Glass sponges (Hexactinellida) are a unique class of marine animals known for their delicate, glass-like silica skeletons. They are highly specialized filter feeders, extracting sustenance from the surrounding water column with remarkable efficiency. Understanding their diet and feeding mechanism requires examining their unusual biological structure and deep-sea habitat.
Unique Characteristics and Deep-Sea Habitat
Glass sponges are defined by their intricate, rigid skeletons composed of siliceous spicules that often fuse to form a complex lattice structure. This skeleton provides a sturdy framework that allows them to thrive in the stable, deep-sea environments where they are predominantly found. These animals typically inhabit cold, dark waters, often below 450 meters.
The deep-sea habitat is characterized by a scarcity of suspended food particles, requiring resident filter feeders to be exceptionally efficient. Conditions are stable, with low temperatures and high pressure, which influences the slow metabolism and longevity of these animals. The stable environment permits the glass sponge’s highly efficient, energy-intensive feeding mechanism to operate continuously.
The Filtered Diet: What Glass Sponges Consume
Glass sponges are dedicated suspension feeders, meaning their entire diet consists of particles filtered directly from the seawater. They are highly efficient bacterivores, primarily consuming ultra-fine particulate matter dispersed in their nutrient-poor habitat. Their diet is dominated by suspended bacteria, the smallest and most abundant food source in the deep ocean, often measuring 0.2 to 1 micrometer in size.
Beyond bacteria, their filtration system effectively captures picoplankton, which includes microscopic detritus and small, unicellular eukaryotes ranging from 0.2 to 10 micrometers. Studies show that they retain nearly all the bacteria and a high percentage of the small unicellular organisms they encounter. This efficiency is necessary because the deep water they inhabit has low concentrations of these food items.
They preferentially take up particles smaller than eight micrometers, demonstrating a selective feeding ability despite their passive flow of water. Inedible material, such as inorganic silt and clay particles, is expelled as waste. High concentrations of sediment can irritate and eventually clog their filtration system.
The Syncytial Structure and Feeding Mechanism
The mechanism by which glass sponges feed is distinctive within the phylum Porifera, setting them apart from other cellular sponges. Their body is largely composed of a syncytium, a continuous mass of cytoplasm containing multiple nuclei not divided into separate cells by membranes. This unique syncytial tissue forms a vast, cobweb-like network, called the trabecular syncytium, that connects the entire animal.
Within this syncytial network are specialized structures functionally equivalent to the choanocytes found in other sponges, but these are multinucleated, non-cellular extensions called choanosyncytia or collar bodies. These extensions form bell-shaped chambers where flagella beat to create the constant water current necessary for filter feeding. The movement of these flagella draws water through the sponge’s body, and the food particles are trapped by microvilli surrounding the flagellum.
Captured food particles are absorbed as they pass through the channels within the sponge and are incorporated into the syncytium for digestion. This interconnected cytoplasmic structure allows for rapid internal transport of nutrients throughout the entire organism. The syncytium is also responsible for an unusual defense mechanism against irritants like excessive sediment.
When the glass sponge senses an irritant in the water, it uses electrical potentials propagating through the syncytial tissue to coordinate a rapid arrest of the feeding current. This electrical signaling allows the sponge to instantly stop pumping water, preventing the filtration system from becoming clogged. This mechanism allows glass sponges to filter immense volumes of water—hundreds of times their body volume per hour—to sustain themselves in the food-scarce environment.