Fall 2008
OCN 201L
Hawaiian Sea Life
The coastlines surrounding the Hawaiian Islands contain several different marine
communities including coral reef flats, sandy beaches, exposed tidepools, and wavewashed rocky shores. Each of these ecosystems hosts a unique combination of
organisms, each of which has a niche, or functional role, in its habitat. The animals in
these environments are classified into two major groups: invertebrates, or soft-bodied
organisms lacking an internal skeleton, and vertebrates, organisms with an internal
skeleton. Organisms that share similar features and evolutionary history are further
classified into phylum. We will consider 7 of the animal phyla in next week’s lab:
Porifera, Cnidaria, Annelida, Mollusca, Arthropoda, Echinodermata, and Chordata
(both an invertebrate and vertebrate phylum).
Overview of Hawaiian Marine Phyla
Phylum Porifera includes sponges in either encrusting or vase-shaped masses that live
on hard substrates. A sponge is a suspension feeder that strains plankton and small
food particles from the water using the small pores covering its body. A large sponge
may filter 1,500 liters of water each day. Sponges are simple organisms with no
circulatory, respiratory, or nervous systems but do have individual digestive cells to
process food particles. Sponges contain a network of spicules composed of either
silica or calcium carbonate that help deter predators.
(from top left) Phylum Porifera, an encrusting sponge; Phylum Cnidara, Pocillopora meandrina
coral; Phylum Cnidara, Zoanthus pacificus polyps; (from bottom left) Phylum Annelida,
Eurythoe complanata (fire worm); Phylum Mollusca, Hexabranchus sanguineus, (spanish
dancer slug); Phylum Arthropoda, Carpilus maculates crab.
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©2004 Department of Oceanography, University of Hawai‘i at Manoa
Fall 2008
OCN 201L
Organisms in phylum Cnidaria have stinging cells called nematocysts and exhibit
radial symmetry, a body form that radiates from a central disc. Cnidarians have two
body forms. Jellyfish and portuguese-man-of-wars are examples of the mobile medusa
form, while sea anemones and corals are sedentary polyps. Some corals, sea
anemones, and zoanthids contain endosymbiotic algae also known as zooxanthellae.
The symbiotic relationship between the coral and zooxanthellae is mutualistic because
the zooxanthellae photosynthesize to provide oxygen and carbohydrates for the coral,
while the coral provides a house for the zooxanthellae as well as the carbon dioxide and
nutrients necessary for photosynthesis.
Phylum Annelida includes segmented worms of the class Polychaeta, the most
abundant marine worms. Polychaetes are often brightly colored and can have bristly
projections extending from segments or feather duster tops. Polychates are usually
found in the sediment and can produce calcareous tubes that allow only their heads to
emerge.
(from left): Phylum Echinodermata, Heterocentrotus mammillatus (pencil slate urchin) and
Actinopyga mauritiana (speckled sea cucumber); Phylum Chordata, tunicate.
Organisms in phylum Mollusca have a soft-body, internal or external shell, and foot.
Common mollusks include snails, bivalves, squid, and octopus. External shells help
protect their soft tissue that predators find especially tasty. Those without shells have
other adaptations including sea slugs that secrete poison, octopus that change
pigmentation for camoflague, or squid that secrete ink to fool their predator while they
make a fast escape.
Phylum Arthropoda includes an abundance of organisms including crabs, shrimp,
lobsters, barnacles, and copepods. Arthropods are characterized by hard exoskeletons
that fit so snugly that they must molt their exoskeleton during periods of growth.
Arthropods have articulated joints that are able to bend at specific points and allow a
full range of motion.
Spiny-skinned organisms including sea stars, brittle stars, sea urchins, and sea
cucumbers are classified into phylum Echinodermata. Echinoderms have radially
symmetric bodies based on five sections, and reside on hard substrates, sandy
bottoms, and sediments.
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©2004 Department of Oceanography, University of Hawai‘i at Manoa
Fall 2008
OCN 201L
Phylum Chordata includes several invertebrates as well as all vertebrates. During
development, some invertebrates including tunicates (sea squirts) display several
features characteristic of vertebrates and are therefore included in this phylum. About
95% of chordates are true vertebrates that possess backbones. Vertebrate chordates
present in Hawai‘i include fish, mammals (humpback whale, spinner dolphin, monk
seal), reptiles (turtles), and seabirds. The Hawaiian monk seal is an endangered
mammal that is found in northwest Hawai‘ian islands and occasionally Kauai and Oahu.
The monk seal may travel for up to a month, and searches for undeveloped coastline for
breeding. The monk seal, like other seals, can dive as deep as 600 ft for up to 20
minutes! They are able to slow their heart rate from a normal rate or 55-120 beats per
minute (bpm) to 4-15 bpm while diving, a physiology known as bradycardia. Why might
a slowed heart rate be beneficial to the monk seal?
A female monk seal and pup in the Northwest Hawaiian Islands.
Phylum Summary Table:
Phylum
Porifera
Cnidaria
Annelida
Mollusca
Arthropoda
Echinodermata
Chordata
(invertebrates)
Chordata (vertebrates)
Characteristics
pores, spicules, suspension feeder
nematocysts, radial symmetry,
some with endosymbionts
segmented, buried in sediments or
hard substrates
soft-body, external or internal shell,
bilaterally symmetrical, well
developed nervous system
hard exoskeleton (molts), articulated
joints
spiny skin, radial symmetry with 5
sections
notochord, primitive nervous system
Marine examples
sponge
coral, jellyfish, sea
anemones
polychaetes
backbone
fish, mammals (whale,
dolphin, monk seal), birds,
reptiles
snails, bivalves, squid,
octopus
crabs, shrimp, lobsters,
barnacles, copepods
sea stars, sea urchins, sea
cucumbers
sea squirts
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©2004 Department of Oceanography, University of Hawai‘i at Manoa
Fall 2008
OCN 201L
Coral Reef Ecology and Zonation
Boulder zone
Reef bench zone
Moderate light
zone
Diagram modified from Lerman (1986) Marine Biology: Environment, Diversity & Ecology Deep slope zone
Coral reef ecosystems are the most productive marine environment due to their large
diversity of organisms. The physical environment and ecological interactions largely
determine the abundance and distribution of organisms on a reef. The shallowest reef
zone, the boulder zone, experiences high light, high wave energy and terrestrial runoff. This area is dominated by low coverage of cauliflower coral, and algae overgrowing
dead coral skeletons and boulders. Boring sea urchins can often be found in the
boulders. The reef bench zone extends from 2-6 m and usually experiences high
wave action. A large diversity of organisms including lobe coral, cauliflower coral, rice
coral, seaweeds, and shore fish are found here. The moderate light zone extends
from 6-25 m and often contains large lobed and columnar finger corals, sea cucumbers
and snails as well as butterflyfish and wrasses. In the deep slope zone below 25 m,
light is very limited and the coral adapts to this condition with a large plate-like
morphology to maximize light energy received while minimizing energy spent on a
skeleton.
Coral reef communities in Hawai‘i are unique due to their relative young age and
geographic isolation. Reef development is generally proportional to age, and since the
Hawai‘ian Islands are geologically young, they are less developed than Indo-Pacific
reefs. Many Indo-Pacific locations exhibit 300-400 species of coral while Hawaii
contains only 40 species.
Corals are especially sensitive to changes in their environment and display ‘bleaching’
conditions under stress. In response to extreme changes in temperature, salinity, UV
radiation, or toxins, or with onset of disease, corals can expel their zooxanthellae
and/or the zooxanthellae decrease their photosynthetic pigments. The expulsion of
zooxanthellae results in an overall decrease in coral color. Corals that bleach are not
necessarily dead, and many can recover by either taking in a new population of
zooxanthellae, or rebuilding their photosynthetic pigments. Hawai‘ian reefs are also
subject to a number of anthropogenic, or human-induced, impacts including pollution,
over-fishing, and damage by boat anchors.
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©2004 Department of Oceanography, University of Hawai‘i at Manoa