Lecture 17 Invertebrates 4: Annelids & Mollusks Flashcards
Lophotrochozoa is
a diverse group of phyla with similar DNA sequences (monophyletic group). The annelid worms and mollusks are two large phyla in this group
Cubozoans (box jellyfish) vs Scyphozoans
- Were classified as one order Scyphozoa until 1973
- Cubozoans have a more developed nervous system
- Display complex behaviors such as obstacle avoidance and fast directional swimming
- Image forming eyes
- (most important) Distinct life cycle with a planula larva growing into a polyp that crawls around
Trochophore larva is
a type of larva common to marine mollusks, marine annelids, and several other phyla from the clade Lophotrochozoa
Segmentation is
the division of some animal body plan into a series of repetitive segments
Nephridia is
an invertebrate organ which occurs in pairs and performs a function similar to the vertebrate kidney, removing metabolic waste
Phylogeny for Lophotrochozoa
In common:
1. Feeding structure called Lophophore
-Ring of ciliated tentacles that surrounds the mouth and oft coiled. For mobility of larva and direct food towards mouth
2. Trocophore larva: “wheel bearer”
3. Similar DNA sequences
Describe Annelida (segmented worms or ringed worms)
- body divided into segments
- ~17,000 species
- aquatic (freshwater, tidal zone, hydrothermal vent), terrestrial (moist environments)
- 0.5 mm – 3 m
- Coelome, used as a hydrostatic skeleton
Segmentation of polychaete worm and regional differentiation
Segments are similar but each can be modified.
Most segments have appendages.
ex. bristles (setae) on parapodia used for locomotion, parapodia for excretion, ganglion, segmental nerve, and organs for respiration and sensation
Why is segmentation in Annelids advantageous?
- multiple copies of organs, structures
- ex: nephridia, parapodia - efficient nervous control
- ganglion in each segment
- faster responses
- efficient localized movement - increases body size by unit repetition
- regeneration
Segmentation is seen in other animal groups (arthropoda, vertebrates (rib cage, spinal cord)).
Segmentation involved independently in different groups.
Annelid systems
- nervous system
-anterior “brain”
-segmental ganglia
-ventral nerve cord, various thickness, dilates into ganglions in each segment - respiratory system
-skin (epidermis) or gills (polychaetes, some oligochaetes). Gills cluster to end exposed to stronger water flow, most oxygen - excretory system
-tubular nephridia (kidney equiv.) - digestive system
-Complete: mouth, esophagus, crop (storing), gizzard (grinding), intestine, anus
-With distinct regions - circulatory system-“closed”
-dorsal & ventral blood vessels
-aortic arches = hearts
Advantage of closed circulatory system
- Improved exchange between deeper tissues and surface (O2, CO2)
- Faster transport of nutrients and gases
- Permits development of a thicker body (free of diffusion limit)
Notes: in open circulatory system, blood discharged to hemolymph in hemocoel.
Diversity of annelid
- Polychaeta – “many hairs”: marine worms
- Clitellata
- Oligochaeta – “few hairs”: earthworms
- Hirudinea: leeches - Echiura & Sipuncula: unsegmented worms
Describe polychaeta (bristle worms)
- mainly marine
- detritivores, filter-feeders
- crawling, burrowing, swimming, pelagic, tube-dwelling or boring forms
- mobile forms have many eyes, palps, tentacles; sessile forms have reduced organs and heads
- parapodia
- muscular flaps with setae
- locomotion, respiration
- separate sexes with external fertilization (though some are unisex)
- trochophore larvae
Describe Eudistylia, Riftia under Polychaeta
Eudistylia
-live in parchment-like tube, single opening.
-Withdraw tentacles when low tide
Riftia (giant tube worm)
-can live near hydrothermal vents
-symbioses with chemosynthetic bacteria
Describe Oligochaeta
- terrestrial and few aquatic
- 10,000 species (half of Annelida)
- no parapodia, few setae
- light sensitive cells at end of body, vibration sensor
- detritivores
- hermaphroditic
–clitellum: belt like structure, secretes a cocoon for embryo development (similar to leeches, Hirudinia)
–no specialized larval stage
Mollusk Body Plan
Foot: large muscle for movement and hydrostatic skeleton
Mantle: outgrowth of body wall, many made of calcium carbonate
Coelum reduced, replaced by hemocoel (open circulatory system)
Radula with teeth to scrape algae, or hard surfaces. Arrangement of teeth varies across species, a marine predator has radular tooth as poison harpoon
Modifications of the ancestral molluskan body plan
Body plan-primary separation into classes
Describe Polyplacophora “bearer of many plates”
- segmented shell (8 overlapping plates), pliable and flexible-protection and flex (locomotion).
- the most primitive group of mollusks
- Grazing, herbivores
– radula scrapes algae & bryozoans - Separate sexes, external fertilization
– Trochophore larva
Chitons
-From cold waters to tropics, on hard surfaces
-1000 species
-common to intertidal zones
-large muscular foot
-ability to roll into a ball
-gills located in mantle grooves
Describe Bivalvia “two valves”
- Two hinged shells
- reduction of head, no radula
- enlargement of foot (digging in burrowing forms, reduced in free living like oysters)
- mantle cavity modified by siphons
- enlargement of gills, filter feeders
- Relatively sessile
- either dioecious or hermaphroditic
- broadcast spawners,
- external fertilization
- trochophore larvae
Siphons
– extensions of posterior mantle
– water flows in mantle cavity and then across gills that act as filters for food
– one single oyster can filter 200L of water per day
Foot used as anchor and for digging
Describe Gastropoda “belly feet”
- most diverse group of mollusks (~70,000 species)
- terrestrial or aquatic
- shelled (snails) or shell-less (slugs, nudibranchs)
- herbivores, predators, scavengers
- more complex head and eyes
- Reproduce sexually or asexually (by parthenogenesis)
- internal or external fertilization
- either dioecious or hermaphroditic
– trochophore larvae into veliger larvae (marine forms) - torsion (twisting) of body (some sinistral-cw, some dextral-ccw)
– 180° rotation of visceral mass
– mantle cavity, anus moved over head - coiling of visceral mass
- developed radula
Land snails (Pumonata):
-snails with pallial lung well vascularized, eyes on tips of tentacles.
-Feed on plants, decaying plant matter (detritus), lay eggs in soil
-Important food source of other wildlife in trophic chain
Nudibranchs: “naked gill”: secondary de-torsion
Some prey on tentacles of cnidarians and re-use the cnidocytes
Bright colors warn potential predators of presence of toxins
Describe Cephalopoda “head foot”
- subdivided foot –> arms, tentacles
- enlarged head, reduced shell (inside), complex eyes, powerful radula, subdivided foot with arms
- 800 Species
- most evolved invertebrates
- Strong jet from mantle cavity for locomotion
- Decapodiforms (8 arms and 2 tentacles), squids, catttlefishes
- Octopodiforms (8 arms, no tentacles), octopuses
- arms and tentacles equipped with suckers &/or hooks
- predators with excellent vision (lens)
- complex behaviour
– visual communication with colour & texture (chromatophores or photophores)
– cryptic coloration (masters of disguise)
-some produce bioluminescence using photoreceptors
– Inking sacs for smoke effect, escape predation
Reproduction
* Separate sexes with elaborate courtship
* Internal fertilization, where males transfer sperm via specialized arm = hectocotylus
* Females die after laying eggs or after eggs hatch, thus fisheries impact Cephalopod greatly
* no trochophore larva
