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Scaphopodans, commonly referred to as 'tusk shells', are small mollusks that with enlongated shells that open at the top and are tapered like an elephant tusk. They live buried in the sand with one end of their shell protruding above the surface and are most common in deeper waters. This allows them to remain protected while extending their tentacles for feeding. Unlike many other mollusks, scaphopodans don't actually have gills. They perform gas exchange across the surface of their body, supported by their burrowing lifestyle keeping them in contact with oxygenated water in the sediment. Tusk shells play subtle roles in benthic ecosytems by controlling small invertebrate populations and nutrient recycling.
Defining Characteristics
Defining Characteristics
Bilateral symmetry
Triploblastic body plan: endoderm, mesoderm, ectoderm
Elongated, tubular shells that open at the top
Tapered (curved) similar to an elephant tusk
Benthic burrowers
Head reduced with no true cephalization
Numerous thin, sticky tentacles called captacula
use to capture prey and bring to mouth for ingestion
Perform gas exchange across body surface
Complete digestive tract: separate openings for mouth and anus that allows for maximized digestive efficiency
Closed Circulatory System: blood is contained within vessels as opposed to freely thoughout the body
Simple nervous system and sensory organs
Reproductive Adaptations
Reproductive Adaptations
Sexes separate (dioecious), no hermaphroditic species
Fertilization typically external with both eggs and sperm released into the water
Egg stage: fertilized eggs develop into embryos before hatching as larva
Trochophore larva: Tiny, ciliated larval stage that feeds on plankton
Helps disperse animal throughout water column
Metamorphosis: The larva settles into substrate and transforms into a juvenile tusk shell
Some species have direct development skip this larval stage
Adult stage: Reproductive system fully developed with the capability of mating; burrows into sediment and utilizes captacula for feeding
Where are they found?
Where are they found?
Only reside in marine environments
Most common in deeper waters
Benthic habitats
soft sediments
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