phylum hemichordata marine worms share characteristics with echinoderms and chordates deuterostomes...
TRANSCRIPT
Phylum Hemichordata
• Marine worms
• Share characteristics with echinoderms and chordates
• Deuterostomes (radial cleavage)
• Pharyngeal gill slits
• Some have dorsal, hollow nerve cord
Class Enteropneusta
• Acorn worms ~ 70 species
• Live in burrows in fine sediments
Acorn worms
• Proboscis for burrowing and feeding
Acorn worms
• Mostly deposit feeders
Digestion
• Mouth – esophagus – pharynx – intestine - anus
Respiration
• Gill slits connected to pharynx
• Cilia move water through mouth, out pharynx
Circulation
• Closed system with blood and vessels
Nervous system
• Similar to echinoderms – no brain
• Nerve cords connect all regions
Reproduction
• Dioecious = separate sexes
• Gonads produce gametes
• External fertilization
Tornaria larva
• Similar appearance to echinoderm larva
• Ciliated bands
Class Pterobranchia
• Colonial zooid
• Construct rigid tubes
• Ciliated tentacles
• U-shaped gut
• Pharyngeal gill slits
New topic: invertebrate evolution
• Original prediction for larva type:– Ancestral condition = external fertilization,
non-feeding larvae– More recently evolved = internal fertilization,
feeding larvae
• Both conditions are scattered across phyla
Evolution across the Central American Isthmus
• Formed 2-3 mya
• Enough time for evolution?
Different environments?
• Western Atlantic• water depth varies among islands, different
climate – no upwellings, less El Nino effects, higher evaporation rate, lower productivity
Different environments?
• Eastern Pacific– Deeper, upwellings bring nutrients, El Nino
effects strong, stronger tidal effects
Different environments?
• Western Atlantic: sea grass beds common
• Not in EP
Different environments?
• Western Atlantic: coral reefs large – hundreds of acres, higher S
• Small outcrops in EP, lower S
Different taxa?
• Large swimmers can still disperse around continents
• Not smaller-bodied taxa
Genetic differences?
• Similar mitochondrial DNA differences for all crustracea
• Other groups – variation high
Life history differences?
• Echinoderm, bivalve eggs smaller in open ocean EP = higher productivity
Speciation?
• Many examples of incompatibility, but not distinct species yet