1B BIS (Winter 2008) Study question answers: Echinodermata Lecture 19 1. Why would it be difficult for an echinoderm to live in freshwater or terrestrial habitat? Echinoderms lack osmoregulatory organs; thus, they would face major problems with osmoregulation in a freshwater (hypotonic) environment. Echinoderms have cutaneous, gas exchange; thus, they would face major problems with water loss in a terrestrial environment. 2. How does a starfish extend a tube foot from its body and attach it to the substrate? The foot consists of a bulbous ampulla and a tubular podium, which terminates in a suction cup. It has a classic hydrostatic skeleton: it is filled with a fluid (which is incompressible, but capable of changing shape). The ampulla is surrounded by circular muscles, which can squeeze water from the ampulla into the podium, thereby extending the podium. The sucker is pressed against the substrate and held there by mucus (secreted by glands in the sucker) and by suction (tiny muscles pull the center of the suction cup inwards). 3. List four functions are performed by the water vascular system? The water vascular system functions in (i) support and locomotion, (ii) circulation, (iii) gas exchange, and (iv) excretion/osmoregulation. 4. What are some anti-predator adaptations demonstrated by echinoderms? Being active only at night, when predators are less likely to be hunting for prey Presence of a hard calcareous skeleton armed with sharp (and sometimes poisonous) spines Production of copious quantities of sticky mucus Pedicellaria may produce toxins thdt can paralyze some predators (fish), prevent other organisms from settling on the echinoderm, and grasp and hold objects used for camouflage (to hide from predators) In sea cucumbers: evisceration (rupture and spew out the hindgut) and discharge of the Cuvierian tubules Mutualistic associations with other animals that provide protection (e.g., some brittle stars live inside sponges) 5. Why don't many hermaphrodites self-fertilize?
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Some hermaphrodites do self-fertilize, and this can be advantageous when it is difficult or impossible to find a mate. However, most hermaphrodites mate with other members of the population, and avoid self-fertilization. Self-fertilization is disadvantageous first because it causes lineages to become evolutionarily isolated from all other lineages (in this regard, self-fertilization is similar to asexual reproduction). This causes a loss of some of the advantages of sexual reproduction. Second, self-fertilization causes levels of heterozygosity to decrease by 50% each generation, so that lineages rapidly become completely homozygous. This means that (1) all the offspring from a single parent are identical, and some of the advantages of sexual reproduction are lost and (2) deleterious recessive alleles will be expressed, causing a decline in fitness. 6. Briefly describe the function of the water vascular system in echinoderms, covering locomotion and feeding. The water vascular system in echinoderms is essentially a system of tubes, beginning with the madreporite (which opens to the outside), moving to the stone canal, and then to the ring canal (which surrounds the mouth). Five radial canals branch from the ring canal and each radial canal has many short side branches called lateral canals. Each lateral canal ends blindly as a tube foot or podium. Above each podium is an ampulla, which serves as a water reservoir for the tube foot. A valve in the lateral canal controls water flow to and from the ampulla; when the valve closes and muscles in the ampulla contract, water is pushed into the tube foot, extending it. The tube foot is extensible, and when filled with water, becomes stiff, and can bear the body weight (over all tube feet). Movement is accomplished through the combined action of many tube feet. The tube feet may create suction. The tube feet are used for locomotion and for feeding in many asteroids. Action of the tube feet opens bivalve shells (clams, mussels, etc.); in other echinoderms, tube feet are used to sift sediment or to hold mucus strands that collect food. 7. From the text whether discover the circulatory system is open or closed in each of the four major classes of molluscs, the 3 extant subphyla of the arthropods, the 3 classes of annelids, and the echinoderms in general. Can you see any pattern of association between habit or life style and the nature of the circulatory system? Mollusca: Polyplacophora open; Bivalvia open; Gastropoda open; Cephalopoda closed Arthropoda: Chelicerata open; Crustacea open; Uniramia open Annelida: Oligochaeta closed; Polychaeta closed; Hirudinea modified from a closed ancestor to open sinuses Echinodermata: all closed
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The circulatory system is closed in animals with active locomotion (the oligochaetes, polychaetes, and cephalopods), in predators like the cephalopods and some polychaetes, and it is closed in the only deuterostomes on the list. However, one would expect a closed system in active crustaceans and insects and there isn't one. Notice though that in the insects, the circulatory system doesn't deliver oxygen to the cells. The trachea do this and are quite effective. Small animals may also be able to do without an extensive circulatory system. 8. What are the essential features of the protostomedeuterostome division and explain what this division tells us about the relationships among phyla. The protostome phyla all share features of early development (spiral determinate cleavage, the blastopore becomes the mouth, and the coelom forms by schizocoely) which differ from those of the deuterostome phyla (radial indeterminate cleavage, blastopore becomes the anus, and the coelom forms by enterocoely). We presume that these similarities are homologous features, derived from shared ancestors, and that the protostome phyla are more closely related to each other than to any of the deuterostome phyla. 9. Do hermaphrodites reproduce asexually or sexually? Sexually (some can also reproduce asexually, but that has nothing to do with being a hermaphrodite). 10. Why do we think that the ancestral forms of the echinoderms were bilaterally symmetrical? Why would sand dollars be considered secondarily bilaterally symmetrical? Because they have bilaterally symmetrical larvae Because these echinoids have altered symmetry from their immediate ancestors (radially symmetrical echinoids like the sea urchin) to become bilateral. The term secondary implies that the trait was not found in the immediate ancestor of the group, but instead that the animals were secondarily modified to have radial symmetry. 11. Do echinoderm larvae feed? Are they radially symmetrical? What are the essential body features that give them such odd shapes? yes; no; They are basically just larvae with several bands of cilia that wrap around the outside (sort of trochophorelike), but they have internal calcite rods that push the ciliary bands out into weird shapes (greater feeding surface area). These internal calcite rods are just an early part of the calcareous endoskeleton. 12. The water vascular system of sea stars serves the same function as the what structure of gastropods.
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foot 13. Define secondary radial symmetry. Which of the following phyla contains species that illustrate this concept (more than one answer is possible)? a. b. c. d. e. Porifera Cnidaria Annelida Arthropoda Echinodermata
The term secondary implies that the trait was not found in the immediate ancestor of the group in question. Hence, secondary radial symmetry implies that members of a radially symmetrical group came from ancestors with bilateral symmetry. Answer "e" is obviously right (based on the fact that the larvae are bilateral and larvae suggest that the echinoderms had a bilateral ancestor. However, in the sand dollars and the sea cucumbers, there is evidence of another transition back to bilateral symmetry from currently radial ancestors, so these echinoderms are secondarily bilateral. We should probably say tertiary bilateral symmetry, but we don't.). Answer "c" is correct since some of the annelids are at least superficially radially symmetrical (e.g., the Christmastree worm or the sabellid polychaetes in your text).
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