Earth Day Greetings from the Blue Marble

Blue Marble

by B. N. Sullivan

Behold, our favorite image of  Earth --  the 'Blue Marble'.  This now-famous photograph was taken on December 7, 1972 by the astronauts of Apollo 17, which was the last manned mission to the moon.  The photograph, listed in the Apollo 17 Image Library as AS17-148-22727, is described this way:
A Full Earth from the Apollo 17 Command Module at about 5 hours 6 minutes, shortly after separation of the docked CSM-LM from the S-IVB at 4 hours 45 minutes. Note that the trajectory is far enough south that Antarctica is visible.
That simple description belies the stunning impact of this iconic image.  If anything can remind us of the interconnectedness -- the oneness -- of the Earth's seas, land masses, and atmosphere, the evocative imagery of the Blue Marble  is it!

This Earth Day, regardless of what else you do, we encourage each of you to pause and reflect on the Blue Marble. That's our home --  the only one we'll ever know.  Each of us individually, and all of us together, must do whatever we can to look after it.

Purple sea slug (Hypselodoris apolegma)

by B. N. Sullivan

Isn't this a pretty little critter?  It is  Hypselodoris apolegma, a nudibranch from the family Chromodorididae.  It doesn't have a standard common name, so we just call it the purple sea slug. This attractive nudibranch lives in the western tropical Pacific region.

H. apolegma feeds on sponges. It seems to prefer a dysideid sponge of the genus Euryspongia, but it may feed on other sponges as well.

Like many nudibranchs, the purple sea slug lays its eggs in a ribbon-like mass. The egg mass of H. apolegma is yellow.

The creature's striking coloration makes it an attractive subject for underwater macro photography.  I photographed this one off the northern coast of the Indonesia island of  Sulawesi, in the Celebes Sea.  This specimen was about 3 cm long (about an inch).

The species fact sheet for H. apolegma on the Australian Museum's authoritative Sea Slug Forum describes this creature's coloring as follows:
The background colour is a rich pinkish purple with a white border to the mantle. At the edge of the mantle the border is solid white but inside this is a region of varying width in which the white forms a reticulate pattern gradually merging in to the pinkish purple. The rhinophore stalks and the base of the gills is an intense purple, the rhinophore clubs and the gills are orange yellow.
Yep, that's our purple sea slug!

Visit the Hypselodoris apolegma species page on the Sea Slug Forum for more information and photos, including feeding records, mating, and the egg mass of this species.

Seeing stars in the Mediterranean Sea

by B. N. Sullivan

Echinaster sepositus
If you go diving or snorkeling in coastal areas of the Mediterranean region, you are very likely to see this Red Sea Star (Echinaster sepositus), which is the most common starfish species in that area.  Divers and snorkelers have a good chance to spot this sea star resting on the bottom along rocky coastlines.  Although there are records of Mediterranean Red Sea Stars found at depths of more than 200 meters, they are seen most often in relatively shallow water -- less than 10 meters.

Mediterranean Red Sea Stars are actually red-orange in color, rather than pure red.  As sea stars go, they are relatively large.  The one in the top photo on this page was about 20 cm (8 in) in diameter.  The individual in the photos below was even larger -- nearly 30 cm (12 in) across.

Like many common sea star species, Echinaster sepositus has five rays, or arms.  One time, however, we found a six-armed sea star that looked just like E. sepositus, except that it had an extra arm.  At first I was not certain that it was indeed the same species, so I took the time to photograph it carefully.  Someone who knows much more than we do about Echinoderms looked at the photos and assured us that the six-armed sea star was a less common, though well-known, morphological variant of E. sepositus.

The two macro photos below are of the unusual six-armed Mediterranean Red Sea Star.  In these macro photos you can see the characteristic surface of the sea star, which is uneven -- sort of dimpled.  In the first of the two photos below, you can see the tiny structures, called papullae, which protrude from the 'dimple' indentations on the sea star's surface. In the second photo below you can see the animal's tube feet protruding from grooves on the underside of each ray.

Both the tube feet and the papullae facilitate water exchange, and are involved in the animal's respiration and excretion.  The sea star 'breathes' by extracting oxygen from sea water.  It excretes some dissolved waste matter through the papullae and tube feet, too.

The tube feet also aid the sea star in locomotion.  The tube feet in each row move successively, in a wave, using hydraulic pressure from the animal's water vascular system.  This allows the critter to crawl along.  By a similar process, the tube feet also can be used to pass bits of food from the distal ends of the rays to the sea star's mouth, which is at the center of its underside.

If you look closely at the second macro photo below, you can see that there is a little suction cup at the tip of each tube foot.  These structures help the animal stay put after it situates itself on a rock or other hard surface.  If a sea star gets overturned, it can right itself by twisting one or more of its rays so that the tube feet can grab hold of the surface and turn itself right-side up again.  (This process can take quite awhile and consumes a lot of the critter's energy, so if you pick up a sea star to look at it, be kind enough to return it to its normal position!)

The sea star lounging in an algae bed in the photo at the top of this page was photographed in the shallows of Aedipsos Bay, on the coast of the Greek island of Evia.  The two macro images below, of the six-armed Mediterranean Red Sea Star, were photographed at Cape Greco, Cyprus.

Echinaster sepositus

Echinaster sepositus