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Phylum Porifera

The content and activities in this topic will work towards building an understanding of the phylum Porifera.

Introduction to Phylum Porifera

The phylum Porifera comprises the sponges. Sponges are simple invertebrate animals that live in aquatic habitats. Although the majority of sponges are marine, some species live in freshwater lakes and streams. They are found in shallow ocean environments to depths as great as five kilometers (km). All adult sponges are sessile, meaning they live permanently attached to rocks or other submerged objects and do not move about on their own. Some sponges grow in thin encrusting layers over surfaces (Fig. 3.18 A). A few species can even bore into hard surfaces like clam shells, coral skeletons, and rock (Fig. 3.18 B). Many sponge species grow upright in branching tree-like (Fig. 3.18 C) or tubular vase-like (Fig. 3.18 D) forms. While some sponges, like the giant barrel sponges of the Caribbean, reach several meters in diameter, most sponges are small organisms that often go unnoticed on the reef or seafloor because they don’t look like other, more familiar, animals or noticeably move. Many of the small sponges that hide under rocks or live on coral reefs are colored in vivid hues of red, yellow, orange, purple, crimson, sky blue, and ultramarine. Figure 3.18 shows some examples of different sponge morphologies.

<p><strong>Fig. 3.18.</strong> (<strong>A</strong>) Encrusting freshwater sponge (<em>Spongilla lacustris</em>)</p><br />
<p><strong>Fig. 3.18.</strong>&nbsp;(<strong>B</strong>) Damage on a clam shell produced by a boring sponge</p><br />

<p><strong>Fig. 3.18.</strong>&nbsp;(<strong>C</strong>) Branching tree-like red toxic finger sponge (<em>Negombata magnifica</em>) positioned on the right side of a hard coral</p><br />
<p><strong>Fig. 3.18.</strong>&nbsp;(<strong>D</strong>) Tubular branching vase sponge (<em>Callyspongia vaginalis</em>) with pink color variation</p><br />


The phylum name Porifera means pore-bearing. Sponges take their name from small holes that cover their bodies. The history of life is written on the bodies of the animals that previously inhabited and continue to inhabit earth. As we learn about the different invertebrate phyla, we will be able to chart some of this history. Sponges are considered to be one of the simplest animals, primarily because their bodies are not organized in organ systems or even tissues. Rather, sponges are made up of a grouping of cells that work together to contribute to meeting the daily needs of the sponge.


<p><strong>Fig. 3.19.</strong> Anatomy of three different simple, vase-like sponges showing (1) spongocoel (2) osculum (3) radial canal (4) flagellated chamber (5) incurrent pore and (6) incurrent chanel.</p><br />

Epithelial cells form a skin-like layer on the outer surface of a sponge (Fig. 3.20 D). These cells protect and enclose the sponge; they can contract and shorten, moving the sponge body slightly. All sponges are filled with the pores that give the phylum its name. The tiny holes are actually the hollow insides of porocyte cells (Fig. 3.20 C), a special type of epithelial cell. Porocytes are narrow and elongated cells that connect the outside of the sponge to the inside cavity. These pores are sometimes also referred to as ostia, and they provide openings for water, which carries planktonic food and oxygen, to enter the sponge body (Fig. 3.19).


Simple vase-like sponges have a single large top opening, called the osculum through which water leaves the sponge. Most compound sponges have many oscula all over the body of the sponge. The oscula are surrounded by cells and are bigger than the ostia. Epithelial cells around the osculum can contract enough to close the opening, but the process is slow (up to several minutes).


<p><strong>Fig. 3.20.</strong> Cells in a sponge (<strong>A</strong>) Choanocyte, (<strong>B</strong>) Amoebocyte (<strong>C</strong>) Porocyte (<strong>D</strong>) Epithelial cells.</p><br />

The inner surface of the sponge is lined with cells called collar cells, also known as choanocytes (Fig. 3.20 A). The collar is made of fine tubes surrounding a long whiplike thread called a flagellum. As flagella (plural of flagellum) in the collar cells move back and forth, they create a current of water that moves into the ostia and out the osculum. Several gallons of water can circulate through a fist-sized sponge in a single day, bringing in tiny food particles such as suspended bacteria, bits of plant and animal matter, and tiny drifting planktonic organisms. As the water circulates, the fine tubes of the collar cells filter out the food particles and take them into the cells for digestion. For this reason sponges are described as filter feeders.


Between the outer surface of epithelial cells and the inner surface of collar cells is a jellylike material. In this jelly are the structures that support the sponge. There are also free-moving cells called amoebocytes (Fig. 3.20 B), which can move throughout the jelly layer. During feeding, some of the particles taken in by the collar cells are passed on to amoebocytes, which carry them to other cells of the sponge. Several kinds of amoebocytes serve special functions, like producing the sponge skeleton, digesting and transferring nutrients, or reproducing themselves.


The skeletal elements of the sponge are produced by the amoebocytes. The amoebocytes produce spongin, the soft fiber that forms natural bath sponges. These sponges feel soft and springy to the touch because they have soft skeletons made of flexible fibrous spongin. Other sponges have a stiff skeleton that feels prickly because it is made of hard, sliver-like spicules, which are also built by the amoebocytes. Some sponges have both spicules and spongin and feel both prickly and flexible. Many species of sponges can be identified by the shape and composition of their spicules (Fig. 3.21 A). Siliceous sponges have spicules made of silicon. Calcareous sponges have spicules made of calcium. Spicules also have many shapes and sizes. While some sponges have no spicules, others have so many that they look and feel like lacy skeletons of glass (Fig. 3.21 B).

<p><strong>Fig. 3.21.</strong> (<strong>A</strong>) Scanning electron microscope (SEM) images illustrating the wide diversity of sponge spicule shapes</p><br />
<p><strong>Fig. 3.21.</strong>&nbsp;(<strong>B</strong>) Microscopic spicule lattice from Pachastrellid sponge</p><br />


All cells in a sponge are in contact with or near to seawater. Because each cell exchanges oxygen and carbon dioxide and discharges waste products into the seawater, a sponge has no respiratory, circulatory, or excretory system.


Sponges can reproduce either asexually or sexually. Asexually reproduction (without eggs and sperm) often occurs by budding, similar to growing a new branch on a tree. Cells on the side or base of the parent begin to bulge out and form a new organism. The buds may remain attached to the parent, or they may detach and settle down nearby to form a separate organism. Sponges also reproduce sexually when specialized gametocyte cells produce sperm and eggs. Sponges undergo synchronous spawning and eject sperm and egg cells into the water. If gametes (sex cells; either sperm or egg) from the same species meet, they form a larval sponge. After a period of planktonic drifting, the larva settles to a suitable location on the bottom and grows into an adult sponge. The drifting larval stage means that sponges can colonize new locations, even though as adults they remain attached in a sessile lifestyle.


Freshwater sponges can live in areas that are subject to cyclical wet and dry periods. They have a special strategy to help them deal with these harsh conditions. Freshwater sponges can produce a “resting” stage called a gemmule. A gemmule is a small, encysted bud that can tolerate being dried out for a long period of time. When the gemmule is exposed to water, it can resume development as a sponge. Organisms that can undergo a phase where they are dormant to survive harsh conditions are said to be in cryptobiosis (from the root words crypto meaning hidden and bio meaning life), because they do not appear to be living. In reality, these organisms are in a state of suspended animation. See more information about cryptobiosis at Weird Science: Cryptobiosis.


Symbiosis in Sponges

Many species of plants and animals live on or in some other organism in a close association of symbiosis (from Greek root words meaning “living together”). Large sponges have many small chambers where other organisms can live symbiotically (Fig. 3.22). Although the sponges rarely benefit from this arrangement, they do not seem to suffer harm, and their symbionts, the organisms that live in them, do gain benefits. This type of symbiosis is called commensalism. For example, certain species of shrimp live in the chambers of sponges and feed on the particles that are flowing through the chambers.

<p><strong>Fig. 3.22.</strong> (<strong>A</strong>) Two&nbsp;<em>Stenopus hispidus</em>&nbsp;banded cleaner shrimp (phylum Arthropoda) on a&nbsp;barrel sponge (<em>Xestospongia muta</em>)</p><br />
<p><strong>Fig. 3.22.</strong>&nbsp;(<strong>B</strong>) Stalked deep-water sponge with small shrimp</p><br />

<p><strong>Fig. 3.22.</strong>&nbsp;(<strong>C</strong>) Deep sea glass sponge (<em>Polipogon</em> sp. D) with echinoderms</p><br />
<p><strong>Fig. 3.22.</strong>&nbsp;(<strong>D</strong>) Sponge crab (<em>Dromia</em> sp.) using a sponge as camouflage</p><br />

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.