Up to this point, arthropods have dominated the Invertebrate of the Week series so I am going to give them a rest and head over to a different invertebrate lineage: the sponges (phylum Porifera).
Sponges hail from a very, very old evolutionary heritage and this week’s invertebrate comes from the oldest of the old: Hexactinellida. According to the Natural History Museum in London, this group of sponges represents the oldest multicellular organisms in the fossil record and they enjoyed “…their maximum diversity during the Cretaceous (99.6 – 65.5 million years ago) when these sponges formed the vast reefs of the Tethys Sea.”
Known as glass sponges, they are exclusively marine organisms and enjoy a worldwide distribution at depths between 10 and 6000 meters, though they are most commonly encountered in cold waters (2-11o C) on deep-sea (>200 meters) substrates where they survive by filter feeding.
As their name suggest, Hexactinellida have a close association with ‘glass’ in the form of silica and possess internal skeletons composed of aggregations of 6-pointed silica spicules which form a rigid mesh. As described by researcher Martin Dohrmann, these spicules “…have a triaxonic and cubic symmetry, (i.e. they are composed of three axes that are arranged at right angles to each other.) The basic spicule form is the hexactin, which has all six rays (two per axis) fully developed – hence the taxonomic name, Hexactinellida.” The strength imparted by this design is presumably one of the reasons these sponges are able to survive at such crushing depths.
While the skeletal structure of theses sponges is clearly defined, the surrounding adult soft tissues are syncytial. This means that their cells are fused resulting in a single ‘mega-cell’ with many nuclei but few (if any) internal borders. Such intimate connectivity among cells has its advantages and it has been demonstrated that sponges can use syncytial tissues to propagate electrical impulses much like a nervous system.
This week, I am focusing on a particularly conspicuous representative of the Hexactinellida called Euplectella aspergillum. Colloquially dubbed ‘Venus’s Flower Basket’, this sponge can be found in the western Pacific Ocean near the Philippine Islands at depths between 100 – 1000 meters with a preferred habitat being deepwater rocky substrates >500 meters down. It is radially symmetric and exhibits a beautiful mesh-like ‘vase’ morphology that typically reaches a height between 10 – 30cm.
The interior (spongocoel) of this remarkable organism sometimes plays host to abyssal shrimp which enter the cavity as larvae and eventually grow too large to exit. Occasionally, a male and female shrimp will mature and become trapped within the same spongocoel and anecdotal reports suggest that sponge specimens collected with such shrimp pairings have traditionally been presented as wedding gifts in Japan to symbolize the vow of “til death do us part.”
References and Further Reading:
- Eupectella aspergillum (Venus’ flower basket) – Natural History Museum, London
- Euplectella aspergillum – Encyclopedia of Life.
- Hexactinellid Sponges – Encyclopedia of Life
- Dohrmann, M., A. G. Collins, and G. Wörheide. “New insights into the phylogeny of glass sponges (Porifera, Hexactinellida): monophyly of Lyssacinosida and Euplectellinae, and the phylogenetic position of Euretidae.”Molecular phylogenetics and evolution 52.1 (2009): 257-262.
- Dohrmann, Martin, et al. “Phylogeny and evolution of glass sponges (Porifera, Hexactinellida).” Systematic Biology 57.3 (2008): 388-405.
- Dohrmann, Martin, et al. “Systematics and spicule evolution in dictyonal sponges (Hexactinellida: Sceptrulophora) with description of two new species.”Zoological Journal of the Linnean Society 163.4 (2011): 1003-1025.
- Soares, Beau McKenzie. Euplectella aspergillum. – Animal Diversity Web, University of Michigan.
- Wörheide, G., et al. “1 Deep Phylogeny and Evolution of Sponges (Phylum Porifera).” Advances in marine biology 61 (2012): 1.