Fall in Rhode Island

posted by Casey Dunn / on September 30th, 2009 / in Arthropods, Parasites

Manduca

The leaves are starting to turn and the garden is getting thin as most fruits and vegetables are harvested. There are some fun surprises among the plants that remain, including this tobacco hornworm (Manduca sexta) above that was chewing on our tomatoes. It stayed in one spot, and over the course of two days more and more parasitoid wasp larvae, probably Cotesia congregata, emerged to spin their cocoons. When the mother wasp injected her eggs into the young caterpillar, she also injected a virus that had been multiplying in her ovaries. This virus continued to reproduce in its new host, castrating the caterpillar and preventing it from metamorphosing. This trick provides the the perfect feeding ground for the wasp babies.

Other organisms are also at their peak, and the woods are full of beautiful and delicious fungi. The specimen below is Laetiporus, also known as chicken of the woods because it is so common and quite eatable.

Chicken

Photos by Casey Dunn. Thanks to Doug Morse, Alan Bergland, and Erika Edwards.

Glowing worms in the deep sea

posted by Orla O'Brien / on September 14th, 2009 / in Annelids

Swima

Bioluminescence can be used for myriad purposes in different species—this recently discovered species of annelid, Swima bombaviridis, probably uses bioluminescence to escape from predators. It was described by Karen Osborn and friends. The worm carries eight fluid-filled packets near its head that it can release at will. When these packets are released, they bioluminesce a bright green for several seconds. Since the worms live in the deep sea, these flashes are a contrast to the dark environment and may distract predators—instead of getting a bite of worm, they are left with nothing. The mechanism for releasing these bioluminescent bombs is unclear—in addition to the lack of light at the depths the worms live at they are without eyes—but the release is probably related to a tactile sensory system, as they release their bioluminescent organs when touched.

Photo by Casey Dunn. The head is to the left, and the green bioluminescent packets can be seen attached to the body just behind it.

Hiding submarines beneath jellyfish

posted by Casey Dunn / on September 9th, 2009 / in Jellies, Siphonophores

Nanomia

With the advent of submarine warfare, the ability to locate large underwater objects with SONAR became of prime strategic importance. Active SONAR detects objects by listening for echos from pulses of sound. As SONAR became more widely used, though, some very strange things were seen in the open ocean. At times, the SONAR suggested that the ocean floor was much shallower than maps and direct depth measurements indicated. Ships sitting in one place would also find that the depth of the ocean would appear to change through the course of the day, as if the sea floor were heaving beneath them.

Something was creating a false bottom that the SONAR couldn’t see through. Submarines found that they could dive right through this layer, hiding beneath it and rendering the SONAR above useless. Details about these false bottoms in the open ocean were closely guarded military secrets during World War II.

It had been suspected that the false bottom was made of large groups of animals, but nets sent to this region usually came up empty. Then, in 1963, Eric Barham, a scientist at the US Navy Electronics Laboratory, reported his first-hand observations form aboard the research submarine Trieste. His dives were coordinated with ships above that monitored the position of the false bottom with SONAR. When Trieste arrived at the false bottom it did find animals, and lots of them. They were siphonophores, extremely fragile colonial jellyfish that are notoriously difficult to collect. They are so fragile that they usually turn to slime in nets and pass right through the mesh.

How could something so delicate and gelatinous have such a strong signature on the SONAR, powerful enough to hide entire submarines? Many species of siphonophores have a gas filled float that serves to regulate buoyancy, and possibly to sense which way is up. The siphonophore that was found in the false bottom, Nanomia bijuga (see photo above), has a float that is about a milimeter in diamater, which is predicted to resonate at a frequency very close to the sound pulse used by SONAR. This resonance scatters the sound, and when there are lots of siphonophores the scattering is so thorough that the SONAR can’t penetrate through the swimming jellyfish.

Besides revealing the important impacts of a poorly-known colonial jellyfish on military technology, these findings also indicate how difficult it can be to measure the abundance of jellyfish. They weren’t detected in nets sent to the false bottom, but there were enough of them to hide entire warships. This measurement problem is compounded when we try to establish whether jellyfish are rising or falling in abundance through time. Because they are so difficult to observe, their abundance has likely been dramatically underestimated in the historical record. As we see more jellyfish with improved sampling methods, it is hard to know if they are more numerous than they used to be.

The Nanomia bijuga photo above was taken by Claude Carré at Villefranche. The float is at the upper right of the image. More information on siphonophores can be found at siphonophores.org.

Retractable spots

posted by Casey Dunn / on September 7th, 2009 / in molluscs

cyphoma

The marine snail Cyphoma gibbosum browses on the polyps of soft corals (top). It appears to have a brightly spotted shell, but when disturbed the spots begin to move (middle) and then retract within the white shell, along with the rest of the animal (bottom). This is possible because the spots are not part of the shell at all. They are patterns on the thin mantle tissue that extends out of the shell opening and up around the snail’s back. Photos by Casey Dunn.