Ch. 4

Question 1

Phylum Porifera

Answer 1

(Latin: pore bearing)

Defining Characteristic:’ Microvillar collars surround flagella, with units arising from either single cells or syncytia

Question 2

Sponges:

Answer 2

• 2% fresh-water.
• 98% marine
• None terrestrial
• Lack nerves
• Lack a conventional musculature
• All feed on food particles suspended in the water.
• Typically amorphous, asymmetrical creatures (although are some very beautiful exceptions),
• Have no specialized reproductive, digestive, respiratory, sensory, or excretory organs.
• No adult sponge has anything corresponding to anterior, posterior, or oral surfaces.
• About 20 morphologically distinct cell types can be recognized in individual sponges.
• The cells functionally independent to extent that an entire sponge can be dissociated into its constituent cells with no long-term impact.
  — The cells dedifferentiate to an amoeboid form, reaggregate, and at least in some species, can redifferentiate to reform the sponge.
• One sponge can distinguish between its own cells and those of different individuals in same species
• Major components of aquatic communities
• Compete impressively with other sedentary metazoans
• Provide homes for a variety of animals from many other phyla, and for a variety of bacteria and cyanobacteria.
• Some freshwater colonial protozoans (freshwater choanoflagellates) bear very definite morphological similarities to the simplest sponges, and as recently as 100 years ago —> suggesting that sponges be classified as colonial protozoans.
  — However, recent molecular data support a common evolutionary origin for sponges and more structurally complex animals.

From molecular analyses:

• Demosponges and glass sponges may not have given rise to further lineages
• Calcareous sponges along with Homoscleromorpha group, may be the ancestors of other multicellular animals, making the phylum Porifera paraphyletic.
• Their Fossil record extends back about 580 million years, and molecular evidence of chemicals uniquely produced through sponge metabolism have been reported from sediments at least 50 million years older than that.

Question 3

Porifera General Characteristics

Spongocoel, choanocytes, collar cells

Answer 3

sponge is a fairly rigid, perforated bag whose inner surface is lined with flagellated cells.

The empty space within this bag is called the spongocoel.

The flagellated cells lining the spongocoel are called choanocytes (literally, “funnel cells”), or collar cells in recognition of the cylindrical arrangement of the cytoplasmic extensions (collars) surrounding the proximal portion of each flagellum.

Question 4

Porifera General Characteristics

Collar cells perform the following functions:

Answer 4

The collars of sponge choanocytes may be homologous with those of protozoan choanoflagellates.

1. They generate currents that help maintain circulation of seawater within and through the sponge.
2. They capture small food particles.
3. They capture incoming sperm for fertilization.

Sponges typically have thousands of collar cells per cubic mm of tissue.

Question 5

Porifera General Characteristics

mesohyl layer

Answer 5

Adjacent to the choanocyte layer (formally known as the choanosome) is a gelatinous, nonliving layer of material called the mesohyl layer (meso = middle; hyl = stuff, matter).

Although the mesohyl is itself acellular, it contains live cells.

Question 6

Porifera General Characteristics

Archaeocytes

Answer 6

Amorphous, amoeboid cells called archaeocytes wander throughout the mesohyl by typical cytoplasmic streaming, which involves the formation of pseudopodia as in amoeboid protozoans.

• perform a number of essential functions within sponges
• develop into more specialized cell types when necessary
• responsible for digesting food particles captured by the choanocytes (= digestion is entirely intracellular.)
• some store digested food material
• may give rise to both sperm (flagellated) and eggs, although gametes may also arise through morphological modification of existing choanocytes.
• probably play an active role in non-self-recognition reactions in response to contact with other sponges.
• play a role in eliminating wastes
• can become specialized to secrete the supporting elements located in the mesohyl layer (Spicules- by sclerocytes) or they may be fibers composed of a collagenous protein (spongin- by spongocytes).

Question 7

Porifera General Characteristics

spicules and sclerocytes

Answer 7

Calcareous or silicious formations present in the tissues and generally serving protective or supportive functions.

archaeocytes become specialized to secrete these supporting elements located in the mesohyl layer.

cells secreting spicules are termed sclerocytes

Question 8

Porifera General Characteristics

spongin and spongocytes

Answer 8

fibers composed of a collagenous protein

cells producing spongin fibers are termed spongocytes

Question 9

sclerocytes and spongocytes are of great importance:

Answer 9

(1) to sponges, which generally depend upon the support elements for maintaining shape and, possibly, for discouraging predation;

(2) to systematists, as an indispensable factor in species identification.

Question 10

Porifera General Characteristics

gemmules

Answer 10

At certain times of the year, many freshwater sponge species (and a few marine species) produce dormant structures called gemmules.

Archaeocytes accumulate nutrients by phagocytizing other cells and then cluster together within the sponge.

Then Certain cells surrounding each cluster secrete a thick, protective covering; the gemmule consists of the cluster plus its surrounding capsule.

Gemmules are typically far more resistant to desiccation, freezing, and anoxia (lack of oxygen than are the sponges that produce them.

Question 11

Porifera General Characteristics

gemmule vernalization

Answer 11

the gemmules of many species must spend several months at low temperatures before they become capable of hatching; that is, they require a period of vernalization.

Under appropriate environmental conditions, the living cells leave the gemmule (hatch) through a narrow opening and differentiate to form a functional sponge.

Gemmule formation thus allows sponges to withstand unfavorable environmental conditions by entering a stage of developmental arrest, a period of dormancy.

Because each sponge produces many gemmules, gemmule formation can be an effective means of asexual reproduction, resulting in numerous genetically identical oftspring.

Question 12

Instead of forming gemmules

Answer 12

some freshwater and marine species undergo pronounced tissue regression during unfavorable periods, with the sponge becoming reduced to little more than a compact cellular mass with an outer protective covering.

The cells reactivate when Evironmental conditions improve, regenerating all of the structures present before the regression.

Question 13

Porifera General Characteristics

pinacocytes

Answer 13

The outer layer of most sponges is composed of flattened contractile cells called pinacocytes forming a layer called the pinacoderm.

Also line the incurrent canals and the spongocoel in places where choanocytes are lacking.

Contraction of pinacocytes enables sponges to undergo minor to major shape changes and also may play a role in regulating water flow through the sponge by varying the diameter of the incurrent openings.

Some pinacocytes are ciliated, but not in any known calcareous sponges.

Question 14

Porifera General Characteristics

basal lamina

Answer 14

In most animals, epithelial cells rest on a collagenous sheet of extracellular matrix that they secrete, called the basal lamina (or basement membrane); the pinacocytes of most sponges lack a basal lamina.

Some sponges have functional epithelial tissues capable of preventing small molecules from diffusing into and out of the animal, and of transporting specific ions to control the membrane potential.

Question 15

Porifera General Characteristics

dependence on water flow

Answer 15

Because they lack muscles, nerves, and deformable bodies, sponges are utterly dependent upon water flow for food, gas exchange, waste removal, and the dissemination and collection of sperm.

Question 16

Porifera General Characteristics

ostia and oscula

Answer 16

• Partly as a consequence of choanocyte activity and partly as a consequence of sponge architecture:
  — water flows into the spongocoel through narrow openings (ostia)
  — and then water exits the spongocoel through larger openings (oscula).
• Ostia always numerous on the sponges body
• there may be only one osculum present per individual.
• Rapid flow past a food-capturing surface is incompatible with efficient particle capture.
• water slow down within the choanocyte chambers, allowing time for efficient particle removal, and then speed up on the way out of the sponge to dissipate wastes (and sperm) effectively.
  — simply because of internal sponge architecture.

Question 17

Porifera General Characteristics

chemical compounds used for protection

Answer 17

Natural products chemists have become extremely interested in sponges for their

chemical compounds produced uniquely by particular sponge species to protect against predation and to discourage the larvae of barnacles and other “fouling” organisms from attaching to their surfaces.

• chemicals are produced by the sponges themselves
• some are produced by bacterial and cyanobacterial (blue-green algal) symbionts.
• Some of chemicals released may not discourage fouling by the larvae of other animals directly, but rather by modifying the species’ composition of adjacent bacterial communities.
  _{—biomedical applications, useful in discouraging larvae from attaching to ship hulls and other underwater surfaces}

Question 18

three basic levels of sponge construction

Answer 18

asconoid, syconoid, and leuconoid, in order of increasing complexity.

Each form simply reflects an increased degree of evagination of the choanocyte layer away from the spongocoel, increasing the extent of flagellated surface area enclosed by the sponge.

Most leuconoid.

Question 19

Poriferan Diversity

Answer 19

sponges are immobile, members of a few sponge species can move presumably by the coordinated cytoplasmic movements of individual, amoeboid cells.

Sponges are distributed among the following four classes, based largely upon the chemical composition and morphology of the support elements: Calcarea, Demospongiae, Hexactinellida, and Homoscleromorpha.

Homoscleromorph species previously contained within Demospongiae.

Question 20

Class Calcarea

Answer 20

• bear spicules composed only of calcium carbonate (CaCOz).
• only sponge class to include all three types of body construction; indeed, the only living asconoid forms are found among the Calcarea.

Question 21

Class Demospongiae

Answer 21

• contain at least 80% of all sponge species
• nearly all of leuconoid construction
• supporting spicules and fibers may be composed of spongin and/or silica, but never of CaCO3.
• skeletal fibers of this class and glass sponges also contain chitin.
• small number of species possess neither fibers nor spicules.
• All freshwater sponges found in this clade
• freshwater species possess contractile vacuoles, which are organelles specialized for eliminating water from cytoplasm.

<sub>bizarre group of deep sea sponges, spicules of these “cladorhizid” sponges place them within the this class.
- no ostia
- no oscula
- no internal canal system
- no choanocytes.
- feed as carnivores, entrapping small swimming crustaceans on numerous long, thin filaments that cover much of the body surface.
- Captured prey are gradually surrounded by adjacent epithelial cells and the growth of new filaments, and are digested externally</sub>

_{other cladorhizid sponges do have a functional internal canal system, complete with choano-cytes, and yet they still feed primarily by carnivory.}

Question 22

Class Hexactinellida

“glass sponges”

Answer 22

• bodies are supported entirely by interconnected six-rayed spicules of silica and chitin.
• some live in soft sediment, anchored by tufts of spicules
• some live attached to solid substratum.
• canal systems may be either syconoid or leuconoid
• entire sponge, including the outer layer, is syncytial (having many nuclei contained within a single plasma membrane) rather than cellular
• lacks contractile elements; thus, there is no pinacoderm layer
• The inner, flagellated layer is also syncytial
• long and thin silica fibers secreted by these sponges at their base may have light-guiding properties superior to those found in commercially produced fiber-optic cables, and are less prone to fracture.

Question 23

Class Homoscleromorpha

Answer 23

• most species lack spicules
• when spicules are present, they are entirely siliceous
• have a clear, distinct basal membrane underlying the epithelium
• all their epithelial cells bear cilia.

Question 24

hermaphroditic and reproduction of sponges

Answer 24

• a single individual producing both types of gametes.
• Fertilization and early development are typically internal.
• In at least some species, choanocytes capture the incoming sperm, dedifferentiate to amoeboid form, and then transport the sperm to the mesohyl, where the eggs are fertilized.
• Most sponge species retain (i.e., “brood”) the developing embryos for a time, releasing them through the oscula as swimming larvae.
• smaller number of species are oviparous; newly fertilized eggs (or gametes themselves) are shed into the seawater, so embryonic development is external.

-

Question 25

coeloblastula

Answer 25

In calcareous sponges and in some demosponges: - embryo develops into a hollow blastula (coeloblastula; coelo = hollow).

Question 26

amphiblastula

Answer 26

In some calcareous sponges inversion occurs: - rapidly dividing cells at one end of the embryo become flagellated, with flagella directed into the blastocoel rather than toward the outside of the embryo. - other cells of the embryo divide more slowly and remain unflagellated. - The blastocoel opens to the outside in the middle of this group of relatively large, slowly dividing cells - as development continues, embryo demonstrates a process called inversion and turns inside out through this opening - internal flagella thus come to lie on the outer surface, where they propel the larva forward (flagellated end leading) once the individual is discharged through the excurrent canal system. - Even after turning inside out, the embryo remains only one cell thick

Question 27

stereoblastula (stereo = solid)

Answer 27

In some other calcareous sponge species, the initially hollow embryo (coeloblastula) becomes solid as numerous cells detach from the wall of the blastula and completely fill the blastocoel.

Question 28

embryos and larvae demosponges

Answer 28

- Embryos develop directly into stereoblastulae, which then differentiate to form extensively flagellated parenchymella (also called parenchymula larvae); each cell usually bears one flagellum. Embryos of glass sponges and many freshwater sponges: - parenchymella larvae possess spicules and choanocytes, and even rudimentary canal systems. - Hexactinellid larvae become highly differentiated, possessing both siliceous spicules and choanocyte chambers; only cells in the larval midsection are flagellated, and each cell bears a number of flagella.

Question 29

Sponge larvae

Answer 29

- typically incapable of feeding, and swim for less than 24 hours before metamorphosing. - Before losing the ability to swim, the larvae attach to a substrate. In at least some calcareous and demosponge species: - choanocytes develop directly from the flagellated cells of the larva. In some other sponge species: - the flagella are degraded and choanocytes then differentiate de novo from archaeocytes. In at least one freshwater species: - the flagellated cells first dedifferentiate into amoeboid cells, which later redifferentiate into choanocytes.

Question 30

Phylum Placozoa

Answer 30

(flat plate animal) Defining Characteristic: Small, multicellular, amor-phous, mobile, animals lacking a body cavity, digestive system, and nervous system and composed of two layers of ciliated epithelial cells with a layer of multinucleated contractile cells in between. Only a single placozoan species has been described, Trichoplax adhaerens.

Question 31

Placozoa General Characteristics

Answer 31

- no front or back - no right or left - lack organs and tissues. - no digestive system - no nervous system - no true musculature. - lack specialized sensory structures - lack cells that are artificially dissociated will reaggregate to reform a functioning animal. - fully mobile - planktonic for part of their lives, b/c they collect on microscope slides suspended in the water - seem adapted for gliding across hard substrates on thousands of motile cilia and through coordinated contractions of fiber cells, changing shape markedly, amoeba-style, as they travel. - don't get much larger than 2 mm across - smallest nuclear genome of any known animal, at 98 million base pairs - are flat, with two distinct layers of epithelial cells, each layer containing perhaps a thousand or so cells. - lack Basal lamina - The ventral layer is composed of columnar cells, each bearing a single flagellum. — Associated glandular cells apparently secrete digestive enzymes beneath the animal as it sits atop the algae and protozoans on which it apparently feeds - digestion seems to be entirely extracellular, as there is no mouth and no sign of phagocytosis. - The much thinner, upper layer of the animal bears flagellated cells along with numerous "shiny spheres" that seem to play a role in chemical defense against predators, but no gland cells. - Between the upper and lower cell layers is a fluid-filled space containing a dense network of fibrous cells that may be contractile. - Asexual reproduction by budding, fragmentation, or binary fission - could also reproduce sexually - individuals have no difficulty regenerating pieces that are cut off. - mitochondrial genome is the largest ever documented, at 43,079 base pairs, not because of especially complex coding systems, but mostly because it contains numerous intragenic spacers and introns - genome includes genes that code for various transcription factors and signalling genes that are involved in early embryonic development and cell fate determination in most other animals. - Possibly, placozoans are secondarily simplified from more complex animals. _{- If placozoans are secondarily simplified from more complex animals, then placozoan evolution must reflect loss of the nervous system, and other degenerative events.}