What are sponges (Metazoa: Porifera)?

Source: Janussen & Reiswig (2009)
Source: Janussen & Reiswig (2009)

We already looked at the characteristics of animals. The currently-known basalmost animals are the Porifera Grant 1836, the sponges. Like the hypothetical basal animal described in the post, sponges don’t have any organs or even epithelia. They have no true cell-cell interactions – their cells are very loosely-organised and only a few tight junctions are found. They also undergo external fertilisation.

The above features are termed plesiomorphies. They are not evolutionary novelties, but are merely inherited from the last common ancestor. It follows logically that they can’t be used to characterise sponges: for that, we need autapomorphies, evolutionary novelties unique to them. The main ones for sponges are:

  • Water canal system and an osculum.
  • Various unique cell types.
  • A biphasic life cycle, with a sessile adult developing from a coeloblastular larva.
Source: Storch & Welsch (2009)
Source: Storch & Welsch (2009)

The above diagram shows a sponge body. A sponge will soak up water through the pores, leading it through the water canal system into the central cavity. The water is then expelled through the osculum at the top. In case you are wondering, this is not entirely passive: if you take time-lapse videos of sponges, you will see that they can actively contract and expand. Somewhat irrelevant sidenote: colleagues know that I have a pet and laughed-at hypothesis that sponges have a rudimentary nervous system; this is one my evidences.

The body is not entirely defenceless: spiculae are produced by the sponge. These are needles that stick out of the body as a deterrent to predators, and their enormous morphological diversity makes them the most suitable taxonomic markers for sponges – it’s often so that recognising a species is done solely through looking at its spiculae. Sponges are also the subjects of intense pharmacological study, as they are biochemically very active, producing diverse toxins that keep the sponge predator-free, and may potentially give us new drugs to play around with.

Source: Storch & Welsch (2009)
Source: Storch & Welsch (2009)

Sponges come in three major body types. From a to c above: ascon, sycon, leucon. If you read older zoological books, you might come across texts explaining that the evolution of sponges follows a path from the “simple” ascon through the intermediate sycon to the “advanced” leucon body type. This is bunk – the apparent complexity of the body types is not an evolutionary marker: you can have basal sponges with a leucon body type, and derived sponges with an ascon body type.

Instead, the systematics of the sponges is based on the spiculae. The three sponge taxa are:

  • Calcarea Bowerbank 1864
  • Hexactinellida Schmidt 1870
  • Demospongiae Sollas 1885

Calcarea have calcareous spicules, hexactinellids and demosponges siliceous ones. The latter two differ in the structure of the spicules. That said, sponge phylogenetics is a bit of a mess. That each of these groups is monophyletic is clear. The consensus is that siliceous spicules evolved only once, uniting the hexactinellids and demosponges as the Silicea, although there is also a minority view thatCalcarea and Demospongiae belong together as Cellularia (Reiswig & Macke, 1983).However, the Calcarea cause some problems, with some studies showing them as more closely related to the rest of the animals rather than with the other sponges (Botting & Butterfield, 2005; Medina et al., 2001). I personally hold the traditional view that the sponges are monophyletic since the evidence is still inconclusive for either end; however this may change a few studies later. The implications of a shake-up and a paraphyletic Porifera is quite significant, as we would have to alter our already fragmentary view of the animalian last common ancestor.

With this out of the way, and assuming a monophyletic Porifera, let’s get back to the autapomorphies list. The next point was several unique cell types. Sponges may not have the differentiated cells we know from the rest of the animal kingdom, like nerve cells, muscle cells, gland cells, etc., but on the other hand, they have their own bunch of cells not found anywhere else.

Source: Storch & Welsch (2009)
Source: Storch & Welsch (2009)

Of these, the archaeocytes and the choanocytes are universal among all sponges, and are thus assumed to be definite sponge characters. Choanocytes, one diagrammed above, are cells that lie in groups in the pores of the water canal system, and use the ring of microvilli to trap food. Archaeocytes are unique sponge stem cells.

There are several other cell types, but they may only be found in some groups of sponges, not all. As examples: pinacocytes line the outer layers of the sponge body; endopinacocytes surround the water canals; scleroblasts build the spicules, but with different processes and companion cells in the Calcarea and Silicea.

The final autapomorphy is the lifestyle of the sponge. The sponge life cycle begins with a coeloblastular larva – so named because of its striking resemblance to a blastula (“ontogeny recapitulates phylogeny” fans, unite!). The larva is responsible for geographic dispersal and eventually settles to metamorphose into the sessile adult. This two-staged life cycle was a sponge novelty, a departure from the hypothetical metazoan ancestor. Adult sponges are filter-feeders, although some have evolved to be carnivores.

References:

Botting JP. & Butterfield NJ. 2005. Reconstructing early sponge relationships by using the Burgess Shale fossil Eiffelia globosa, Walcott. PNAS102, 1554-1559.

Grant RE. 1836. Animal Kingdom. In: Todd RB (ed.). The Cyclopaedia of Anatomy and Physiology. Volume 1.

Janussen D & Reiswig HM. 2009. Hexactinellida (Porifera) from the ANDEEP III Expedition to the Weddell Sea, Antarctica. Zootaxa2136, 1-20.

Medina M, Collins AG, Silberman JD & Sogin ML. 2001. Evaluating hypotheses of basal animal phylogeny using complete sequences of large and small subunit rRNA. PNAS98, 9707–9712.

Reiswig HM. & Mackie GO. 1983. Studies on hexactinellid sponges. III. The taxonomic status of Hexactinellida within the Porifera. Phil. Trans. R. Soc. B301, 419-428.

Storch V & Welsch U. 2009. Kükenthal Zoologisches Praktikum, 29thed.

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