How do sponges and hydra attach to their substrate?
Sponges have the amazing ability to settle on diverse substrates, including living and dead material. You may see sponges attached to the shells of mussels and crabs; literally stuck between a rock and a hard place, including reef nooks and crannies; or even anchored on the bottom of the sandy ocean. How do these organisms manage to attach themselves to such a wide variety of substrates?
Sponges are animals containing various specialized cell types, but they lack true tissues. One noteworthy sponge cell is the scleroblast. Scleroblasts form spicules, which, in addition to collagen fibers, support the sponge body in its underwater environment. Different types of sponges possess spicules made of distinct materials—for example, hexactinellida (glass sponges) have silica spicules, and calcarea (calcareous sponges) have calcium carbonate spicules. Note that some sponges lack spicules all together.
Some sponges use spicule arrangements or fused spicule lattices to attach to hard rocks as well as looser surfaces, such as the ocean floor. The spicules, when organized into a complex structure buried in the ocean floor, form a solid foundation for the sponge body to rest on. These spicule arrangements (sometimes referred to as basal plates) can be thought of as “anchors” or “roots” that hold the sponge body in place. They are so stable that amazing reefs can develop on them.
Off the coast of British Columbia, Canada, scientists are currently studying reefs of hexactinellid sponges that reach depths of hundreds of meters below the surface. These reefs are particularly interesting because as the sponges die, their remaining spicule lattice provides a great surface for larval settlement and new sponge development. (A good description of this process can be found on the porifera.org site referenced below.) In this way, these reefs are sometimes described as “living fossils” because they provide a glimpse into the past and provide some evidence of prior conditions.
Sponges possess other characteristics in addition to spicules that allow them to adhere and attach to substrates. When a sponge begins its life, it is a small free-swimming larvae in the water column. That tiny sponge has to establish a home for itself. When it does, the larvae metamorphose and begin to transform their cellular organization into the adult body plan. During this process, the outermost layer of cells that covers the metamorphosing sponge (and adult body too) is called the pinacoderm. These cells secrete a mixture of collagen and complex carbohydrates (a fibrillar collagen-polysaccharide complex) that allows the animal to attach to a substrate. The pinacoderm cells at the base of the sponge are called basopinacocytes, and the collagen-carb glue secreted by these cells holding the sponge in place is called the basal lamina.
The polyp forms of many cnidarians (including anemones and hydra) are also capable of settling on diverse substrates. The structural anatomy of cnidarians matches the type of surface upon which the animal settles. For example, those that settle on rocks have a flattened pedal disc, those settling on sandy floors utilize a rounded structure called a physa, and colonial cnidarians attach as a group by using a common anchor such as a hydrorhiza (Brusca and Brusca 2003). The hydrorhiza functions like a root system connecting the different polyps and joining each with the substrate (think of rhizoids in fungi).
Some cnidarians, such as scleractinian corals, have specialized cells called scleroblasts that are capable of secreting hard, calcium carbonate sclerites. These sclerites fuse and organize themselves into a tough skeleton that attaches the corals to a substrate and provides ample support for individuals and colonies. The living part of the coral rests above this basal calcium carbonate tubelike skeletal foundation. As coral polyps grow, the nonliving skeletons become sectioned off and sealed at the bottoms, creating new substrate for the coral and allowing individuals and colonies to take various shapes and forms. This process is similar to the one mentioned for sponges: scleroblasts help the sponge produce spicules, which provide structural support for the animal, and when left behind, provide foundations for new animals and living reefs.
For more information on sponge reefs:
For more information on sponge and cnidarian biology:
Brusca, R., and Brusca G. 2003. Invertebrates. 2nd ed. Sunderland, MA: Sinauer Associates.