Marine Biology: Nautilus Internal Structure

Nautilus

• Has a huge, chambered (septate), and buoyant shell
• Shell has a structure called a siphuncle, which helps in buoyancy
• Has tentacles and highly developed eyes
• Moves using the siphuncle, which sucks water inside and helps the animal move in a smaller chamber

Tusk Shell or Scaphopoda

• Shell is open at both ends, elongated, and has different diameters (left is wider than the right)
• Has a narrow portion of the shell in water, and the rest of the shell/animal is buried in the substrate
• Moves using the structure inside the shell called the siphuncle

Squid

• Divided into classes such as Protobranchia, Pteriomorphia, Palaeoheterodonta, Heterodonta, and Anomalodesmata (based on dentition or teeth)

Aplacophora

• Worm-like, absent of shell
• Cuticle is covered with calcareous spicules called aragonitic sclerites made of calcium carbonate
• Has two subclasses: Chaetodermomorpha and Neomeniomorpha
• Chaetodermomorpha are deep-water species, usually borrowers, and detritivores
• Neomeniomorpha feed on small cnidarians

Polyplacophora and Monoplacophora

• No additional information provided

Opisthobranchia

• Members have highly reduced shells
• Have a chemosensory organ called a rhinophore
• Exhibit detorsion; reduction or loss of shell; gills located behind the heart; hermaphroditic; mostly marine

Anatomy of an Opisthobranch

• External appearance of a doridacean nudibranch
• Has rinophores, mantle, gills, food, and mouth
• Colorful appearance, lacks shells and radula

Mollusks

• Capable of secreting a pigment for camouflaging or evading predators
• Exhibit torsion, the rotation of the visceral mass, mantle, and shell 180˚ with respect to the head and foot of the gastropod

Bivalves

• Foot adapted for burrowing (in soft substrates e.g., muddy or purely sandy)
• Some are attached to surfaces via byssal thread (byssus), or by cementing themselves to hard substances through production of calcium carbonate on their shell’s lower part
• Scallops (highly mobile) can swim by clapping their shells

Phylogeny of Pteriormphia

• Pectinida are closely related to Limida (not that common); sister group to Arcida
• Pteriida and Ostreida are closely related; sister group to Mytilida

Subclass Pteriormphia (Bivalves)

• Epibenthic (not infanual); most are attached by byssus threads; with lamellibranch gills
• Includes economically important mussels, scallops, clams, and oysters
• Comprises the following orders: Mytilida (mussels), Pteriida (pearl oysters and winged oysters), Ostreida (true oysters; edible; with central adductors like scallops), Arcadia (ark clams, taxodont dentition), Limida (file shells), and Pectinida (scallops)