CK-12 Life Science
Page 27
Bivalvia 8,000 Marine (some clams live in deep ocean around hot hydrothermal vents) and freshwater. Most bivalves are filter feeders (mechanism whereby suspended matter and food particles are strained from the water, typically by passing the water over a specialized filtering structure); bivalves include clams, oysters, scallops, and mussels.
Scaphopoda 350 Marine Tusk shells
Figure 12.6
A chiton and sea anemones at a tide pool.
Figure 12.7
An example of a gastropod species, the ostrich foot.
Figure 12.8
A Caribbean reef squid, an example of a cephalopod.
As you can see, the majority of mollusk species live in marine environments, and many of them are found intertidally in the shallow subtidal zone and on the continental shelf. Freshwater species are represented in the bivalves and gastropods, and some gastropods, like land snails, and slugs, live on land.
Importance of Mollusks
Mollusks are important in a variety of ways, including as food, for decoration, in jewelry, and in scientific studies. They are even used as roadbed material and in calcium supplements.
Edible species of mollusks include numerous species of clams, mussels, oysters, scallops, marine and land snails, squid, and octopus. Many species of mollusks, such as oysters, are farmed in order to provide additional food sources (Figure below).
Figure 12.9
An oyster harvest in France.
Two natural products of mollusks used for decorations and jewelry are pearls and nacre, or mother of pearl. A pearl is the hard, round object produced within the mantle of a living shelled mollusk. Fine quality natural pearls have been highly valued as gemstones and objects of beauty for many centuries. The most desirable pearls are produced by oysters and river mussels.
Nacre is an iridescent inner shell layer produced by some bivalves, some gastropods, and some cephalopods, and has been used in sheets on floors, walls, counter tops, doors, and ceilings. It is also inserted into furniture; it can be found in buttons, watch faces, knives, guns, and jewelry; and is used as decorations on various musical instruments.
Several mollusks are ideal subjects for scientific investigation, especially in the area of neurobiology. The giant squid has a sophisticated nervous system and a complex brain for study. The California sea slug, also called the California sea hare, is used in studies of learning and memory, since it has a simple nervous system, consisting of just a few thousand large, easily identified neurons, but also a variety of learning tasks.
Lesson Summary
The mollusk body often has a head with eyes or tentacles, a muscular foot, a mass housing the organs, and a mantle, which secretes the external shell.
Other mollusk structures include a gill or gills for absorbing oxygen, a complete digestive tract, and a radula.
Mollusks are divided into ten living classes, including the familiar gastropods, cephalopods, and bivalves.
Mollusks live in marine and freshwater habitats, as well as on land.
Mollusks are important as food, for decoration, and in scientific studies.
Review Questions
What are the main characteristics of mollusks?
What evidence shows that mollusks and annelids are related? How are they different?
What habitats do marine mollusks live in?
What makes the California sea slug ideal for studies of learning and memory?
Oysters, one of the bivalve filter feeders, filter up to five liters of water per hour. Sediment, nutrients, and algae can cause problems in local waters, but oysters filter these pollutants and either eat them or shape them into small packets that are deposited on the bottom where they are harmless. When there is a high concentration of bacteria in the water from sewage run-off, this can make filter feeders, like clams and mussels, risky to eat. What do you think happens to the pollutants in this case?
Further Reading / Supplemental Links
http://www.manandmollusc.net/links_educational.html
http://www.oceanicresearch.org/education/wonders/mollusk.html
http://www.manandmollusc.net/links_medicine.html
http://en.wikipedia.org
Vocabulary
chitin
A tough, semitransparent substance that is the main component of the radula.
filter feeders
A mechanism whereby suspended matter and food particles are strained from the water, typically by passing the water over a specialized filtering structure.
mollusca
The phylum containing ten living classes of mollusks.
nacre
The iridescent inner shell layer produced by some bivalves, some gastropods, and some cephalopods; also known as mother of pearl.
pearl
The hard, round object produced within the mantle of a living shelled mollusk.
radula
A molluscan feeding structure, composed mostly of chitin.
Points to Consider
Many mollusks demonstrate bilateral symmetry. How do you think this differs from the radial symmetry evident in echinoderms, in the next lesson?
As we have seen, some species of mollusks live in the deep ocean around hot hydrothermal vents. In the next lesson we will learn that many echinoderms also live in the deep sea. What adaptations do you think both groups might have for living in such a unique environment?
Mollusks have an exoskeleton, which is primarily external and composed of calcium carbonate. As a result many of these are preserved in the fossil record. How do you think this compares to the type of skeleton that an echinoderm has and to its fossil record?
Lesson 12.2: Echinoderms
Lesson Objectives
Discuss the traits of echinoderms.
List the types of echinoderms.
Explain the roles echinoderms play.
Check Your Understanding
What is meant by body symmetry?
What is radial symmetry?
What is bilateral symmetry?
Introduction
We’re all familiar with starfish (Figure below), and also maybe sea urchins (Figure below) and sand dollars (Figure below). The radial symmetry is what hits us right away, a symmetry in which the body is arranged in five parts around a central axis. Much of the perceived beauty of this group resides in that design. Later in this lesson, learn how symmetry takes advantage of the animal’s habitat.
Figure 12.10
A starfish, showing the radial symmetry, characteristic of the echinoderms.
Figure 12.11
Another echinoderm. a sea urchin, showing its calcareous spines.
Figure 12.12
An echinoderm, the keyhole sand dollar.
The other things that stand out, quite literally, are the calcareous (containing calcium carbonate) spines of the sea urchin. If you’ve gone snorkeling or walked on a sandy beach you’ve most likely learned to watch out for those sharp spines. Think about how this adaptation might benefit the sea urchin in terms of predation and colonization by other organisms. Can you think of another use of these structures?
These and other adaptations will be explored in more detail as we examine this most fascinating group of invertebrates. Next time you take a walk on the beach, you’ll have appreciation for these organisms and how they are adapted for their environment.
What are Echinoderms?
Echinoderms belong to the phylum Echinodermata, which contains marine animals living at all ocean depths. It consists of about 7,000 living species, the largest phylum without freshwater or terrestrial members. Also, few other groups are so abundant in the deep ocean as well as the shallower seas.
As mentioned earlier, echinoderms are radially symmetric. In spite of their appearance, they do not have an external skeleton. Instead, a thin outermost skin covers an internal endoskeleton made of tiny calcified plates and spines, contained within tissues of the organism, and which forms a rigid support. Some groups, such as the sea urchins (Figure below)
, have calcareous spines, referred to earlier, which protect the organism from predation and colonization by encrusting (covering or coating) organisms. The sea cucumbers also use these spines for locomotion.
Figure 12.13
An echinoderm, the giant California sea cucumber.
Echinoderms have a unique water vascular system, a network of fluid-filled canals, which function in gas exchange, feeding, and also in locomotion. This system allows them to function without gill slits found in other organisms. Echinoderms possess a very simple digestive system, often leading directly from mouth to anus. They also possess an open and reduced circulatory system, but no heart. Their nervous system consists of a modified nerve net (interconnected neurons with no central brain).
In most species, eggs and sperm cells are released into open water, where fertilization takes place. The release of sperm and eggs is coordinated temporally (to occur at the same time) in some species and spatially (to occur within the same location) in others. Internal fertilization takes place in a few species. Some species even have parental care!
Many echinoderms have amazing powers of regeneration. Some sea stars are capable of regenerating lost arms, and in some cases, lost arms have been observed to regenerate a second complete sea star! Sea cucumbers often discharge parts of their internal organs if they perceive danger. The discharged organs and tissues are then quickly regenerated.
Feeding strategies vary greatly among the different groups of echinoderms. Some are passive filter-feeders, absorbing suspended particles from passing water; others are grazers; others are deposit feeders, which feed on particles of organic matter, usually in the top layer of soil, and still others are active hunters.
Types of Echinoderms
The echinoderms are subdivided into two major groups, the Eleutherozoa, which contains the more familiar, motile classes, and the Pelmatozoa, which contains the sessile (permanently attached and not freely moving) crinoids, including the feather stars (Figure below), which have secondarily developed a free-living lifestyle.
Figure 12.14
This passion flower feather star is an echinoderm.
The following table summarizes the four main classes of echinoderms present in the Eleutherozoa Group:
Echinoderm Class Representative Organisms
Asteroidea Starfish and sea daisies
Ophiuroidea Brittle stars (Figure below)
Echinoidea Sea urchins and sand dollars
Holothuroidea Sea cucumbers
Figure 12.15
The giant red brittle star, an ophiuroid echinoderm.
Echinoderms are distributed all over the world at almost all depths, latitudes, and environments in the ocean. They are in highest diversity in reefs but are also widespread on shallow shores, around the poles (where crinoids are at their most abundant) and throughout the deep ocean, where bottom dwelling and burrowing sea cucumbers are common, sometimes accounting for up to 90% of organisms.
While almost all echinoderms are benthic (live on the sea floor) some sea-lilies can swim at great speeds for brief periods of time, and a few deep-sea sea cucumbers are fully floating. Some crinoids attach themselves to floating logs and debris and some sea cucumbers employ a similar strategy, attaching to the sides of fish.
Roles of Echinoderms
Echinoderms play roles both ecologically and economically. Ecologically, sea urchin grazing reduces the colonizing of bare rock by such organisms as mussels and barnacles, and sand dollar and sea cucumber burrowing provides more oxygen at greater depths of the sea floor, thus allowing a more complex ecological community to develop. In addition, starfish and brittle stars prevent the growth of algal mats on coral reefs, so that the coral can more effectively filter-feed.
Echinoderms are also the staple diet of many organisms, including the otter. Many sea cucumbers provide a habitat for parasites, including crabs, worms, and snails. The extinction of large quantities of echinoderms appears to have caused a subsequent overrunning of ecosystems by seaweed, causing the destruction of entire reefs.
Economically, in some countries echinoderms are regarded as delicacies. Around 50,000 tons of sea urchins are captured each year, and certain parts are consumed mostly in Japan, Peru, and France. Sea cucumbers are considered a delicacy in some southeastern Asian countries.
Some sea cucumber toxins slow down the growth rate of tumor cells, so there is an interest in using these in cancer research. The calcareous external covering of echinoderms is used as a source of lime by farmers in some areas where limestone is unavailable and 4,000 tons of the animals are used each year for this purpose.
Lesson Summary
Echinoderms belong to the phylum Echinodermata, the largest phylum without freshwater or terrestrial members.
Echinoderms are radially symmetric, they have an endoskeleton, some have calcareous spines, they have a unique water vascular system, a simple digestive system, an open and reduced circulatory system and a modified nerve net.
Fertilization is generally external; regeneration is fairly common among echinoderms; feeding strategies vary greatly.
Echinoderms consist of two main subdivisions: the motile Eleutherozoa and the sessile Pelmatazoa.
Echinoderms are distributed all over the world at almost all depths, latitudes, and marine environments.
Echinoderms play an important role in the ecological community. Economically, they are eaten as delicacies in different countries, they play a role in cancer research, and they are used as a source of lime to fertilize crops.
Review Questions
What are the characteristic features of echinoderms?
What feeding strategies are represented in the echinoderms?
What protection do echinoderms have against predation?
Chemical elements within the skeleton makes it stronger and more resistant. How could this be an advantage in grazing echinoderms?
The larvae of many echinoderms, especially starfish and sea urchins, are pelagic (of or pertaining to the open ocean). How does this relate to the fact that echinoderms are distributed globally?
Further Reading / Supplemental Links
http://dictionary.reference.com
http://www.oceanicresearch.org/education/wonders/echinoderm.html
http://www.junglewalk.com/info/echinoderm-information.htm
[http://invertebrates.si.edu/echinoderm/ http://invertebrates.si.edu/echinoderm/
http://en.wikipedia.org
Vocabulary
echinodermata
The phylum of the echinoderms; contains about 7,000 living species, the largest phylum without freshwater or terrestrial members.
nerve net
Interconnected neurons that send signals in all directions.
pelagic
Of, or pertaining to, the open ocean.
sessile
Permanently attached and not freely moving.
water vascular system
A network of fluid-filled canals; functions in gas exchange, feeding, and also in locomotion.
Points to Consider
Echinoderms’ water vascular system functions in gas exchange via a network of fluid-filled canals. Terrestrial arthropods have internal surfaces that are specialized for gas exchange, via air sacs. How might these systems compare and differ?
Echinoderms possess an open and reduced circulatory system, consisting of a central ring and five radial vessels but no heart. Arthropods also have an open circulatory system but the blood is propelled by a series of hearts into the body cavity where it comes in direct contact with the tissues. Why might there be an advantage to having a heart as part of the circulatory system?
Lesson 12.3: Arthropods
Lesson Objectives
Explain what arthropods are.
Describe the features of crustaceans.
Describe the characteristics of centipedes and millipedes.
List the features of arachnids.
Describe why arthropods are important.
Check Your
Understanding
What is an invertebrate?
What do mollusks and echinoderms have in common?
Introduction
With over a million described species in the phylum containing arthropods, chances are you encounter one of these organisms every day, even without leaving your house. As much as we would like to eliminate all insect pests from our dwellings, for example, there is a great probability you will see an ant, a spider, a fly, or a moth inside. Even if you don’t, you will most likely see such creatures in your yard or on a walk around your neighborhood.
Wherever you observe these animals, you will see a tremendous amount of diversity and adaptations. You will also learn, despite how you feel about how annoying some of these organisms may be, how beneficial in fact they are both ecologically and economically.
What are Arthropods?
Arthropods belong to the phylum Arthropoda, which means “jointed feet,” and includes four living subphyla. These are chelicerates, including spiders (Figure below), mites, scorpions (Figure below) and related organisms; myriapods, comprising centipedes (Figure below) and millipedes (Figure below) and their relatives, who are hexapods, including insects and three small orders of insect-like animals; and crustaceans, including lobsters (Figure below), crabs (Figure below), barnacles (Figure below), crayfish (Figure below), and shrimp.