Sponges and Placozoans Chapter 12

Origin of Metazoa Evolution of the eukaryotic cell was followed by diversification into many lineages including: Modern protozoans Plants Fungi Animals Multicellular animals are called metazoans.

Dendrogram of Major Phyla Porifera Actinopoda Cnidaria Apicomplexa Ciliophora Mollusca Annelida Euglenozoa Bryozoa Platyhelminthes Rhizopoda Rotifera Granuloreticulosa Nematoda Priapulida Kinorhyncha Loricifera Phoronida Brachiopoda Echinodermata Hemichordata Chelicerata Uniramia Crustacea Protochordata Vertebrata

Choanoflagellates Choanoflagellates are solitary or colonial protozoans with a flagellum surrounded by a collar of microvilli.

Choanoflagellates Choanoflagellates resemble sponge feeding cells (choanocytes). Scientists are studying colony formation and cell- to-cell communication in choanoflagellates in search of clues to the evolution of multicellularity.

Colonial Flagellate Hypothesis Colonial Flagellate Hypothesis – metazoans descended from ancestors characterized by a hollow, spherical colony of flagellated cells. Individual cells became specialized for different functions. Radially symmetrical, similar to a blastula. First proposed by Haeckel in 1874

Phylum Porifera Sponges, Phylum Porifera, are multicellular heterotrophs. They are asymmetrical. They lack true tissues and organs. Molecular evidence suggests they do share a common ancestor with other animals. Kingdom Animalia is monophyletic.

Phylum Porifera Sponges are sessile animals that have a porous body and choanocytes. Supported by a skeleton of tiny needlelike spicules and protein. They live in both fresh and marine waters.

Phylum Porifera Sponges range in size and shape. Up to 2 meters in diameter! Encrusting, boring, finger, tube or vase shaped.

Neighbors Many organisms, including crabs, nudibranchs, mites, bryozoans, and fish live as commensals or parasites in sponges.

Skeletal Framework The skeletal framework of a sponge may be fibrous or rigid. The fibrous part comes from collagen fibrils in the intercellular matrix. Spongin Rigid skeletons consist of needlelike spicules. Calcareous Siliceous Composition and shape the spicules forms the basis of sponge classification.

Suspension Feeders Sponges are suspension feeders capturing food particles suspended in the water that passes through their body.

Suspension Feeders Water flows in through incurrent pores called dermal ostia. It flows past the choanocytes where food particles are collected on the choanocyte collar.

Suspension Feeders Choanocytes take in small particles by phagocytosis. Protein molecules are taken in by pinocytosis. Sponges can also absorb nutrients dissolved in the water.

Canal Systems Asconoid – the simplest canal system. Choanocytes line the spongocoel. Water enters through the ostia and exit through the large osculum. Usually tube shaped. Found only in the Class Calcarea.

Canal Systems Syconoid – tubular body and singular osculum like asconoids. The walls of the sponge are folded to form choanocyte lined canals. Increased area for feeding. Class Calcarea.

Canal Systems Leuconoids – most complex, permits an increase in sponge size. Choanocytes line the walls of small chambers where they can filter all the water that flows through. Most sponges.

Types of Cells Absence of tissues & organs means that fundamental processes occur on the cellular level. Respiration and excretion occur by diffusion in each cell. Mesohyl is the gelatinous matrix containing skeletal elements & amoeboid cells.

Types of Cells Choanocytes, flagellated collar cells, generate a water current through the sponge and ingest suspended food.

Types of Cells The choanocytes pass food particles to archaeocyte cells for digestion. Digestion occurs entirely within cells, there is no gut. Other cell types secrete spicules (sclerocytes), spongin (spongocytes), & collegen (collenocytes).

Types of Cells Pinacocytes are thin, flat, epithelial- type cells that cover the exterior and some interior surfaces of the sponge. Almost a true tissue.

Reproduction Sponges have remarkable regeneration capabilities. Regeneration following fragmentation is a form of asexual reproduction. External buds can break off to form new sponges. Internal buds (gemmules) in freshwater sponges can remain dormant in times of drought.

Reproduction Most sponges are hermaphrodites meaning that each individual functions as both male and female. Monoecious Gametes are derived from choanocytes or sometimes archaeocytes.

Reproduction Most sponges are viviparous. After fertilization, the zygote is retained and is nourished by the parent. Ciliated larvae are later released. Some are oviparous releasing gametes into the water.

Reproduction Sponges in the class Calcarea and a few Demospongiae have an unusual developmental pattern where the embryo turns inside out. Flagellated cells become choanocytes & archaeocytes. Larger cells become pinacocytes.

Class Calcarea Calcareous sponges (Class Calcarea) have spicules composed of calcium carbonate. Small, usually vase shaped. Asconoid, syconoid, or leuconoid in structure.

Class Hexactinellida Glass sponges (Class Hexactinellida) are mostly deep sea forms. Spicules are six-rayed and made of silica. Hexactinellids lack a pinacoderm or gelatinous mesohyll. Chambers appear to correspond to both syconoid and leuconoid types.

Class Hexactinellida Some advocate placing hexactinellids in a subphylum separate from other sponges. Trabecular reticulum made of a fusion of archaeocyte pseudopodia - forms the chambers opening to spongocoel. Trabecular reticulum is largest continuous syncytial tissue known in Metazoa. Choanoblasts are associated with flagellated chambers. Collar bodies do not participate in phagocytosis – this is the function of the primary and secondary reticula.

Class Demospongiae Class Demospongiae contains most of the sponge species. Spicules are siliceous, but not six-rayed. Spicules may be bound together by spongin, or absent. All leuconoid, mostly marine.

Class Homoscleromorpha A fourth class, Homoscleromorpha, was formed to contain sponges without a skeleton or with siliceous spicules without an axial filament.

Cladogram of Sponge Classes

Phylogeny and Adaptive Diversification Sponges appeared before the Cambrian. Glass sponges expanded in the Devonian. One theory - sponges arose from choanoflagellates. However, some corals and echinoderms also have collar cells, and sponges acquire them late in development.

Phylogeny and Adaptive Diversification Molecular rRNA evidence suggests a common ancestor for choanoflagellates and metazoans. Sponges and Eumetazoa are sister groups with Porifera splitting off before radiates and placozoans.

Phylum Placozoa Trichoplax adhaerens is the sole species of phylum Placozoa (marine). No symmetry No muscular or nervous organs Placozoans glide over food, secrete digestive enzymes, and absorb nutrients.

Phylum Placozoa Cell layers Dorsal epithelium Thick ventral epithelium of monociliated cells and nonciliated gland cells. Space between the epithelia contain fibrous “cells” within a contractile syncytium. Grell considers it diploblastic. Dorsal epithelium represents ectoderm and ventral epithelium represents endoderm.