Page 279 - (DK) Ocean - The Definitive Visual Guide

MOLLUSKS 277

Sense Organs HUMAN IMPACT

Touch, smell, taste, and vision are well developed in many mollusks. GRAFTING OYSTERS
The nervous system has several paired bundles of nervous tissue
(ganglia), some of which operate the foot, and interpret sensory Pearls form in oysters when a grain
information such as light intensity. Photoreceptors range from the of sand or other irritant lodges in
their shells. The oyster coats the
simple eyes (ocelli) seen along the edges of the mantle or on bivalve grain with a substance called nacre,
siphons, to the sophisticated image-forming eyes of cephalopods. forming a pearl. Today many pearls
Cephalopods are also capable of rapidly changing their color. are cultured artificially: the shell is
opened just enough to introduce
PIGMENTED SKIN CELLS HELP CUTTLEFISH TO CHANGE COLOR an irritant into the mantle cavity.
SEEDING AN OYSTER
The best-shaped artificial pearls are produced by
“seeding” oysters with a tiny pearl bead and a
piece of mantle tissue from another mollusk .

1 The giant cuttlefish’s color change is due 2 When the cuttlefish passes over Movement
to skin cells called chromatophores. It is a darker background, it disperses the
pale when pigment is confined to colored pigments throughout each of its Mollusks move in many different ways. Most gastropods glide across
a small area of each cell. chromatophores, and the animal darkens. surfaces using their mucus-lubricated foot. Exceptions include the
sea butterfly, which has a modified foot with finlike extensions for
MOLLUSCAN BEAUTY swimming. Some bivalves, such as scallops, also swim, producing jerky
Displaying fabulous warning colors, this nudibranch is
a shell-less example of the many thousands of movements by clapping the two halves of their shell together. Other
marine species of gastropods (slugs and snails). bivalves burrow by probing with their foot and then pulling themselves
downward by muscular action. Cephalopods are efficient swimmers;
some have fins on the sides of their bodies
that let them hover in the water, and they
can accelerate rapidly by squirting
water out through their siphons.

siphon AIDED BY MUCUS
REDUCING DRAG Muscular contractions ripple
Swimming backward reduces drag through the fleshy foot of this
from the tentacles. The siphon, used marine snail. It secretes a
for jet propulsion, is clearly visible lubricating mucus that helps
in this Humboldt squid. it to move on rough surfaces.

Respiration
Most mollusks obtain oxygen from water using gills, called ctenidia,
which are situated in the mantle cavity. These are delicate structures
with an extensive capillary network and a large surface area for gaseous
exchange. In species that are always submerged, water can continually be
drawn in and over the gills. Those living in the intertidal zone are exposed
to the air for short periods and must keep their gills moist. At low tide,
bivalves clamp shut and some gastropods close their shell with a “door”
(called an operculum) to retain moisture. Pulmonate snails have a simple
lung formed from the mantle cavity instead of ctenidia and are mostly
terrestrial but others live on the seashore and can absorb oxygen through
their skin when immersed.The respiratory pigment in most molluscan
blood is a copper compound called hemocyanin. It is not
as efficient at taking up oxygen as
hemoglobin and gives mollusks’ external gills
blood a blue color. (ctenidia)

COLOR CODING
Nudibranchs (sea
slugs) have feathery
external gills toward
the rear of their OCEAN LIFE
bodies. The warning
coloration of this
species includes the
bright orange gills.