Seashells (mollusks) &
beachcombing
Maia McGuire, PhD
Florida Sea Grant Extension Agent
Mollusks
• Includes Gastropods (snails), Bivalves (e.g.
clams) and Cephalopods (e.g. octopus)
• Second largest invertebrate phylum (after
arthropods)
• Generally have a muscular
foot, a calcareous shell and a
feeding organ called a radula
• Have gills, blood vascular
system, stomach, heart,
sensory organs (e.g. eyes)
Molluscan reproduction
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Sexual reproduction
Most are single sex (gonochoric)
Fertilization is external
Larvae are often planktonic (most common
type is called a veliger)
Gastropods
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Snails and sea slugs
“Head foot”
Carnivores and herbivores
There are gastropods in salt water, fresh water
and on land, but all are restricted to that
particular habitat. Land snails have lungs.
• Many have an operculum (“trap door”) to seal
the entrance to their shell.
Gastropod adaptations
• Operculum (‘trap door”)
– Used to help protect snail from predators
– Used to help prevent snails from drying out
(especially for intertidal snails)
– Different types of snails have different types of
opercula (range from thin and flimsy, to made of
calcium carbonate)
Some snails are predators
• Remember that radula?
• It becomes a drill or rasp that some snails use
to drill through the shells of other snails or
bivalves (e.g. clams)
www.seaslugforum.net
Common gastropods
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Slipper shell
Moon snail/shark’s eye
Olive
Auger
Wentletrap
Baby’s ear
Whelks
Tulips
Gastropod egg cases
• Sand collar (moon snail)
www.jaxshells.org
• Whelk and tulip egg cases
Baby snails
• Often planktonic (carried by ocean currents)
• Have a tiny shell, which becomes the tip of the
shell after the animal settles and grows.
Snail growth
• Snails add to their shell (at the opening/lip,
and in thickness) as they grow.
• The mantle used calcium and bicarbonate ions
from the water to create calcium carbonate.
• We cannot age a shell based on growth lines
(unlike trees and corals)
Sea slugs
• No external shell
• Variety of defense mechanisms
• Some are herbivores, others are carnivores
Bivalves
• Have two, hinged shells
• Examples include oysters, clams, mussels,
scallops
• Many can live 20-30 years; some more than
100 years!
• Most are filter-feeders
• Many attach to hard
surfaces; some have
threads or cement
Bivalve shells
• Some bivalves have a periostracum (thin,
often brown covering over the shell)—
provides camouflage.
Common bivalves
• Ark clams
• Pen shells
• Coquina clams
• Jingles
• Cockles
• Scallops
Cephalopods
• Octopus, squid, nautilus, cuttlefish
• Swim by producing a jet of water through a
funnel
• Most have 2 tentacles with suction cup ends;
8 arms with suction cups
• Horny, parrot-like beak; radula functions as a
tongue
• Have pigment cells in skin
Cephalopod biology
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Many can squirt ink as a defense
Eyes and nervous system are well-developed
Carnivores
Poisonous saliva
Have gills, heart, brain
Gonochoric
Squid die after mating
Female octopus die after
brooding their eggs
Cephalopods
• Paper nautilus
www.manandmollusc.net
www.seabean.com
• Ram’s horn shell
www.seabean.com
Activities
• “I Have a Shell” (Monterey Bay Aquarium)
• “CSI: Clamshell Investigation” (NJ Sea Grant)
Weird and Wonderful things…
• Mermaid’s purse
• Sand dollar
• Sea stars
Jellyfish
• Portuguese man-o-war
www.enature.com
– Don’t touch!
– Associated companions—”By-the-wind
sailor”, “blue buttons”, purple sea snail,
man-o-war slug, man-o-war fish
www.amonline.net.au
Cannonball jellyfish
www.jaxshells.org
Moon jellyfish
www.jaxshells.org
Sharks’ teeth
Sea beans
• www.seabean.com
• Any of a number of seeds
that are adapted to
germinate even after
spending a period of time
floating in water
• Mostly from tropical plants,
many from vines
Activity
• Beach Treasures