A photo of a crab-eater seal diving beneath Antarctic ice won Best of Show.
Despite their name, these seals eat mostly krill, not crabs. Photographer Greg Lecoeur spotted this one near the Antarctic Peninsula, the northernmost part of the continent.
Other winning photos showed smaller life forms, like these clownfish eggs.
Photographer Paolo Isgro spotted them at a scuba-diving site in Tulamben, Indonesia.
A rare shot of pink whip rays riding on a small-eyed ray near Queensland, Australia won the Marine Life Behavior category.
Scientists think the small rays do this to save energy, protect themselves from predators, and eat leftovers from the larger rays. Paula Vianna saw the rays near an Australian shipwreck site.
In the Red Sea off the coast of Egypt, another winning photo captured a different pair of fish friends: a grouper and a cleaner wrasse.
The small cleaner wrasse eats parasites, dead skin, and extra food off the grouper — a win-win situation.
Six rowdy seahorse babies pointed their faces in the same direction just long enough for novice photographer Jules Casey to snap a shot.
"I'm absolutely delighted to be able to share such a split second in time before this scene changed dramatically," Casey wrote of the photo.
Nudibranchs, a vast group of marine mollusks sometimes called sea slugs, get their own contest category.
The winning photo shows a bright purple "flabellina lotus" in Australia's Mooloolah River, which hosts over 350 species of nudibranch.
Coral reefs, like the Indonesian one shown here, offer endless opportunities for underwater photographers.
The image that took first place in the Reefscapes category shows a school of tiny glass fish flowing over a handful of ribboned sweetlips.
This pair of lemon goby parents watched over eggs that they'd laid on a piece of discarded glass.
"I decided to take shot to record this because it presented how fish can coexist with human garbage," photographer Stan Chen wrote.
Warning: The next image shows a dead animal, so it may be disturbing to some readers.
But garbage is generally harmful to marine life. The winner of a new category, Conservation, was this haunting image of a long-dead sea turtle tangled in fishing line.
Photographer Shane Gross said his diving partner came to him in tears after encountering the dead sea turtle.
"She didn't have time to remove the line, so she told me where it was and I went back. I didn't want any scavengers to also become entangled," Gross wrote in his contest entry. "I took my camera because images like this can become warnings for the future."
Another new category, Blackwater, encouraged photographers to go diving at night. That's when this young snaketooth fish made an appearance.
According to photographer Fabien Michenet, snaketooths begin their lives near the surface, then spend adulthood in deep-sea sediment.
This yellow crested weedfish hid in the kelp at Shelly Beach, Australia. Snapping this expert camouflager required patience.
"Its movements sway like the seaweed it buries itself in, its color almost identical, making it the ultimate master of disguise," photographer Talia Greis wrote. "The only way to capture this moment was to hang back, remain still, and wait for the perfect moment it decided to surface and analyze my presence."
This seahorse photo won first place in the Portraits category.
"I was immediately surprised by his punk hairstyle," photographer Virginia Salzedo wrote.
Photographer Nicholas More said a shot of this school of ribboned sweetlips was "at the top" of his wish list.
"To allow the sweetlips to be center of attention, I used a slow shutter speed and accelerated panning to blur the background," More wrote. "This effect also helps to reinforce the unity of the school moving as a group, in the same direction."
Though the following images didn't win first place in any category, they're just as captivating and surprising.
Photographer Pedro Carillo Montero caught these young sea lions playing under a rock ledge. In this image, he wrote, they almost look like they're singing.
Predators on the prowl made for awe-inspiring images. Photographer Emry Oxford saw this crocodile in Cuba.
Photographer Dave Johnson took a picture of this banded sea snake as it rested alone in the sand in Indonesia's Lembeh Strait.
This curious leopard shark kept approaching photographer Jake Wilton. The species is rarely aggressive toward humans.
Photographer Fabien Martinazzo captured this pelagia noctiluca jellyfish on camera as it made a rippling trail through the water.
Another fish, a blenny, took a similar approach, fitting perfectly inside empty beer bottles on a sunken ship.
The peacock mantis shrimp has notoriously complex eyes, with 16 color receptors. Humans, by comparison, have three.
Photographer Fabien Martinazzo is convinced that this tompot blenny saw its reflection in his camera lens.
"She remained still for almost a minute," Martinazzo wrote. "It was like we were having a long discussion."
