Dominant sponge growth forms used in a purely functional context can reflect prevailing ambient conditions – which can be used for community classification and can be a great source of information for environmental conditions. Sponge morphological strategies respond most strongly to flow and sedimentation.
The need to study sponge communities in comparatively inaccessible habitats led to a sponge classification system that relies on the strictly functional interpretation of traditional sponge morphologies. The aim is to deliver a standardised approach that can optionally be based on imagery and can be applied across all oceans and to any water depth. The system is designed to recognise community-level changes across time and space. The functional context allows a basic interpretation of environmental conditions and may thereby inform on the reasons for observed differences in prevailing morphologies. In terms of growth form sponges appear to respond most strongly to the flow regime and to sediments. Strong turbulent flow will favour low-relief, morphologically simple sponges that are often structurally reinforced and well attached, such as crusts and simple-massive forms. Laminar flow selects for two-dimensionally erect, vertically flattened, usually flexible sponges that are aligned broadside to the current, inhalant openings (ostiae) pointing upstream, and exhalant openings downstream (oscula). Flow strength is generally inversely related to number of erect sponges, to body height (except in globular sponges), oscular diameter, branch number and branch complexity. Where flow conditions reduce or limit access to water exchange and nutrients, sponges tend to separate in- and exhalants in cup-like forms, reach into the water column as erect and even stalked forms, and in cases of extreme nutrient limitation the community will consist predominantly of carnivorous sponges. Globular and fistular sponges are usually abundant where the substrate is dominated by sediments, and where sediment deposition or movement is high. Fine sediments will often exclude sponges with much horizontal surface area. Based on these insights, the proposed scheme uses four basic morphologies: functional 1 – crusts, 2 – massives, 3 – cups and 4 – erect sponges. These are further divided into sponges that function as 1 – true crusts, endolithic-bioeroding, and creeping sponges, 2 – simple-massive, globular massive, composite-massive, and fistular sponges, 3 – cups, tubes, and barrels, and 4 – one-dimensionally, two-dimensionally and three-dimensionally erect forms, stalked, and carnivorous sponges.