CHAPTER 12 Sponges and Placozoans

Powerpoint revised by Franklyn Tan Te

12-1 Copyright © 2013 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. A Caribbean demosponge, Aplysina fistularis.

12-2 Origins of Multicellularity

A. Cells are the elementary units of life 1. Nature’s experiments with larger organisms without cellular differentiation are limited such that large, single celled marine algae are rare 2. Sponges are the simplest multicellular animals but their cell assemblages are distinct from other metazoans. 3. Sponges have cells embedded in an extracellular matrix supported by a skeleton with needle-like spicules and

12-3 protein. Origins of Multicellularity

◼ Increasing the size of a cell causes problems of exchanging molecules with the environment. ◼ Multicellularity prevents surface-to-mass problems as smaller units greatly increase surface area for metabolic activities ◼ Highly adaptive towards larger body size ◼ Sponges neither look like or behave as animals but molecular evidence demonstrates that they are phylogenetically grouped with animals 12-4 Origin of Metazoa

◼ Evolution of the Metazoa (animals) ◼ Eukaryotic cells evolved and diversified into many lineages that led to modern day descendants ◼ Includes all unicellular protozoans, colonial and multicellular plants, animals, and fungi ◼ Multicellular organisms were collectively called metazoans and are now also termed “animals” ◼ Metazoans are placed in the Opisthokont clade which include fungi, choanoflagellates, and other groups 12-5 Origin of Metazoa

◼ Choanoflagellates ◼ Solitary or colonial aquatic eukaryotes ◼ Each cell has a flagellum surrounded by a collar of microvilli ◼ Flagellum beats and draws water into collar where the microvilli collect particles like bacteria ◼ Most are sessile but one species attaches to floating diatom colonies and feed midwater ◼ Strongly resemble sponge feeding cells called choanocytes, which have been argued to be ancestral to choanoflagellates

12-6 Origin of Metazoa

◼ Evidences of common ancestry between Choanoflagellates and Metazoans ◼ By comparing the genomes or proteomes of sponges with more complex taxa, scientists can discover what cell transmitters or morphogens the first metazoans possessed. ◼ Shared characteristics would have been inherited from the most recent common ancestor of animals. ◼ Molecular phylogeny indicates that colonial bodies evolved early in the lineage.

12-7 Origin of Metazoa

◼ Recent research indicates proteins used by colonial choanoflagellates for cell communication and adhesion are homologous to those that metazoans use in cell-to-cell signaling. ◼ Sponge genome contains elements that code for regulatory pathways of more complex metazoans ◼ Sponges have proteins that code for spatial patterning that specify anterior and posterior pole of larvae.

12-8 Origin of Metazoa

◼ Sponges today are less complex than their ancestors ◼ Sponges have simple bodies composed of aggregates of several cell types held together by extracellular matrix ◼ Sponge bodies are not symmetrical and have no mouth or digestive tract ◼ Placozoans share features with other animal groups. ◼ Have small nuclear genome and the largest mitochondrial genome in the animal kingdom ◼ Placozoan bodies are as puzzling as sponges: they also have no heads or tails 12-9 Phylum Porifera

◼ General Features of Sponges ◼ Mostly sessile ◼ Body designed for efficient aquatic filter feeding ◼ Porifera means “pore-bearing”; sac-like bodies are perforated by many pores ◼ Use flagellated “collar cells”, or choanocytes, to move water to bring food and oxygen while removing wastes ◼ Most of the 8600 sponges are marine, found in all seas and all depths, while few live in brackish water and 150 live in fresh water

12-10 Figure 12.1 Some growth habits and forms of sponges. 12-11 Figure 12.2 Sponge choanocytes have a collar of microvilli

12-12 surrounding a flagellum. Phylum Porifera

◼ Sponges vary in size from a few millimeters to over 2 meters in diameter ◼ Many species are brightly colored because of pigments in dermal cells ◼ Embryos are free-swimming while adult sponges always attached ◼ Some appear radially symmetrical but many are irregular in shape ◼ Some stand erect, some are branched, and others are encrusting ◼ Growth patterns depend on shape of substratum, direction of water, speed of flow

12-13 and availability of space Phylum Porifera

◼ Many animals like crabs, nudibranchs, fish, and other species do live as commensals or parasites in or on sponges ◼ Sponges can also grow on a variety of other living organisms with some crabs using sponges for camouflage and protection ◼ Sponges and microorganisms living on them often have a noxious odor and produce a variety of bioactive compounds ◼ Certain sponge extracts have manifested medical and pharmaceutical effectiveness. 12-14 Phylum Porifera

◼ Skeletal structure of a sponge can be fibrous and/or rigid consisting of calcareous or siliceous spicules ◼ Fibrous portion comes from collagen protein fibrils in intercellular matrix ◼ There are several types of collagen, which vary in chemical composition; sponges contain spongin ◼ Composition and shape the spicules form the basis of sponge classification ◼ Modern materials science view spicules for possible nanoparticle products ◼ The simplistic exterior of sponges often mask their chemical and functional sophistication 12-15 Figure 12.3 Diverse forms of spicules that support a sponge body.

12-16 Phylum Porifera

◼ Sponges date back to the early Cambrian and maybe even Precambrian period ◼ Traditionally grouped in three classes based on spicules and chemical composition ◼ Calcarea: calcium carbonate spicules with one, three, or four rays ◼ Hexactinellids: glass sponges with six-rayed siliceous spicules ◼ Demospongiae: siliceous spicules around an axial filament, spongin fibers, or both ◼ Homoscleromorpha, was formed to contain sponges without a skeleton or with siliceous spicules without an axial filament 12-17 Figure 12.4 Cladogram depicting evolutionary relationships among the four classes of sponges.

12-18 Phylum Porifera

◼ Form and Function ◼ Body openings consist of small incurrent pores or dermal ostia in the outer layer of cells called pinacoderm ◼ Sponges feed by collecting suspended particles from the water through internal canal systems ◼ Water is directed past the choanocytes, which are flagellated collar cells that keep the current flowing via beating of flagella ◼ Microvilli in the collar trap and phagocytize food particles that pass by.

12-19 Phylum Porifera

◼ Sponges non-selectively consume food particles (detritus, plankton, and bacteria) ◼ The smallest particles (80%) are taken into choanocytes by phagocytosis ◼ Protein molecules may be taken in by pinocytosis ◼ Two other cell types, pinacocytes and archaeocytes, facilitate feeding ◼ Dissolved nutrients can also be absorbed by sponges ◼ Efficiency of food capture is dependent on water movement through the sponge body

12-20 Phylum Porifera ◼ Three types of sponge body designs ◼ Asconoids ◼ Simplest body organization ◼ Small and tube-shaped to allow water to flow directly across cells so no “dead space” ◼ Choanocytes are in a large internal chamber, the spongocoel ◼ Choanocyte flagella pull water through the pores and extract food particles ◼ Used water is expelled through a large single osculum ◼ All Calcarea are asconoids ◼ Leucosolenia sp. and Clathrina sp, for example 12-21 Figure 12.5 Three types of sponge structure.

12-22 Figure 12.6 Clathrina canariensis (class Calcarea) is a common Asconoid on Caribbean reefs.

12-23 Phylum Porifera

◼ Syconoids ◼ Resemble asconoids but larger and with a thicker more complex body wall ◼ Body wall is folded outwards with choanocyte- lined radial canals that empty into spongocoel ◼ Water enters through dermal ostia and move into tiny openings called prosopyles into the radial canals ◼ Food is ingested by choanocytes and used water is pumped through internal pores called apopyles then outwards via osculum ◼ Spongocoel is lined with epithelial cells rather than choanocytes as in asconoids

12-24 Phylum Porifera

◼ Developmental evidence of being derived from asconoid ancestors ◼ Syconoids pass through an asconoid stage in development but do not form highly branched colonies ◼ Flagellated canals form by evagination of the body wall ◼ Syconoid body plan is not homologous among all sponges that have it ◼ Classes Calcarea and Hexactinellida have syconoid species (ex: Sycon sp.)

12-25 Figure 12.7 Cross section through wall of sponge Sycon sp., showing choanocytes in canals within the wall but do not line spongocoel.

12-26 Phylum Porifera

◼ Leuconoids ◼ Most complex and larger, for more food- collecting regions ◼ These regions have choanocytes lining in small chambers that effectively filter all water present ◼ Clusters of flagellated chambers are filled from incurrent canals and discharge to excurrent canals which lead to osculum ◼ After food is removed, used water is pooled to form an exit stream that leaves through an exit pore at very high velocity ◼ This high rate of exit flow prevents the sponge from re-filtering used water and wastes

12-27 ◼ Most sponges are leuconoid type Phylum Porifera

◼ The leuconoid system has high adaptive value to efficiently meet high food demands of larger body size ◼ Has the highest proportion of flagellated surface per volume of cell tissue ◼ More collar cells can filter more particles ◼ Water flow slows down inside due to greater surface area within the chambers ◼ Large sponges filter 1500 liters of water per day for maximum food collection ◼ The leuconoid system has evolved independently many times in sponges

12-28 Figure 12.8 This orange demosponge, Mycale laevis, often grows beneath plate-like colonies of the stony coral Montastrea annularis.

12-29 Phylum Porifera

◼ Types of Cells in Sponges ◼ Sponge cells are arranged in a gelatinous extracellular matrix called mesohyl or mesenchyme ◼ The connective “tissue” of sponges found in fibrils, skeletal elements, and amoeboid cells ◼ Absence of organs requires that all fundamental processes occur at the individual cell level ◼ Respiration and excretion via diffusion and water regulation via contractile vacuoles in the archaeocytes and choanocytes

12-30 Phylum Porifera

◼ Visible activities seen in sponges include slight alterations in shape, local contraction, propagating contractions, and closing and opening of incurrent and excurrent pores ◼ Sponges can close their osculum due to heavy sediment load ◼ Movements occur very slowly but they suggest a whole body response in organisms lacking complex organization above the cellular level ◼ Apparently excitation spreads from cell to cell by mechanical stimuli and signaling molecules

12-31 like hormones or via electrical impulses Figure 12.9 Small section through sponge wall, showing four types of sponge cells.

12-32 Phylum Porifera

◼ Choanocytes ◼ Oval cells with one end embedded in mesohyl and exposed end has one flagellum surrounded by a collar ◼ Collar consists of microvilli connected to each other by fine microfibrils ◼ Forms a fine filtering device to strain food ◼ Particles too large to enter collar are trapped in mucous and slide down to base to be phagocytized ◼ Food is passed to archaeocytes for intracellular digestion with no need for gut cavity

12-33 Figure 12.10 Food trapping by sponge cells. A) Cutaway section of canals showing direction of water flow. B) Two choanocytes, and C) structure of the collar.

12-34 Phylum Porifera

◼ Archaeocytes ◼ Amoeboid cells that move about in the mesohyl with many functions ◼ Phagocytize particles in the pinacoderm ◼ Receive particles for digestion from choanocytes ◼ Can differentiate into many other more specialized cell types ◼ Sclerocytes: secrete spicules ◼ Spongocytes: secrete spongin ◼ Collencytes: secrete fibrillar collagen ◼ Lophocytes: secrete large amounts of collagen 12-35 Phylum Porifera

◼ Pinacocytes ◼ Thin, flat, epithelial-like cells that cover the exterior and interior surfaces of sponges almost like real tissues ◼ Form pinacoderm with a variety of intercellular junctions but no basal membrane for most sponges ◼ Ingest food by phagocytosis and are contractile to regulate surface area of sponge ◼ Form myocytes that are circular bands around oscula and help regulate flow of water

12-36 Phylum Porifera

◼ Cell Independence: Regeneration and Somatic Embryogenesis ◼ Sponges have a great ability to regenerate lost parts and repair injuries ◼ Complete reorganization of the structure and function of participating cells or bits of tissue occurs in somatic embryogenesis ◼ Process of reorganization differs in sponges of differing complexity ◼ Regeneration following fragmentation is one means of asexual reproduction

12-37 Phylum Porifera

◼ Types of Asexual reproduction ◼ Fragmentation ◼ Sponge breaks into parts that are capable of forming a completely new sponge ◼ Bud formation ◼ External buds ◼ Small individuals that break off from parents that have reached a certain size ◼ Internal buds or gemmules ◼ Formed by archaeocytes that collect in mesohyl and coated with tough spongin and spicules that can survive harsh

12-38 environmental conditions Phylum Porifera

◼ How gemmules work? ◼ When parent sponge dies, gemmules survive and remain dormant during the harsh situations ◼ Live cells within gemmules escape through special opening called micropyles and develop into new sponges ◼ Gemmulation is a adaptation to changing seasons and for colonization of new habitats ◼ Gemmules are controlled by weather, internal chemicals, and by remaining inside the parent sponge.

12-39 Figure 12.11 Section through a gemmule of a freshwater sponge (Spongillidae).

12-40 Phylum Porifera ◼ Sexual Reproduction ◼ Most are monoecious (both male and female sex cells in one body) ◼ In some Demospongiae and Calcarea ◼ Gametes develop from choanocytes ◼ Some gametes from archaeocytes ◼ Most sponges are viviparous where zygote is retained within parent and provided with nourishment until it is released as a ciliated larva ◼ One sponge releases sperm which enter the pores of another sponge 12-41 Phylum Porifera

◼ Different types of fertilization and zygote formation exits in sponges ◼ Viviparous sponges have choanocytes that phagocytize the sperm and transform into carrier cells that transport sperm through the mesohyl and to oocytes to form zygotes ◼ Oviparous sponges release both sperm and oocytes into water for external fertilization ◼ The free-swimming larva of most sponges is a solid-bodied parenchymula; six other larval forms exits. ◼ The outwardly directed flagellated cells of

12-42 the parenchymula become choanocytes Phylum Porifera

◼ Unique development patterns in Calcarea and some Demospongiae ◼ Hollow stomoblastula develops with flagellated cells oriented toward the interior ◼ Blastula then turns inside out (inversion) and the flagellated cells now turn outside ◼ Small flagellated cells or micromeres located at anterior end while larger non-flagellated macromeres located at posterior end ◼ Macromeres overgrow invaginating micromeres during metamorphosis and settlement ◼ Micromeres become choanocytes, archaeocytes, and collencytes while macromeres give rise to 12-43 pinacoderm and sclerocytes Figure 12.12 A) Development of demosponges, B) Development of the calcareous syconoid sponge Sycon sp..

12-44 Phylum Porifera

◼ Class Calcarea (Calcispongiae) ◼ Calcareous sponges with spicules of calcium carbonate ◼ Spicules are straight (monaxons) or have three or four rays ◼ Most are small with tubular or vase shapes ◼ Many are drab in color, but some are bright yellow, green, red, or lavender ◼ Leucosolenia sp. and Sycon sp. are marine shallow-water ◼ Asconoid, syconoid and leuconoid body 12-45 forms Figure 12.13 Some sponge body forms. 12-46 Phylum Porifera

◼ Class Hexactinellida (Hyalospongiae) ◼ Glass sponges with six-rayed spicules of silica bound together to form network ◼ Nearly all are deep-sea forms ◼ Most are radially symmetrical with vase or funnel shaped bodies attached by stalks of root spicules onto the substrate ◼ Have syncytial cell structure that have many nuclei with a large cell which were produced by the fusion of many cells or division of nuclei without dividing the cytoplasm

12-47 Phylum Porifera

◼ Most Hexactinellids have trabecular reticulum that is bilayered, sheet-like and tubular with collagenous mesohyl cells ◼ Cytoplasmic bridges connect choanoblasts and archaeocytes with trabecular reticulum ◼ Choanoblasts are unusual cells that make flagellated outgrowths called collar bodies whose flagella beat to move water like choanocytes ◼ Food is collected by directing water through the primary and secondary reticulum layers 12-48 Figure 12.14 Diagram of part of a flagellated chamber of hexactinellids.

12-49 Phylum Porifera

◼ Class Demospongiae ◼ Contains 95% of living sponge species include most large sponges ◼ Spicules are siliceous but not six rayed and may be absent or bound together by spongin ◼ Leuconoid body form for all species ◼ All marine except for Spongillidae, the freshwater sponges ◼ Marine demosponges are highly varied in color and shape, with some growing to

12-50 several meters in diameter. Phylum Porifera

◼ Freshwater demosponges ◼ Widely distributed in well-oxygenated ponds and streams ◼ They encrust plant stems and submerged wood ◼ Look like wrinkled scum, pitted and porous with brown and green colors ◼ Flourish in summer and in early autumn ◼ Reproduce sexually, but existing genotypes may also reappear annually from gemmules ◼ Sponges die by late autumn and asexually release gemmules to prepare for next year’s population. 12-51 Figure 12.15 Marine Demospongiae on Caribbean coral reefs. A) Pseudoceratina crassa, B) Aplysina fistularis, C) Monanchora unguifera

12-52 Phylum Porifera

◼ Class Homoscleromorpha ◼ Mostly marine with a variety of colors but live in cryptic habitats ◼ Generally found near shore but have deep water forms ◼ Separated from Demospongiae due to presence of true basement membrane under pinacoderm or extracellular matrix ◼ Also have adherens cell junctions that from true tissues unlike other sponges ◼ Divided into two clades based on absence

12-53 or presence of spicules Phylum Porifera

◼ Phylogeny and Adaptive Diversification ◼ Sponges appeared before the Cambrian and two calcareous sponge-like organisms were in Paleozoic reefs. ◼ Sponges share many traits with other animals and are considered sister taxon ◼ Proteins for cell adhesion and cell-signaling are homologous to other animals ◼ Some sponges have basement membrane with collagen and adherens junctions with cadherin molecules that connect epithelial cells ◼ Sponge have blastula and some form gastrula 12-54 stages like many animals Phylum Porifera

◼ Adaptive Diversification ◼ Poriferans are a highly successful group with thousands of species in diverse habitats ◼ Diversification centers on their unique water-current system and its degree of complexity ◼ New feeding mode has evolved for sponges found in deep water caves with low nutrients ◼ Illustrates the non-directional nature of evolution 12-55 Phylum Porifera

◼ Unique features of deep water sponges ◼ Many tiny hook-like spicules cover highly branched body ◼ Spicule layer can entangle the legs of crustaceans that come near sponge ◼ Filaments of the sponge body grow over prey, slowly enveloping it and later digesting it ◼ Most of the group are carnivores and not suspension feeders ◼ Some have symbiotic methanotrophic bacteria ◼ Contain siliceous spicules, but lack choanocytes and internal canals so very 12-56 different than regular sponges Figure 12.16 The carnivorous sponge, Chondrocladia lyra , is commonly called a “harp sponge.”

12-57 Phylum Placozoa

◼ Proposed by K. G. Grell (1971) based on a single species- Trichoplax adhaerens ◼ Tiny (2-3 mm) marine form that is plate-like and has no symmetry ◼ No major organs, no muscular or nervous system ◼ Lacks basal lamina beneath epidermis and no extracellular matrix but has genes for it ◼ Body has dorsal epithelium to cover cells and have thick ventral epithelium of monociliated cells and nonciliated gland

12-58 cells Phylum Placozoa

◼ Space between the epithelia contain multinucleated fibrous “cells” within a contractile syncytium ◼ Placozoans glides over food, secretes digestive enzymes, and absorb nutrients ◼ Divide asexually and produce “swarmer” stages by budding. ◼ No sexual stages have been seen but have isolated eggs in the laboratory ◼ Considered diploblastic with dorsal epithelium representing ectoderm and ventral epithelium representing endoderm 12-59 Figure 12.17 A) Trichoplax adhaerens is a marine placozoan, B) Section through Trichoplax adhaerens, showing histological structure.

12-60