Optical fibers in sponges?

Grantia is a genus of calcareous sponges belonging to the family Grantiidae. Grantias contain spicules and spongin fibers. Spicules are structural elements found in most sponges. They provide structural support and deter predators. Large spicules, visible to the naked eye are referred to
as megascleres, while smaller, microscopic ones are termed microscleres.

Research on the Euplectella aspergillum (Venus' Flower Basket) demonstrated that the spicules of certain deep-sea sponges have similar traits to optical fibre. In addition to being able to trap and transport light, these spicules have a number of advantages over commercial fibre optic wire. They are stronger, resist stress easier, and form their own support elements. Also, the low-temperature formation of the spicules, as compared to the high temperature stretching process of commercial fibre optics, allows for the addition of impurities which improve the refractive index. In addition, these spicules have built-in lenses in the ends which gather and focus light in dark conditions.

It has been theorized that this ability may function as a light source for symbiotic diatoms (as with Rosella racovitzae) or as an attractor for shrimp which live inside the Venus' Flower Basket. However, a conclusive decision has not been reached; it may be that the light capabilities are simply a coincidental trait from a purely structural element. Spicules funnel light deep inside sea sponges.
Source: Wikipedia, Antarctic Field Guide