Section 2: Distinguishing Features, General Body Plan, and Anatomy

{{active_language_id_uc}} {{settings.doi}}

Introduction
Sponges (Phylum Porifera) are simple, sessile animals with a body structure optimized for filter feeding. Unlike most animals, sponges lack true tissues and organs. Instead, their bodies consist of specialized cells embedded within a gelatinous matrix called the mesohyl, which allows for a flexible yet functional architecture. Sponges are generally asymmetrical, though some species exhibit a degree of radial symmetry. This lack of defined body symmetry, along with the absence of organs, highlights their ancient evolutionary lineage.

The structure of a sponge is defined by its aquiferous system, a network of canals and chambers that allows water to circulate through the sponge’s body. Water enters through tiny pores called ostia, flows through internal channels where food particles are filtered by choanocytes (collar cells), and exits through one or more large openings called oscula. This simple yet effective system allows sponges to filter large volumes of water relative to their size, capturing bacteria, plankton, and organic matter as food.

Sponge bodies come in three basic forms—asconoid, syconoid, and leuconoid—each representing an increasing level of complexity and filtration efficiency. In addition to the aquiferous system, sponges are supported by skeletal structures composed of spicules and collagen fibers, which provide shape, protection, and resilience. These basic structures, combined with a diverse range of specialized cell types, make sponges highly efficient filter feeders and essential components of marine and freshwater ecosystems.

Body Plans
Sponges exhibit three main body plans—asconoid, syconoid, and leuconoid—which represent different levels of complexity in the arrangement of water channels and filtration chambers. These adaptations maximize the surface area available for choanocytes (collar cells) to filter food particles efficiently.

• Asconoid Body Plan:
  The asconoid body plan is the simplest sponge form, found primarily in smaller species, especially within the class Calcarea. In asconoid sponges, water enters through small pores called ostia (singular: ostium) on the outer surface, flows directly into a central cavity called the spongocoel (also known as the atrium), and exits through a single large opening called the osculum. The inner walls of the spongocoel are lined with choanocytes, whose flagella create water currents that trap food particles on the microvilli of their collars.

Due to the limited surface area of choanocytes, asconoid sponges are restricted to smaller sizes. The direct pathway from ostia to spongocoel limits filtration efficiency, so asconoid sponges typically inhabit environments rich in suspended organic matter. They are usually small, tubular, or vase-shaped to allow efficient water movement through this simple structure.

• Syconoid Body Plan:
  The syconoid body plan is more complex and supports larger body sizes by increasing the surface area available for choanocytes. In syconoid sponges, water flows through ostia into a network of incurrent canals, which lead to radial chambers lined with choanocytes. These chambers filter water before it passes into the spongocoel, which is smaller than in asconoid forms. Finally, water exits through the osculum.

This arrangement, with choanocyte-lined radial chambers, increases filtration efficiency by allowing more surface area for food capture. Syconoid sponges are often cylindrical or vase-shaped and are common in Calcarea. The folding of the body wall into radial chambers represents an evolutionary step that enables syconoid sponges to filter larger volumes of water.

• Leuconoid Body Plan:
  The leuconoid body plan is the most complex and efficient, allowing for the largest sponges, particularly within the class Demospongiae. In leuconoid sponges, the spongocoel is replaced by a network of flagellated chambers lined with choanocytes. Water flows through numerous ostia into a system of incurrent canals that lead to these multiple chambers, where filtration occurs.

After passing through the flagellated chambers, filtered water is directed into excurrent canals and exits through multiple oscula. This system maximizes the area available for choanocytes, allowing leuconoid sponges to process large volumes of water efficiently. Leuconoid sponges have highly varied shapes, including massive barrel forms, branching structures, and encrusting growths, enabling them to thrive in diverse habitats and filter water on a large scale.

Cell Types
Sponges lack true tissues, but they possess a variety of specialized cell types that work together to perform essential functions such as feeding, structural support, regeneration, and defense. These cell types operate independently within a gelatinous matrix called the mesohyl.

• Choanocytes (choane, “funnel”): Known as collar cells, choanocytes line the inner chambers or canals and play a central role in filter feeding. Each choanocyte has a flagellum surrounded by a collar of microvilli that traps food particles. The flagellum generates water currents that draw water into the sponge, enabling choanocytes to capture bacteria, plankton, and organic matter. Choanocytes are critical to the sponge’s feeding process and resemble choanoflagellates, unicellular protists considered the closest relatives to animals.
• Pinacocytes (pinax, “tablet”): Flat, epithelial-like cells that form the outer layer of the sponge and line some internal surfaces. Pinacocytes help regulate the sponge’s surface area and contribute to maintaining its structure. They can be divided into:
• Ectopinacocytes: Cover the outer surface, forming a protective layer.
• Endopinacocytes: Line internal canals, assisting in water regulation.
• Basopinacocytes: Anchor the sponge to its substrate.
• Porocytes (porus, “pore”): Tube-like cells that form the ostia, allowing water to enter the sponge’s body. Porocytes contract and expand to regulate water flow into the sponge and are most prominent in asconoid and some syconoid sponges.
• Sclerocytes (scleros, “hard”): Cells responsible for secreting spicules, the rigid skeletal elements that provide structural support. Sclerocytes produce silica or calcium carbonate spicules, which vary by sponge class and are crucial for identification.
• Spongocytes (spongia, “sponge”): Cells that secrete spongin, a collagen-based protein that provides flexible support in demosponges. Spongin fibers allow for elasticity and resilience, helping sponges withstand environmental stressors.
• Collenocytes (colla, “glue”): These cells secrete fibrillar collagen fibers, providing additional structural support within the mesohyl and allowing the sponge to maintain its form under physical stress.
• Amoebocytes/Archeocytes (amoibe, “change”): Highly versatile cells that move within the mesohyl, performing digestion, nutrient transport, and repair. They can transform into other cell types as needed, making them essential for regeneration.
• Myocytes (myo, “muscle”): Contractile cells surrounding the osculum and certain canals, enabling the sponge to control water flow by opening and closing these structures.
• Spherulous Cells: Cells containing bioactive chemicals, such as toxins and antimicrobial compounds, which help protect the sponge from predators and fouling organisms.

Cell Plasticity: Pluripotency and Totipotency
One of the unique and fascinating features of sponges is the pluripotency and totipotency of their cells, meaning that many sponge cells have the ability to transform into other cell types as needed. This cellular plasticity is crucial for the sponge’s survival, allowing it to adapt to changing environmental conditions and to repair and regenerate damaged tissue. Amoebocytes (also known as archeocytes) are particularly important in this regard, as they are totipotent cells capable of differentiating into any other cell type within the sponge. This versatility enables them to replace damaged or lost cells, maintain tissue integrity, and facilitate growth and reproduction.

Other sponge cells, such as choanocytes and pinacocytes, exhibit pluripotency, meaning they can differentiate into multiple cell types but with a more limited range than totipotent archeocytes. For example, choanocytes can transform into reproductive cells, becoming either sperm or eggs, during sexual reproduction. This cellular flexibility is an ancient evolutionary trait that contributes to the sponge's remarkable resilience and regenerative capabilities. Through these processes, sponges can efficiently repair injuries, adapt to environmental pressures, and survive in dynamic and sometimes harsh aquatic ecosystems.

Skeletal Structures
The skeletal framework of sponges is composed of spicules and collagen fibers, both of which vary in composition and arrangement, providing structural support, shape, and defense. These elements help classify sponges and enable them to inhabit diverse aquatic environments.

• Spicules:
  Spicules are rigid, needle-like structures that contribute to the sponge’s skeletal support and are secreted by sclerocytes. They vary in composition and complexity according to the sponge class.
• Siliceous Spicules: Found in Demospongiae and Hexactinellida, siliceous spicules are composed of silica (silicon dioxide). In demosponges, they may appear as monaxon (single-rayed) or tetraxon (four-rayed) structures. Hexactinellids, or glass sponges, are characterized by six-rayed silica spicules that interlock to create lattice-like frameworks, which provide both strength and flexibility suitable for deep-sea habitats. This skeletal structure supports these sponges in cold, high-pressure environments.
• Calcareous Spicules: Found in Calcarea, calcareous spicules are composed of calcium carbonate, making them strong but less intricate than silica-based spicules. Calcareous spicules are typically monaxon or triaxon (three-rayed) and are adapted for shallow, nutrient-rich waters, where they maintain structural stability in turbulent environments.

The arrangement and form of spicules vary across species and are essential for sponge identification and classification. Spicules serve defensive functions, deterring predators with their sharp edges, and can assist in light transmission in photosynthetic sponges, allowing light to reach symbiotic algae.

• Collagen Fibers:
  In addition to spicules, sponges rely on collagen-based fibers for flexibility and strength. Collagen fibers vary in thickness and structure and are essential for maintaining sponge shape and resilience.
• Fibrillar Collagen Fibers: These fine fibers create a supportive network within the mesohyl, providing tensile strength and allowing the sponge to withstand bending and compression. Fibrillar collagen is present in most sponges and is vital for structural integrity, especially in high-flow environments.
• Spongin Fibers: Found primarily in Demospongiae, spongin fibers form a dense, interconnected network within the mesohyl. Spongin provides elasticity and durability, enabling demosponges to grow to large sizes and adopt complex shapes, such as massive barrels and branches. The flexible structure of spongin fibers allows these sponges to absorb and dissipate mechanical forces from currents, making them well-suited for habitats with dynamic water conditions.

The combined support from spicules and collagen fibers enables sponges to adapt to various environments, from shallow, high-energy reefs to the depths of the ocean floor. These skeletal elements allow sponges to withstand physical stress, deter predators, and contribute to their role as essential filter feeders and habitat providers in aquatic ecosystems.