Encrusting sponges

Sponges are the reason a Maltese cave wall looks like someone attacked it with poster paint. They are animals without organs: a skeleton of mineral spicules and protein fibres holding open a plumbing system that pumps water in through thousands of tiny pores and out through the larger, obvious craters called oscula. Look for those oscula; they are the surest sign that an orange smear on rock is a sponge rather than an alga. A handful of species do most of the colouring around Malta and Gozo. Crambe crambe is the flat scarlet-to-orange crust, only a few millimetres thick, spreading over shaded rock like spilled paint with a fine network of surface channels. Petrosia ficiformis is the hard, stony, grey-lilac 'fig sponge' of caves and overhangs, thick-walled and studded with volcano-like oscula. Chondrosia reniformis forms smooth, rubbery, kidney-shaped beige-brown lobes with no visible texture at all.

Add Ircinia variabilis, a tough dark grey-brown sponge with a surface of small conical points, and Agelas oroides, a vivid orange lobed sponge of dim water, and you have most of what coats the walls of Billinghurst, the Inland Sea tunnel, the Blue Hole chimney and the shaded plating of Maltese wrecks. In caves, the deeper you go from the entrance the more the community shifts, with pale yellow Aplysina finger clusters and delicate white mesh sponges taking over in permanent darkness.

They run from just under the surface down to well beyond diving depth, but the real displays are on vertical and overhanging surfaces between about five and sixty metres, wherever light is low and water moves. A patch may be a few centimetres across or spread over half a metre of rock, and it grows slowly, in millimetres per year.