6.1 Diversity and biology of marine sponges

Sponge Anatomy and Physiology

Sponges (phylum Porifera, meaning "pore-bearing") are among the oldest and simplest multicellular animals in the ocean. Despite lacking true tissues and organs, they're remarkably effective filter feeders with specialized cells that handle everything from structural support to digestion. Understanding sponge biology gives you a foundation for comparing more complex invertebrate body plans later in this unit.

Anatomy and Physiology of Marine Sponges

Sponges are asymmetrical, sessile, multicellular organisms. They don't have tissues or organs, but they do have several types of specialized cells that divide up the work.

Body structure has three main components:

• Pinacoderm — the outer cell layer, made of flattened cells called pinacocytes that cover the sponge's exterior surface
• Choanoderm — the inner cell layer, composed of flagellated choanocytes (collar cells) that line the interior canals and chambers. These cells beat their flagella to create water currents and trap food particles during filter feeding.
• Mesohyl — a gelatinous matrix that fills the space between the pinacoderm and choanoderm. This is where amoebocytes move around, transporting nutrients and producing skeletal elements.

Skeleton is made of one or both of these components:

• Spicules — small structural elements made of either silica (siliceous) or calcium carbonate (calcareous). They provide support and defense against predators.
• Spongin — flexible protein fibers that form a softer, more pliable skeleton (bath sponges are a classic example).

Canal system is how water flows through the sponge body:

1. Water enters through ostia (tiny pores on the surface)
2. Incurrent canals transport water inward toward the choanocyte chambers
3. Choanocyte chambers are spaces lined with choanocytes that filter food particles from the water
4. Excurrent canals carry the filtered water back outward
5. Water exits through the oscula (large openings on the sponge surface)

A single sponge can filter thousands of liters of water per day through this system.

Feeding and Reproduction of Sponges

Feeding: Sponges are filter feeders. Choanocytes create water currents with their flagella and trap food particles (bacteria, phytoplankton, dissolved organic matter) using their collar of microvilli. Pinacocytes and amoebocytes then digest the captured food and distribute nutrients throughout the mesohyl.

Asexual reproduction occurs through three main methods:

1. Budding — a new individual grows as an outgrowth from the parent sponge and eventually detaches
2. Gemmules — resistant capsules containing totipotent cells (cells that can develop into any cell type). Gemmules survive harsh conditions like drought or freezing and later develop into new sponges. This is especially common in freshwater species.
3. Fragmentation/Regeneration — sponges can regrow from small broken-off pieces, which is possible because of their totipotent amoebocytes

Sexual reproduction: Most sponges are hermaphroditic, producing both sperm and eggs (though usually not at the same time, which prevents self-fertilization). Two reproductive strategies exist:

• Oviparous — eggs are fertilized internally, and free-swimming larvae are released into the water column
• Viviparous — eggs are fertilized and develop into larvae within the parent sponge before release

In both cases, the planktonic larval stage allows sponges to disperse to new locations before settling on a substrate and metamorphosing into sessile adults.

Ecological Roles of Marine Sponges

Sponges punch well above their weight in marine ecosystems. Their ecological contributions fall into several categories:

• Habitat providers — Sponge bodies shelter crustaceans, worms, echinoderms, and many other small organisms. They also contribute to the structural complexity of coral reefs and other benthic communities.
• Water filtration and nutrient cycling — By filtering massive volumes of water, sponges remove particulate matter and dissolved organic carbon. This couples the benthic (bottom) and pelagic (water column) environments, converting suspended particles into forms that other organisms can use. This process is sometimes called the sponge loop.
• Bioerosion — Some species (like Cliona) bore into coral skeletons and calcareous substrates, helping maintain the balance between reef growth and erosion.
• Microbial symbiosis — Many sponges host dense microbial communities within their mesohyl. These symbionts contribute to the sponge's metabolism and chemical defense. In some species, microbial cells make up nearly half the sponge's total biomass.
• Bioactive compounds — Sponges produce a wide array of secondary metabolites with antiviral, antibacterial, antifungal, and anticancer properties. These compounds are a major area of interest in pharmaceutical and biomedical research.

Classes of Marine Sponges

There are four recognized classes of sponges. You should know the key features and representative examples of each:

• Calcarea (calcareous sponges) — Skeleton of calcium carbonate spicules. Exclusively marine. Tend to be small with simple body plans, using asconoid, syconoid, or leuconoid canal systems. Examples: Sycon, Leucosolenia.
• Hexactinellida (glass sponges) — Skeleton of siliceous spicules with six-rayed (triaxonic) symmetry, often fused into intricate lattice structures. Mainly found in deep-sea environments. Unique syncytial tissue organization, where cells share continuous cytoplasm rather than being fully separated. Examples: Euplectella (Venus' flower basket), Aphrocallistes.
• Demospongiae (demosponges) — The largest and most diverse class, containing roughly 90% of all sponge species. Skeleton of siliceous spicules and/or spongin fibers. All have a leuconoid canal system (the most complex and efficient type). Extremely diverse in morphology and habitat. Examples: Aplysina (yellow tube sponge), Cliona (boring sponge), Xestospongia (giant barrel sponge).
• Homoscleromorpha — Skeleton of siliceous spicules with four-rayed (tetraxonic) symmetry. Simple canal systems. Cellular organization more similar to other metazoans than other sponge classes, which makes them interesting for studying early animal evolution. Examples: Oscarella, Plakina.