Oct. 23, 2012 --
For 37 years the Nikon Small World photography competition showcases the beauty and extreme details captured using light microscopes. Every year the world below the waves provides many interesting subjects for the Small World competition. Most examples of aquatic life are entered by professionals in, or students of marine biology, but many of these images were provided by entrants from other disciplines who simply find marine life fascinating. Here, photomicrographer Arlene Wechezak of Anacortes, Wash., and 10th place winner of the 2009 Small World Contest, magnified 10x an Obelia species of hydrozoa with extruded medusae as a fresh sea water mount using a darkfield.
Here we see the same species as before only this time magnified 40x its original size. The extruded medusae of the Obelia hydrozoa contain tentacles that sting and capture the animal's prey.
Bruno Pernet and Russell Zimmer, California S
In this image, biologists magnified 20x a species of bryozoan of the genus Membranipora found on seaweed and kelp using stereomicroscopy.
James H. Nicholson, Hanian Lang, and Sylvia G
Almost transparent tissue covers the hard skeleton "cups" of each polyp in this brightfield photo of brain coral in the genus Goniastrea magnified 25x.
Marine biologist Alvaro Migotto of the University of São Paulo in Brazil used stereomicroscopy and a darkfield to capture this photo of a brittle star magnified 8x.
Photographer David Maitland of Feltwell, UK, zoomed in with 100x magnification on coral sand over a brightfield.
Tomasz Kozielec, Nicolaus Copernicus Universi
Surface of shark skin tanned with a chromium compound magnified 40x with reflected light.
ANALYSIS: US Shark Fin Soup Appalling and Widespread
John Dolan, CNRS/University of Paris Laborato
On its side this marine ciliate (Rhabdonella spiralis)looks like a trumpet; when the image is turned vertical it looks like a champagne flute - a fine photo from French oceanographer John Dolan, who used a differential interference contrast technique and 40x magnification.
Robert Brons Insula College Dordrecht in Spij
A freshwater Bryozoan, or moss animal, Cristatella mucedo in a darkfield magnified 6.5x.
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Biologist Diana Lipscomb of George Washington University in Washington, used Nomarski Interference Contrast and a magnification of 400x the original size to showcase a suctorian ciliate, Acineta tuberosa, in the phylum Ciliophora.
Using Nomarski Interference Contrast and a magnification of 400x the original size this photo shows a ciliate in the genus Sonderia that preys upon various algae, diatoms, and cyanobacteria.
PHOTOS: Evolution Before Your Eyes
A sea slug's unusual mating behavior -- it discards its penis after copulation, grows a new one, and has sex the next day -- is reported in the latest issue of Biology Letters.
The sea slug, Chromodoris reticulata, is a type of soft-bodied marine mollusk, and its disposable penis is very rare in the animal kingdom.
As for what happens after sex ... "The penis just falls off," lead author Ayami Sekizawa told Discovery News.
Sekizawa, a researcher at Osaka City University, and colleagues made the discovery after studying C. reticulata individuals that they collected during scuba diving trips in shallow coral reef areas near Okinawa, Japan.
The researchers set up an experiment tank and watched as the sea slugs copulated 31 times.
These animals are "simultaneous hermaphrodites," meaning each performs both the "male role" of donating sperm to a mating partner and the "female role" of receiving sperm from the partner simultaneously during copulation.
A typical mating episode involves two individuals touching each other with their genital orifices. They then "project" their penises and each insert them into the other’s vagina and start copulation. After a short time, one removes its penis from the partner. Later, the other mate removes its penis too.
Both individuals then crawl away, with their elongated penises still dangling. The sexual organs, which feature backward-pointed spines for possibly trapping rival sperm, would then suddenly sever from their bodies and float away.
"The sea slug sheds 1/3 of the internal penis length after each copulation," Sekizawa said. "The sea slug is able to grow the penis gradually to its original length."
The loss and regrowth doesn’t seem to hamper the sea slug’s active sex life.
"In one case," the researchers wrote, "we observed three successive copulations each separated by approximately 24 hours."
Sekizawa said that "we have no idea about the evolutional conditions for this unique mating behavior."
Only a few other animals have been found to "dispose" of their penis, or male reproductive appendages.
One other is Argonauta, a type of octopus. Some orb-weaving spiders will also shed organs used for mating.
Earlier studies on the periwinkle, a type of edible sea snail, found that they shed their penises after the reproductive season "probably to save the cost of maintenance," Sekizawa and his colleagues believe.
That tactic, however, may not apply to the sea slug, since it has to keep regrowing its penis.
Janet Leonard, a research associate at the University of California at Santa Cruz Institute of Marine Sciences, told Discovery News that she agrees with the new paper’s conclusions and that "similar phenomena (occur) in other species."
"Little is known about mating behavior in simultaneously hermaphroditic animals," Sekizawa said. "The disposable penis in our nudibranch (sea slug) study is merely one case of peculiar mating behavior" in these animals."