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