Calcareous sponges

Phylum Porifera Class Calcarea Number of families 22

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Marine sponges with calcareous skeletal elements (spicules)

Photo: A calcareous sponge in a cryptic reef environment in waters near the Little Cayman Islands, at a depth of 82 ft (25 m). (Photo by ┬ęGregory G. Dimijian, M. D./Photo Researchers, Inc. Reproduced by permission.)

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Evolution and systematics

The fossil record of unambiguously identified Calcarea is relatively poor and fragmented. Most calcareous sponges in the fossil record were classified as either stromatoporoids, chaetetids, archaeocyaths, inozoans, pharetronids, or sphinc-tozoans. They are common in the Paleozoic and Mesozoic, however, rare in the Cenozoic. It is now established that many of these forms actually belong to several groups of demo-sponges because of the possession of primary siliceous spicules, and only few to Calcarea (pharetronids and some sphinctozoans). Identification of "true" calcareous sponges in the fossil record is difficult because fossil remains often lack diagnostic spicules at all. Heteractinida, characterized by a spiculate (consisting of six-rayed heteractinid octactines, poly-actines) and aspiculate calcitic skeleton, are now regarded as an extinct order of Calcarea, restricted to the Paleozoic. The oldest probable calcareous sponge with affinities to modern subclass Calcaronea (Gravestockia pharetroniensis Reitner, 1992) was described from the lower Cambrain of South Australia. The assignment of many records of so-called "Pharetronida," calcareous sponges with a rigid calcareous skeleton, to subclasses Calcaronea or Calcinea is difficult if they do not possess characteristic spicules to allow precise assignment. However, most Pharetronids probably belong to subclass Calcaronea. The majority of modern spiculate cal-careans would be found as dissociated spicules in the fossil record; there is only one record from the middle Jurassic at King's Sutton, Northamptonshire, where the form and arrangement of a calcareous sponge was preserved (Leucandra walfordi Hinde, 1893).

Calcarea are regarded as one of four classes of the phylum Porifera (three extant [Demospongiae, Hexactinellida, Cal carea] and one fossil [Archaeocyatha]), distinctive in possessing a spicule skeleton composed exclusively of calcium carbonate and being the only poriferan taxon realizing all three stages of development of the aquiferous system (asconoid-syconoid-leuconoid). There is still dispute about the true phy-logenetic relationships of the three extant sponge classes, including also the relationship of the class Calcarea to other (higher) diploblastic taxa like Ctenophora and Cnidaria. Two competing hypotheses group a) Hexactinellida + Demospon-giae more closely together based on the possession of silicious spicules ("Silicea") in contrast to Calcarea ("Calcispongia") and b) Demospongiae more closely with Calcarea based on the possession of a cellular pinacoderm ("Cellularia"/"Pina-cophora") to the exclusion of Hexactinellida, which possess a cyncitial tissue structure ("Symplasma"). Both proposals, however, assume poriferan monophyly. More recently, several authors have suggested from ribosomal DNA sequence data that Calcarea might be more closely related to the phyla Ctenophora/Cnidaria than to the other two extant classes of Porifera, rendering phylum Porifera paraphyletic. Class Calcarea was elevated to phylum status ("Calcispongia," a term that was already used in the mid-nineteenth century) (Zrzavy, et al., 1998; Borchiellini, et al., 2001), but as yet without robust statistical support (e.g., Medina, et al., 2001). However, this proposal is not followed in the most comprehensive systematic treatment of sponges to date, the Systema Porifera (Hooper and Van Soest, 2002) and the issue of sponge para-phyly is at the time of writing (2003) far from being resolved. Therefore, it should be regarded as still contentious until further corroboratory data, such as a molecular multi-locus approach, is presented. However, new chemotaxonomic data from lipid biomarkers (Thiel, et al., 2002) support a closer relationship of Hexactinellida and Demosponges. Although this

Calcareous sponge anatomy. (Illustration by Kristen Workman)

confirms that Calcarea are chemotaxonomically different from "Silicosponges" or "Silicea" (Demospongiae + Hexa-ctinellida), it does not necessarily imply sponge paraphyly.

Number of classes and families: 1 class (Calcarea); 2 subclasses (Calcinea, Calcaronea); 5 orders (2 in Calcinea: Clathrinida, Murrayonida; 3 in Calcaronea: Leucosoleniida, Lithonida, Baeriida); 22 families; 75 genera; about 500 described species.

Physical characteristics

Calcareous sponges are mostly small and inconspicious; they occur in a variety of forms, as single tubes, sometimes vase shaped, a mass of small tubes ("cormus"), a bushy arrangement of single tubes, or sometimes massive without any apparent symmetry. Three types of aquiferous system are realized in Calcarea: asconoid, all internal cavities are lined by choanocytes (flagellated cells) without folding of the choanoderm; syconoid, simple folding of the choanoderm; and leuconoid, choanocytes are arranged in discrete "choanocyte chambers."

Calcareous sponges range from minute size an inch or less (few millimeters), to about a maximum of about 12 in (30 cm) (Pericharax heteroraphis). They are mostly colorless (whitish to beige), sometimes bright yellow (Leucetta chagosensis), dark greenish-brown (Pericharax heteroraphis), or fluorescent red/ orange (Leucetta microraphis, sometimes).

Calcareous sponges have a skeleton that is made of calcium carbonate (calcite), composed of free diactines, tri-actines, tetracines, and/or polyactine spicules, to which a solid basal calcitic skeleton may be added, with either cemented basal spicules or which is fully embedded in an enveloping calcareous cement. Calcareans are viviparous and have blastula larvae.

Distribution

Calcareous sponges are found globally in all oceans, from intertidal to the deep sea, but not the abyss.

Habitat

Calcareous sponges live in diverse habitats. In tropical coral reefs, they dwell mainly in shaded and/or cryptic habitats and prefer calmer waters.

Behavior

Not applicable; calcareous sponges are sessile filter feeders.

Feeding ecology and diet

Calcareous sponges are sessile filter feeders, whose main diet is dissolved organic matter and small particulate matter (bacteria) filtered from seawater by pumping activity.

Clathrina sponges are usually dull colored and less than 0.16 in (4 mm) long. These were seen in Papua, New Guinea. (Photo by Ron and Valerie Taylor. Bruce Coleman, Inc. Reproduced by permission.)

Reproductive biology

Calcareous sponges have internal fertilization, with egg size ranging from 25 to 100 pm. They are sexual and viviparous, with some species probably asexual by budding.

Conservation status

No species are listed by the IUCN.

This calcareous sponge Pericharax sp. has been eaten by nudibranchs Notodoris. (Photo by Bill Wood. Bruce Coleman, Inc. Reproduced by permission.)

Significance to humans

There is no known significance of calcareous sponges to humans.

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