LECTURE 07 - Spiralia: Rotifera and Lophophorata

Question 1

In Rotifera, how does being small affect locomotion in fluid?

Answer 1

• The physical constraints on movement through a fluid (such as water or air) are different at small size
• Movement in fluids is governed by two sources of resistance:
  
  • the inertial resistance to acceleration a massive body, creating pressure drag
  • the viscous resistance caused by friction with the fluid, creating friction drag
• The ratio of inertial to viscous resistance is Reynold’s Number, Re –> Re = (D/V) uL
  
  • D = density, V = viscosity, u = velocity, L = length
• For water, Re ~= 100 uL with units of cm and s
• The qualitatively different dynamic regimes encountered by organisms of different size necessitate the evolution of different kinds of structure for locomotion

Question 2

What is the kind of structure for locomotion in bacteria?

Answer 2

• At very low Re, a continuously operating helical “corkscrew” filament, the bacterial flagellum, is an effective low-power motor

Question 3

What is the kind of structure for locomotion in flagellates?

Answer 3

• Unicellular eukaryotes use an undulatory flagellum

Question 4

What is the kind of structure for locomotion in rotifers?

Answer 4

In cilitates and micrometazoans, numerous very short flagella (cilia) are arranged in bands or fields as locomotory organs

Question 5

What is the kind of structure for locomotion in minnows?

Answer 5

• Larger animals use muscle-powered flexible oars or body deformation

Question 6

What is the kind of structure for locomotion in whales?

Answer 6

• Very large, fast swimmers have streamlined bodies powered by the thrust from a lunate tail fin

Question 7

Describe the body cavity and organ systems in Rotifera.

Answer 7

• The body wall consists of cuticle, epidermis and subepidermal muscles
• The epidermis is a thin sycytium with scattered nuclei
• There is a spacious body cavity between the body wall and the intestine and other organs
• It is not lined with mesoderm, nor crossed by mesenteries, so is often called a pseudocoel
• There are no respiratory or circulatory systems; gas exchange is by diffusion across the body wall into the fluid-filled pseudocoel

Question 8

What is the corona?

Answer 8

• The defining rotiferan feature is an anterior ciliated field, the corona
• In many species, the corona is developed as two concentric rings of cilia (trochus and cingulum) that beat in a metochronous pattern
• This action, resulting in a an illusion of a rotating wheel, is the reason for the name of the phylum

Question 9

What is the mastax?

Answer 9

• A second critical feature is a muscular pharynx (mastax) with chitinous jaws (trophi)
• Trophi are composed of seven articulating pieces which process food in a variety of ways (e.g., grinding, piercing, pumping, grasping)

Question 10

Describe Rotifera’s nervous system?

Answer 10

• The nervous system somewhat resemble that of a flatworm
• There is a brain (cerebral ganglion), sensory and motor nerves extending to different parts of the body, including additional ganglia, and two main ventral nerve cords
• There are prominent red eye spots in some groups

Question 11

Describe Rotifera’s reproductive system.

Answer 11

• The reproductive system is usually rather simple: a single syncytial ovary bounded by a membrane that continues to the cloaca as a simple tubular oviduct
• Minute animals have disproportionately large eggs

Question 12

Describe the sexual cycle of rotifers.

Answer 12

• Plankton rotifers can switch from amictic (parthenogenetic) reproduction to a sexual cycle, usually when crowded or starved
• This is a haplodiploid system: unfertilized sexual eggs develop into males whereas fertilized eggs develop into diploid females
• Male rotifers are usually smaller than females, may even be dwarfish and morphologically simplified

Question 13

Describe the development of Rotifers.

Answer 13

• Development is unusual
• Early cleavage is not clearly either spiral or radial, although the fates of blastomeres are said to resemble those of typical Spiralia
• Gastrulation is by epiboly, the growth of a rapidly dividing group of cells to enclose a more slowly dividing group
• It is difficult to compare the development of rotifers with that of other groups
• First cleavage is unequal and produces micromere at animal pole and macromere at vegetal pole
• Development is direct, without any larval stage
• Micromere lineage cleaves more or less equally and overgrow the cells at the vegetal pole (epiboly), which proceed to differentiate into organs and tissues

Question 14

What are some peculiarities of Rotifera?

Answer 14

• Most structures except ganglia are syncytial
• There is a fixed number of nuclei in each organ and tissue

Question 15

What are the three main groups of Rotifera?

Answer 15

• Seisonidea: epizoic on marine crusteacean, reduced corona, feed on host eggs, 2 or 3 species
• Bdelloidea: benthic rotifers, crawl in leech-like fashion, feed by ciliary current, temporary freshwater, exclusively asexual, many species
• Monogononta: plankton rotifers, often swim actively, permanent freshwater, feed by ciliary current, haplodiploid sexuality, hundreds of species

Question 16

What are Seisonidea?

Answer 16

• Elongate animals epizooic on the gills of the marine crustecean Nebalia
• Two species only
• Corona greatly reduced; mouth opens in a ciliated field bordered by a row of tufts of cilia
• Mastax present
• Long segmented foot
• Body covering syncytial
• Excretory system similar to other rotifers
• Strictly bisexual with similar male and female; internal fertilization (although it is not known how this is achieved)
• Often said to be primitive, but looks like a secondarily reduced parasite

Question 17

What are Bdelloid rotifers?

Answer 17

• Bdelloid rotifers are sexual
• Bdelloid rotifers produce large amictic eggs
• There is no trace of males or meiosis
• Their genomes are extraordinary
• Alleles have diverged widely in the absence of recombination
• There are very high rates of horizontal gene transfer
• The animals are highly resistant to radiation, perhaps through tetraploidy

Question 18

What is anhydrobiosis?

Answer 18

• At low humidity, bdelloids lose water, shrink and form a resistant structure called a “tun”
• If kept dry, they remain viable in this state for years or even decades

Question 19

What are Monogononta?

Answer 19

• Monogonont rotifers are an important component of the zooplankton of freshwater ponds and lakes
• They consume bacteria and unicellular algae, and are themselves eaten by copepods and fish larvae
• Rotifers also occur in the sea, but are not as diverse or abundant
• The dormant stage of monogononts is a resistant sexual egg rather than a tun

Question 20

Describe the rotifer trap.

Answer 20

• The aquatic fungus Zoophagus grows short pegs off the main hypha
• These trap the cilia of rotifers with an adhesive substance
• The tip of the peg grows into the body of the rotifer and absorb its contents

Question 21

What are Platyzoa?

Answer 21

‘Platyzoa’ and ‘Polyzoa’ are dubious taxa that include a variety of minute ciliated worms and swimmers

Question 22

What are the general features of ‘Platyzoa’ (except Acanthocephala)?

Answer 22

• Minute
• Cilia used for swimming, creeping and feeding
• No coelom (except rotifers)
• No circulatory or respiratory systems
• Excretory system simple protonephridium
• Unusual cleavage pattern in early development
• Direct development without larva

Question 23

What is Gnathostomulida?

Answer 23

• Gnathostomulida is a relatively small group of marine and brackish worms
• Gnathostomulids are vermiform animals measuring 300-3600 µm
• The integument is totally covered with locomotory cilia that all arise from monociliated cells
• The head may have different kinds of sensory structures
• The mouth leads to the muscular pharynx with its jaws
• A very short esophagus extends from the pharynx into the gut
• An anus is apparently missing but it has been shown that some species might have indications of a temporarily present anal pore
• a coelom is absent and fluid wastes are excreted via monociliated protonephridia
• All gnathostomulids are hermaphrodites

Question 24

What are microganthozoa?

Answer 24

• Minute jawed animals
• Limnognatia feeds on bacteria, blue-green algea, and diatoms
• During the search for food the head moves slowly from side to side while the ciliary bands on the head beat food particles toward the mouth
• When a food item reaches the mouth, it is quickly grabbed by the ventral jaws, dragged into the pharynx, and processed by the main jaws
• The jaw apparatus consists of several independent elements that are interconnected by ligaments and fine, cross-striated muscles
• The central part of the jaws consists of the large main jaws that anteriorly form a pair of forceps with long teeth
• Gut either blind or has an intermittently open dorsal anus
• There is no coelom

Question 25

What are Cycliophora?

Answer 25

• A minute animal that lives on the mouthparts of lobsters
• The anterior feeding structure is a ring of cilia
• The mouth ring leads into the intestine via a cilitate buccal cavity
• The stomach occupies most of the anterior part of the body; the gut is U-shaped, with the anus opening to one side of the feeding structure
• The space between body wall and gut is occupied by mesenchyme; there is no coelom
• The animal is anchored to the seta of a lobster by a pedal disc
• The male lacks all alimentary structures

Question 26

What are Gastrotricha?

Answer 26

• Gastrotrichs are minute aquatic animals found in freshwater and marine habitats
• They glide on surfaces using ventral cilia and eat bacteria and unicellular algae and protists

Question 27

Describe the anatomy of Gastrotricha.

Answer 27

• Mouth
• Posterolateral bristles
• Brain - relatively large
• Posterior salivary gland
• Pharynx
• Protonephridium
• Thin cuticle may bear spines or scales; cuticle underlain by syncytial epidermis
• Egg
• Ovary
• Cement gland
• Rectum
• Ventral surface is ciliated and provides locomotion
• Intestine
• Excretory pore
• Mesoderm, including circular and longitudinal muscle strands, no coelom
• Anterior salivary gland
• Anterolateral bristles

Question 28

Describe the reproduction and development of Gastrotricha.

Answer 28

• Chaetonotoid (freshwater) gastrotrichs are hermaphrodites with obligate internal self-fertilization
• They produce a fixed number (often four) of very large eggs
• Macrodasyoid (marine) gastrotrichs are cross-fertilized
• Development is unusual
• Cleavage is not clearly either spiral or radial and involves the movement of blastomeres
• A coeloblastula is formed, with two cells moving to its interior in a process of gastrulation and forming the archenteron
• Development is direct, with no larval stage

Question 29

What are Acanthocephala?

Answer 29

• Acanthocephalans are endoparasitic worms, ranging from minute to about 50 cm in length
• They have complex life cycles, with a crustacean as intermediate host and a vertebrate as final host
• There is no larva
• Instead, the acanthocephalan modifies the behaviour of the intermediate host to make it easier for final host to detect and swallow

Question 30

Describe the anatomy of Acanthocephala.

Answer 30

• The body wall is syncytial, not moulted
• Eversible head region with spines, which is thrust into the gut lining of the host
• The lemnisci are sac-like invaginations of the skin, which seem to hold a fluid reservoir to keep the proboscis erect
• There is no mouth or gut; no circulatory system

Question 31

How have zoologists determine what Acanthocephalans are?

Answer 31

• Classical zoologists used the structure of the body wall to place Acanthocephala close to rotifers, despite the obvious differences between them
• The latest analysis of mitochondrial genomes seems to confirm this by identifying seisonids as sister group of Acanthocephala

Question 32

What are Lophophorata?

Answer 32

• Lophophorata is a clade within Protostomia (maybe)
• The sister group is Nemertea (perhaps)
• Lophophorates have distinctive crown of hollow ciliated tentacles, the lophophore, that acts as food-collecting device

Question 33

What are the three phyla that originate from Lophophorata?

Answer 33

• Phoronida
• Bryozoa (Ectoprocta)
• Brachiopoda

Question 34

What is a lophophore?

Answer 34

• The body cavity of lophophorates is divided into two main regions by a septum attached to the body wall and embracing the oesophagus
• These two regions are:
  
  • The anterior mesosome, including the tentacles of the lophophore, which have a coelomic lumen (mesocoel) and do not communicate with the exterior
  • The trunk metasome, with a spacious coelom (metacoel), including the rest of the body
• The tentacles of the lophophore are borne along a paired ridge of the body wall, curved into a crescent or horseshoe shape
• In many species, they are rolled into a spiral, greatly increasing the number of tentacles that can be borne - up to a thousand or more
• The mouth is midventral between the two ridges, bordered by the fused bases of the tentacles

Question 35

What is Phoronida?

Answer 35

• Elongate animals living in a tube they secrete which incorporates sand grains, spicules, shell fragments, etc.
• Marine, in littoral zone
• Body with spacious coelom in two parts
• Anterior part bears crown of tentacles, the lophophore
• Long U-shaped gut with anus dorsally, outside lophophore
• Closed circulatory system
• Nerve net in base of epidermis, concentrated between mouth and anus but no definite ganglia or brain; nerve fibres extend to each tentacle
• Metanephridia opening beside anus

Question 36

What is the mechanism of the lophophore of Phoronis?

Answer 36

• The lophophore is a device for ciliary-stream feeding, whereby small particles in a current of water generated by coordinated ciliary action are directed to a mucus trap that flows, again by ciliary action, to the mouth
• The lophophore works by mechanical sorting
• Particles of the right size are deposited on the epistome at the base of the tentacles, much as a heavy vehicle will crash out at a sharp bend in the road
• there are also specialized cilia to remove large inedible particles

Question 37

What does the larval stage in Phoronida look like?

Answer 37

• Cleavage is (usually) (more or less) equal and radial, yielding a blastula which gastrulates by invagination
• Blastopore becomes mouth, with an enteron formed by overgrowth of a preoral lobe; enteron elongates and fuses with ectoderm to establish the anus
• Enteron differentiates into stomach and intestine and becomes L-shaped
• Larva then develops hollow ciliated projections that become feeding tentacles
• There is an intraectodermal nerve net with ganglia in the preoral lobe from which nerve tracts issue
• There is also a circulatory system
• Coelomic cavities develop within the growing larva
• The fully developed larve is called an actinotroph, and is an abundant component of the zooplankton

Question 38

Describe the metamorphosis of the actinotroph (the larva of Phoronida)

Answer 38

• The larva becomes sluggish and opaque and sinks to the bottom
• Through convulsive body contractions, the metastome pouch is suddenly fully everted to the exterior, carrying with it the digestive tract, which is thus thrown into the hairpin loop characteristic of the adult
• The preoral lobe shrinks and is cast off
• The larval tentacles are cast off
• Within 15 minutes, the body plan of the individual has changed, transformed to a sac-like trunk with U-shaped gut about to develop the adult lophophore

Question 39

Where can you find Bryozoa (= Ectoprocta)?

Answer 39

Marine and freshwater lophophorates, abundant in littoral zone and to 200m depth in all oceans; extend to abyssal depths

Question 40

Describe Bryozoa (Ectoprocta).

Answer 40

• Minute lophophorates with lophophore and recurved gut, permanently encased, forming colonies produced by budding, usually encrusting on hard surfaces
• Body consists of lophophore and trunk; lophophore very similar to that of phoronids, embracing mouth
• Recurved digestive tract
• Nerve net throughout body wall with circumpharyngeal ganglionic ring
• No circulatory, respiratory or excretory systems (too small to need them)
• Hermaphroditic with definite gonads but no gonoducts
• Development obscure and unusual; cleavage radial; in blastualy, four cells at vegetal pole elongate, and inner halves become cut off and proliferate in cavity to form endoderm; mesoderm appear to arise from ectoderm
• Young usually brooded with release of trochophore-type larvae which settle and metamorphose into founding zooid of colony

Question 41

What is Bryozoa-in-a-box?

Answer 41

• The zooid is permanently encased in a protective box, with chitinous walls in smaller forms which are calcified in more massive forms (the majority)
• It is attached to the floor of this box (unlike phoronoids, which can move freely in their tube)
• Spacious coelom divided into two regions: the cavities of the trunk and lophophore
• U-shaped gut hangs in trunk coelom suspended by mesenteric cords; anus opens outside the tentacle crown
• Lophophore is everted, most simply by contraction of the parietal muscles, using the trunk coelom as a hydrostatic skeleton; it can be withdrawn very rapidly by retractor muscles
• No specialized respiratory or excretory systems
• No heart, though the funiculus contains fluid and may be a circulatory system transporting nutrients between zooids

Question 42

Describe the colony form of Bryozoa.

Answer 42

• All bryozoans form colonies with some degree of physical connection between zooids
• The simplest colonies consist of a creeping linear stolon from which zooids arise at intervals, but most species form more extensive frondose or (more often) encrusting colonies, consisting of a continuous succession of zooids more or less fused to their neighbours

Question 43

Describe the coloniality of Bryozoa.

Answer 43

• Brryozoans can bud, and form a persistent stolon along which nutrients can be translocated, just like hydroids
• This enables them to form a colony to occupy favourable sites, like hydroids; and like hydroids, these colonies form compound individuals by expressing functional differentiation among zooids

Question 44

What are freshwater bryozoans?

Answer 44

• There has been a modest radiation of Bryozoa in fresh water, recognized as a separate order, Phylactolaemata
• Most form gelatinous colonies attached to sticks, plant stems or similar objects
• Zooids are often scarcely separate, with digestive tracts hung in common coelom
• There is no differentiation among zooids
• Development involves a short larval stage with a completely ciliated larva
• Also reproduce asexually by producing statoblasts, a mass of cells enclosed within a protective covering

Question 45

What are Brachiopoda?

Answer 45

• Brachiopods are lophophorates enclosed by a bivalve shell consisting of two dissimilar valves and lined with a mantle lobe of the epithelial body wall
• The shell is attached to the substrate directly or with a long peduncle
• The two main groups are articulate brachiopods, in which the two valves are hinged by a tooth-and-socket device, and inarticulate branchiopods, in which the valves are held together solely by muscles

Question 46

What are Brachiopoda?

Answer 46

• Brachiopods are lophophorates enclosed by a bivalve shell consisting of two dissimilar valves and lined with a mantle lobe of the epithelial body wall
• The shell is attached to the substrate directly or with a long peduncle
• The two main groups are articulate brachiopods, in which the two valves are hinged by a tooth-and-socket device, and inarticulate branchiopods, in which the valves are held together solely by muscles
• Superficially similar to clams
• Extremely abundant in Palaeozoic; many more fossil than extant species
• Severely reduced by end-Permian extinction and largely replaced by bivalves in modern ocean

Question 47

Describe the anatomy of Brachiopoda.

Answer 47

• The trunk of the animal occupies about a third of the space within the shell, closest to the hinge
• The rest of the mantle cavity is occupied by the lophophore
• The lophophore itself is U-shaped and non-retractable
• The mouth is in the centre of the lophophore; the intestine, seldom U-shaped as in other lophophorates, opens by an anus into the mantle cavity in the inarticulates but ends blindly, without an anus, in articualtes
• There is a spacious coelom divided into two (or three) paired compartments, which extends into the lophophore
• Circulatory system is open, with a contractile vesicle
• Excretory system of two pairs of metanephridia, which act as gonoducts

Question 48

How can Brachiopoda borrow?

Answer 48

• Most branchiopods can burrow, using the shell valves as shovels
• They are anchored to the bottom of the burrow, into which they can retreat by contraction of a long peduncle that issues from the posterior of the valves and has a coelomic lumen

Question 49

What is an example of an archaic lophophorate?

Answer 49

• Hyolithina are problematic Palaeozoic animals that appeared in the early Cambrian and disappeared in the end-Permian mass extinction
• They were benthic organisms with a calcareous shell, operculum and characteristic long, recurved anterior structures
• The recent discovery of soft parts suggests that they may be an extinct clade of lophophorates