Among brittlestars and sea urchins we find visual systems that in some ways rival the arthropods in the form of compound eye-like structures.

Many echinoderms (the marine invertebrate group that includes the familiar starfish and sea urchin) detect light to seek shade and escape from predators.  Now at first sight this might come as a surprise because although quite closely related to the fish (all belong to the deuterostomes) echinoderms have a diffuse nervous system and no brain.  Astonishingly, however, some echinoderms are light sensitive and have specific structures and photoreceptors that allow the whole body, or parts of it, to function rather like a compound eye.  It is a fascinating case of convergence.

Visual perception is achieved differently in the various echinoderm groups.  It is among brittlestars (the ophiuroids) and sea urchins (the echinoids) that we find systems that in some ways rival the arthropods in the form of compound eye-like structures.  So it is that along the arms of one brittlestar (Ophiocoma wendtii) we find calcitic ‘microlenses’.  These are composed of modified ossicles, can be shaded using pigmented chromatophore cells, and are underlain by the photoreceptors.  In sea urchins visual acuity appears to be based on photoreceptors in the tips and bases of tube feet, with here shading is not mediated by protective pigments but rather by skeletal elements (spines and tube foot pores).  Just how microlenses and tube foot photoreception provide parallels with compound eyes is detailed in the sections below.  While microlens and tube foot-based eyes both provide remarkably effective directional vision, simpler types of echinoderm eye also occur. For example, in their arms starfish (e.g. Asterias) have translucent calcite plates termed optic cushions.  In association with photoreceptor cells these confer directional light responses.  Brittlestars like Ophioderma brevispinum and Ophiura ophiura have polarising calcite ossicles that allow orientation within 17° of a light source, facilitating movement towards shade and away from predators. As a final illustration of the diversity of echinoderm eyes, synaptid sea cucumbers (holothurians) are worm-like creatures with feeding tentacles at the anterior end and paired eyes (ocelli) at the base of each tentacle.  Ocelli have evolved many times and comprise a photosensitive cell, pigmented shielding cells and underlying nerve fibres.  This simple eye structure is common to many invertebrates, from snails to annelids.

Brittlestar microlenses and trilobite eyes

The brittle star Ophiocoma wendtii has 40-50µm diameter calcite microlenses arrayed along the dorsal surface of its arms, modified from the calcite plates (ossicles) of the skeleton. The microlenses are biconvex with the optic (or c) axis at right angles to the surface of the lens.  This serves to eliminate birefringence whereas in any direction the light travelling through the calcite crystal will be split into two rays: hardly helpful when it comes to clear vision.  Impressively, the lenses also correct for spherical aberration, creating a sharp focus on the equivalent of the retina.  So it is that light is focused onto photoreceptor and nerve cell bundles a few microns beneath each lens.  Although the nervous system is integrated throughout the body, what the animal actually ‘sees’ is pretty conjectural given that it lacks a brain.  As in other eyes, control of light entering the lens is key to their effectiveness.  In this brittle-star pigment-filled chromatophore cells act as shades, shielding the microlenses from excessive levels of light.  The sophistication of this system may be surprising, but as a result these animals are capable of rapid escape reactions and also engage in dramatic colour changes.  The eyes almost certainly play a role in these activities.

When it comes to convergence the story becomes even more interesting.  This is because the calcite microlenses of brittle star Ophiocoma wendtii closely resemble those found in the compound eyes (the schizochroal variety) of certain trilobites.  Compound eyes are, of course, typical of arthropods.  In brief, such eyes comprise a radial array of cylinder shaped visual units called ommatidia, each with a lens and crystalline cone focusing light onto a photosensitive channel (rhabdom) within a group of photoreceptors.  One optic nerve innervates each ommatidium, so each lens contributes discreet information, integrated subsequently in the optic ganglion to form an image.  In most arthropod eyes the lens margins are all in contact, but in the phacopid trilobites the lenses are unusually large, separated from each other (schizochroal) and optimized to correct for spherical aberration and again by appropriate orientation of the optic axis avoid birefringence.  In schizochroal eyes each lens forms a separate image (as can be demonstrated by looking through a trilobite eye) and these images must be not only integrated across the eye but also presumably combined with the images provided by the other eye.  This hints at quite a sophisticated brain.  Plotting the visual field of the eyes also gives important clues as to the original life-habits of these long-extinct animals.  Perhaps the efficient lenses of trilobites conferred similar advantages to those of brittlestars, namely fast responses and visual sensitivity in dim light.  Similarly, a very sharp-eyed and fast moving insect, the male strepsipteran fly also has schizochroal-like compound eyes.  Here too each lens provides a separate image but in other respects the eyes are like those of other insects and not calcitic.  Why only males?  This is because strepsipterans are parasitic and the roving male needs to keep a sharp-eye for the females nestling in their hosts.

Sea urchin spines and tube foot photoreceptors

At first the notion of sea urchin vision may seem odd given their lack of a defined brain and uniform pin-cushion form.  However, a series of studies point to limited spatial vision with resolution of up to 10^o, equivalent to the visual acuity similar to the relative speedy and large-brained cephalopod Nautilus.  How is this achieved?  It seems that shading afforded by both spines and photoreceptors within the tube feet are likely to be involved.

The first hint that sea urchins use their spines for vision came from observations of phototaxis in the tropical urchin Diadema antillarum.  Subsequent experiments on Echinometra and Strongylocentrotus showed that their opaque spines (attached to calcite skeletal plates at intervals) screen light falling between them (to within 24^o in Echinometra and 10^o in Strongylocentrotus).  In this way each inter-spine area receives its own visual information, so the whole body of the sea urchin can act essentially like a compound eye, with visual resolution in each species depending on the distance between spines.  This eye theory was based on a long-held assumption that the whole of the urchin’s dermis (skin layer) is sensitive to light.  Further investigations showed that this is not quite so; rather, the animal’s tentacle-like tube feet add another fascinating dimension to this story.

Molecular studies of shade-seeking urchins Strongylocentrotus purpuratus and Psammechinus miliaris revealed r-opsin-expressing photoreceptor cells at the tips of tube feet and also, significantly, in a cluster at their base.  Furthermore, early in development photoreceptive regions are also associated with expression of Pax-6, a transcription factor conserved in eye specification.  Basal photoreceptor cells are shielded within a depression of the tube foot pore (each tube foot emerges through pores in the calcite skeletal plates).  Photoreceptors at the tube foot tip can receive light from all angles and may facilitate reflex retraction away from sudden shading, such as that caused by a passing predator.  By contrast, light can only fall on basal tube foot photoreceptor cells from a restricted angle.  This is because they are partly enclosed by the tube foot pore.  Tube feet connect to radial nerves such that they can provide neural signals, locating received light and endowing the sea urchin with impressive directional vision.  Note, however, light-shielding pigments are absent in all sea urchins (a case of secondary loss); it seems likely that instead structural shielding of tube foot photoreceptor cells by calcite pores and opaque spines confers the necessary spatial vision.

You will never look at a sea-urchin in quite the same way again, but so too it is looking at you.