata

• form reproductive structure; clitellum

  • mucus secreted by clitellum holds worm together during copulation

• members are derived annelids that lack parapodia, but still have chaetae

Clitellata Reproduction/Development

• hermaphroditic

• direct development

• no trochophore

• copulation:

  • mucus secreted by clitellum holds worms together

  • sperm from each worm (since hermaphroditic) are transported to the seminal receptacles of the other along seminal grooves

  • after mutual copulation, each worm secretes a mucus tube and chitinous band to form a cocoon

  • cocoon passes forward and eggs, albumin and sperm are added

Crassiclitellata

• within Clitellata

• most earthworms (former oligochaetes)

  • mainly terrestrial detritivores

  • remarkably little morphological diversity

• burrow via retrograde peristaltic waves of body/wall muscles

• all have direct development

Hirudinea

• within clitellata

• leeches

  • most in freshwater, but some marine or semiteresstrial

• ectoparasitic (mostly as blood suckers), predacious or scavengers

Hirudinea Anatomy

• clitellum only during copulation

• posterior and anterior sucker

• true leeches always with 34 segments without septa, and no chaetae

Leech Bloodsucking

• muscular pumping pharynx, often pierce skin with cutting jaws

• secrete an anticoagulant and anesthetic to aid feeding

• can drink blood up to 10x of body weight in one meal by storing blood in highly branched gut

• may digest one blood meal for months

Other Annelids

• myostomida

• sipuncula

• thalassematidae (used to be echiura)

• sibloglinidae

MSTS