That would all depend on the amount and thickness of the spicules that the sponge would have. Of course, the tall, columnar like sponges that you see in the ocean would have large, dense, and strong spicules to keep the sponge erect, but an encrusting sponge such as this may even lack a sufficient amount of spicules to keep it "stable".
Here, compare these two definitions from Encyclopedia.com
Tunicate - TUNICATE [tunicate] , marine animal of the phylum Chordata , which also includes the vertebrates. The adult form of most tunicates (also called urochordates) shows no resemblance to vertebrate animals, but such a resemblance is evident in the larva. The most familiar tunicates are the sea squirts, or ascidians (class Ascidiacea). Adult sea squirts are sedentary, filter-feeding, cylindrical or globular animals, usually found attached to rocks, shells, pilings, or boat bottoms. The soft body is surrounded by a thick test, or tunic, often transparent or translucent and varying in consistency from gelatinous to leathery. The tunic (for which the tunicates are named) is secreted by the body wall of the adult animal. It is composed of cellulose, an almost unique occurrence of that material in the animal kingdom. Two siphons project from the animal's body; water enters the incurrent siphon at the top of the body and leaves the excurrent siphon at the side. Food particles are filtered from the water by the pharynx, which occupies most of the body, and are then passed into the digestive system. Some species reproduce by budding, resulting in the formation of colonies of sea squirts, joined at their bases by slender stalks or embedded in a slab of common tunic material. In addition, nearly all species reproduce sexually and are hermaphroditic. The free-swimming larva, called a tadpole, has a muscular tail and is similar in appearance to a frog tadpole. The larva has the characteristic chordate features also found in the embryos of vertebrates: a dorsal, hollow nerve cord; a stiffening rod, or notochord; and gill slits leading into the pharynx. The tadpole eventually settles and undergoes a drastic metamorphosis into the adult form. A common solitary sea squirt of both coasts of North America is the slender, yellow, transparent Ciona intestinalis, about 2 in. (5 cm) tall. The sea peach, Tethyum pyriforme, is a round, peach-colored sea squirt found from Maine north. Sea grapes are clusters of the greenish colonial squirt, Molgula manhattensis, common from Massachusetts south. Golden stars are colonies of various Botryllus species; the bright yellow individual animals are grouped in starlike clusters in a flat, encrusting, greenish tunic. Amaroucium species form colonies of minute animals embedded in a grayish, gristly tunic; chunks of such colonies, often washed ashore, are known as sea pork. There are two other groups of tunicates, both found in the plankton of open oceans. The salps (class Thaliacea) are barrel-shaped tunicates, open at both ends; they swim by muscular contractions that force water through the body. The larvaceans (class Larvacea) retain the larval form, with a tail and a notochord, as adults. A commonly held theory maintains that vertebrates evolved from animals like the larvaceans. Larvaceans have no tunics, but secrete gelatinous containers, called houses; these are used to filter food from the water and are continuously discarded and replaced. Tunicates are classified in the phylum Chordata, subphylum Urochordata.
Sponge - SPONGE [sponge] common name for members of the aquatic animal phylum Porifera , and for the dried, processed skeletons of certain species used to hold water. Over 4,500 living species are known; they are found throughout the world, especially in shallow temperate waters. All are marine except the members of a single freshwater family. Adult sponges are sessile, attaching themselves to rocks, coral, shells, and other substrates. They show so little movement that until the 18th cent. naturalists considered them plants. Most adults are colonial. Sexual reproduction gives rise to a free-swimming larva, which soon settles on a suitable substrate and develops into the adult form. Asexual reproduction also occurs. The individual sponge is saclike in construction; water is drawn into its central cavity through many tiny holes in the body wall and expelled through a large opening at the top of the body. Hard materials of various kinds, depending on the type of sponge, are imbedded in the body wall, forming a skeleton. A colony consists of a mass of many such individuals. Solitary sponges and colonies range in diameter from about 1/2 in. to 5 ft (1-150 cm) and vary greatly in shape. Some are branched, some more or less globular, and some are thin encrustations on rocks and pilings. Brilliantly colored sponges are common. Bath sponges are the skeletons of certain colonial sponges. These skeletons are composed of a fibrous meshwork of spongin, a material related to horn, and owe their absorbent properties to the fineness of the mesh. Sponges have been used to hold liquid since ancient times. The ancient Greeks used them for bathing and scrubbing, and Roman soldiers used them for drinking. Commercial sponges, species of the genera Spongia and Hippospongia, are harvested principally in the Mediterranean and Caribbean seas and off the Florida coast. They are brought up by divers in deep water, or raked in with long-handled forks in shallow water. They are left in water until the living tissue rots away; the skeletons are then cleaned and dried and sometimes bleached. Sponge fishing has declined in recent decades due to the use of synthetic sponges and to a decline in the population of commercially valuable natural sponges. The block-shaped sponges now commonly sold are the synthetic product. Dried natural sponges are light gray or brown and irregular in shape.
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