Hexactinellid

Hexactinellid sponges
"Hexactinellae" from Ernst Haeckel's Kunstformen der Natur, 1904
Scientific classification
Kingdom: Animalia Phylum: Porifera Class: Hexactinellida Schmidt, 1870
Subclasses
Amphidiscophora Hexasterophora

Hexactinellid sponges are sponges with a skeleton made of four- and/or six-pointed silaceous spicules, often referred to as glass sponges.

Overview

They are usually classified in the Class Hexactinellida along with other sponges in the phylum Porifera, but some researchers consider them sufficiently distinct to deserve their own phylum, Symplasma.

Hexactinellids are relatively uncommon and are mostly found at substantial depths, although the species Oopsacas minuta has been found in shallow water. There are big differences between hexactinellids and other sponges. They are often cup-shaped animals with sturdy internal skeletons made up of fused spicules. Much of their body tissues are syncitia, extensive regions of multinucleate cytoplasm. They are fairly common relative to Demosponges as fossils, but this is thought to be, at least in part, because their spicules are sturdier than spongin and fossilize better. Unlike other sponges, they do not possess the ability to contract.

One ability they do possess is a unique system for rapidly conducting electrical impulses across their bodies, making it possible for them to respond quickly to external stimuli. Hexactinellids like "Venus' Flower Basket" have a tuft of fibres that extends outward like an inverted crown at the base of their skeleton. These fibres are between 50 and 175 mm long and about the thickness of a human hair. They work as optical fibres that are surprisingly similar to those used in modern telecommunication networks.

Venus' Flower Basket, Euplectella aspergillum

The earliest known hexactinellids are from the earliest Cambrian or late Neoproterozoic.

These creatures live for a very long time, but the exact age is hard to measure; one study based on modelling gave an estimated age of a specimen of Scolymastra joubini as 23,000 years, which is thought impossible^[1], but is the basis for a listing of ~15,000 years in the AnAge Database.^[2]

The sponges form reefs off the coast of British Columbia and Washington State^[3], which are studied in the Sponge Reef Project.

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See also

• Sponge reef

References