Phylum Porifera, better known as the sponges, are an interesting group of animals. Yes, they are animals, albeit they are the most primitive of all multicellular creatures as they were the first to separate from the common ancestor of animals, but animals nonetheless (Feuda et al., 2017; Giribet, 2016). Porifera are the first group of marine invertebrates I will be chatting about over the next few weeks, so move over megafauna cause the little guys are in town!
Porifera is estimated to contain around 15,000 species of sponge, many of which have not even been described yet (Degnan et al., 2015). Sponges don’t exactly have any true tissues or organs; what they do have is a mesohyl (a gelatinous matrix that resembles a type of connective tissue) sandwiched between two thin layers of cells (more on the different types of cells soon). One of the craziest things about sponges is that they are an aggregation of cells in space and time; at that moment, they are part of that sponge, but they could be part of another sponge at a different time. Lavrov and Kosevich (2016) took this to the next level when they mechanically separated sponge cells and saw them reaggregate again before their very eyes!
Respiration, digestion, and excretion
Sponges have no nervous system, no digestive system, no excretory system, and no circulatory system, so how are they even… alive??? Sponges are sessile and are therefore attached to a substrate via their pinacocytes (contractile cells that line the outer wall). By orienting themselves perpendicular to the water flow to create low pressure at the excurrent opening (osculum), they then open and close incurrent pores (ostia) to regulate water flow; up to 20,000 times the volume of the sponge can be filtered through the sponge in 24 hours and up to 90% of the bacteria in the water may be filtered out. The structure of the sponge maximises the efficiency of water flowing into the ostia through the central cavity, where respiration and digestion occur (Hutchings, Kingsford, & Hoegh-Guldberg, 2019). As water flows through the body, cells absorb oxygen by diffusion and dump waste products into the outgoing current. This water flow also delivers food particles to the sponge. If the food is larger (>50μm), it cannot enter the ostia, so pinacocytes (remember these from earlier?) grab ‘em and digest ‘em. Usually, though, food particles are <0.5μm, so they can easily pass through the ostia to where the choanocytes are waiting. Choanocytes have a flagellum that beats, creating a unidirectional flow of water, drawing in food particles. Choanocytes have a collar of microvilli which filters nutrients from the water; the choanocytes then store the nutrients in vacuoles of adjacent cells, usually amoebocytes which distribute nutrients around the sponge. Some sponges, like Clarohizdae, are carnivorous and will passively capture small invertebrates via their sticky surface, where cells will migrate to and envelop the prey (Hestetun, Tompkins-Macdonald, & Rapp, 2017).
A peculiar characteristic of Porifera is their spicules, structures made from either calcium carbonate (CaCO3) or silica that vary in size and shape from rods to three-dimensional stars (Renard et al., 2013). They are held in place by collagen fibres (one of the places where “marine collagen” comes from) and produced in the mesohyl by sclerocyte cells. Spicules are thought to be a deterrent from predators or to provide skeletal structure or support.
These are calcareous sponges with CaCO3 spicules.
These are the glass sponges that mainly inhabit deep water. They have siliceous spicules that form stable lattices. You may remember these sponges from David Attenborough’s Blue Planet II, a romantic story of 2 shrimp larvae, male and female, that get swept into a Hexactinellid sponge and grow too large to be able to escape; a twist of fate that leaves them stuck together forever… awww.
Some Demospongiae don’t have spicules, but if they do, they are siliceous spicules that are held together by collagen.
Degnan, B. M., Adamska, M., Richards, G. S., Larroux, C., Leininger, S., Bergum, B., … & Degnan, S. M. (2015). Porifera. In Evolutionary developmental biology of invertebrates 1 (pp. 65–106). Springer, Vienna.
Feuda, R., Dohrmann, M., Pett, W., Philippe, H., Rota-Stabelli, O., Lartillot, N., … & Pisani, D. (2017). Improved modeling of compositional heterogeneity supports sponges as sister to all other animals. Current Biology, 27(24), 3864-3870.
Giribet, G. (2016). Genomics and the animal tree of life: conflicts and future prospects. Zoologica Scripta, 45, 14–21.
Hestetun, J. T., Tompkins-Macdonald, G., & Rapp, H. T. (2017). A review of carnivorous sponges (Porifera: Cladorhizidae) from the Boreal North Atlantic and Arctic. Zoological Journal of the Linnean Society, 181(1), 1–69.
Hutchings, P., Kingsford, M., & Hoegh-Guldberg, O. (Eds.). (2019). The Great Barrier Reef: biology, environment and management. Csiro publishing.
Lavrov, A. I., & Kosevich, I. A. (2016). Sponge cell reaggregation: Cellular structure and morphogenetic potencies of multicellular aggregates. Journal of Experimental Zoology Part A: Ecological Genetics and Physiology, 325(2), 158–177.
Renard, E., Gazave, E., Fierro‐Constain, L., Schenkelaars, Q., Ereskovsky, A., Vacelet, J., & Borchiellini, C. (2013). Porifera (sponges): recent knowledge and new perspectives. eLS.