Lancelets and Tunicates Quiz

Lancelets

• Lancelets are marine sedentary suspension feeders that retain characteristics of the chordate body plan as adults.
• They wriggle backward into the sand.
• Water is drawn into the mouth by ciliary action and particles are extracted using mucus-covered pharyngeal slits.
• Water exits the body through the atriopore.
• Mucus net ingested (along with trapped food).

Tunicates

• Tunicates are more closely related to vertebrates than they are to lancelets.
• They are suspension-feeding marine invertebrate chordates (~2,100 spp.).
• Adults do not look like chordates.
• "Tadpole"-like larvae have a brief free-swimming phase.
• They attach to the substrate by the head.
• They metamorphose to adult body plan (lose chordate traits).
• They become sessile suspension feeders.
• Most adult tunicates are sessile, e.g., sea squirts/sea tulips that are permanently attached to rocks; other tunicates are pelagic.
• Tunicate larval stages most closely resemble the chordate body plan.
• The larval stage exhibits the four chordate shared, derived traits.
• Adult tunicates are covered by a tough polysaccharide "tunic".

Chordate Characteristics

• Chordates share a set of derived characters.
• Some traits are only evident during embryonic development for some chordate taxa.
• Shared, derived traits of chordates:
  • Notochord
  • Dorsal, hollow nerve cord
  • Pharyngeal slits or clefts
  • Muscular, post-anal tail
• Segmentation of chordates evolved independently from segmentation of other phyla, e.g., annelids and arthropods.
• Segmentation is considered a primitive character of clade Deuterostomia.

Segmentation

• Most chordates have segmented body plans, at least during embryonic development.
• The segments are often highly specialized into distinct body regions.
• Segmentally arranged muscle blocks are present in all adult chordates except in the Urochordata.
• Muscle blocks develop from blocks of mesoderm (somites) that flank the notochord.
• Muscle blocks are arranged in chevron (>>>) patterns.
• Alternating contractions of the muscle blocks cause the notochord/vertebral column to flex side-to-side → swimming in marine chordates!

Phylogeny of Living Chordates

• Chordates are bilaterian animals that belong to clade Deuterostomia.
• Chordates consist of all vertebrates and two groups of invertebrates, the urochordates and cephalochordates.
• The majority (>95%) of chordate species have vertebral columns (backbones): the vertebrates (subphylum Vertebrata).

Test your knowledge about marine sedentary suspension feeders known as lancelets and tunicates. Learn about their unique characteristics, feeding mechanisms, and body plan features.