Research Document - 2013/074
Effects of Sediment on Glass Sponges (Porifera, Hexactinellida) and projected effects on Glass Sponge Reefs
By S.P. Leys
Abstract
Sponges are dependent on the environment for flow and therefore food. They lack nerves and muscle and are unable to rapidly relocate in response to changing environments. Glass sponges are particularly immobile due to their heavy silica skeleton. They are nevertheless animals which, like other animals, have tissues and sensory systems that allow them to sense changes to their environment and attempt to avoid detrimental effects. They do this either via sensory cilia and contractions of the filtration system (in demosponges) or sensory tissues and electrical signalling to arrest the feeding current (in hexactinellids = glass sponges).
Sponges are suspension feeders: they draw water through pores in their surface tissues, into canals to chambers of choanocytes where food is captured. They extract bacteria (0.2-1µm) and picoplankton (0.2-10µm) (e.g. unicellular algae). Clearance studies show sponges preferentially take up smaller particles (0-8µm), but are selective feeders – they eat the more nutritious items, and expel inedible particles. Sediment >10-11 mg/L irritates and eventually clogs the filtration system of demosponges. Clogging triggers an expulsion behaviour which causes the incurrent canals to inflate and then contract, pushing the sediment out of the sponge; several contractions in a row (about 10-40 min each) effectively expel small amounts of sediment waste.
Smothering by sediment causes increased respiration, decreases in oxygen consumption, and reduced reproductive ability and body weight. Death occurs in 3-6 months.
Glass sponges live in deep water (>30m) and usually on raised topography with accelerated flow. Reef sponges live in turbid water (high suspended solids, 7-8 mg/L) and are efficient bacteriovores. They take up nearly 99% of bacteria and 94% of unicellular eukaryotes from the water they filter. Sediment – clay/silt – is expelled as wastes. Sediment at reefs consists of 44-58% clays, and 75% of reef sediment is less than 3 microns in size. Whereas demosponges contract to expel debris and unwanted particles, Glass sponges if irritated by mechanical stimuli such as sediment use electrical signals to arrest feeding. No experiments have tested long-term effects of smothering of glass sponges by sediment but continued presence of >15-35 mg/L of sediment (grain size <25µm) causes complete and continued arrest of glass sponge pumping and filtration. Longer than 40 minutes exposure to 15-35 mg/L sediment causes clogging of sponge feeding tissues. Clogging by sediment reduces filtration in the glass reef sponge by 50-80% of normal levels.
Glass reef sponges rely on induced current to reduce the energetic cost of filtration. An estimated 2/3 of the sponge’s daily food intake occurs during the maximum flood tides which occur approximately 20% of the time. Trawl activities cause temporary resuspension of bottom sediment (40-120 mg/L – a combination of inedible and edible particulates) during increased ambient current flows. Reef sponges take 6 hours or longer to recover normal filtration levels, which would reduce the daily time to feed by a minimum of 6 hours. Reduced feeding during maximum ambient current would deprive the reef sponges of 2/3 of their daily food intake, compromising growth and future reproductive ability.
- Date modified: