If you’re spending this holiday weekend at the beach or headed there later this summer, take a moment to consider how many amazing life forms inhabit the oceans. From microscopic plankton to apex predators like great white sharks, the marine world teems with bizarre and often beautiful life forms. Here we offer looks at a few of them.

My, what big teeth you have

Readers who grew up with the 1975 hit movie “Jaws” may still hear its ominous two-note theme in the back of their minds when they step into the surf. And it’s true that along many U.S. coastlines you could sight a shark, possibly even a great white. But that’s good news, explains George Burgess, director of the Florida Program for Shark Research and coordinator of operations at the Florida Museum of Natural History:

“Like it or not, sharks are part of a balanced ocean ecosystem. After a decline of up to 90 percent for some species in the United States, they are beginning a gradual rise toward the numbers of a century ago.

"That suggests some of the damage we’ve done to the oceans has been reversed, and that’s something to celebrate.”

Burgess readily acknowledges that great whites and a few other shark species will attack humans. But in his view, the burden is on us to understand that when we step into the ocean, we enter their world, and should understand the risks and take reasonable precautions – just as we do to guard against drowning.

That’s especially true for people who engage in activities like surfing, where the chances of encountering sharks are higher. Policies such as culling sharks near popular surfing beaches won’t work, Burgess warns:

“Sharks are low-density, highly migratory animals that readily recolonize areas denuded of their kind, rendering any attempt to cull an ineffective strategy.”

Moreover, Burgess points out, culling programs kill many nondangerous sharks, thereby harming ocean ecosystems that need these large predators to keep smaller species in check.

Welcome back, otters

If great white sharks are the most-feared species in the oceans, sea otters may be the most-loved. But otters’ congenital cuteness didn’t stop fur traders from hunting them to near-extinction along the Pacific coast in the 18th and 19th centuries.

Since they received international protection in 1911, otters have gradually repopulated the West Coast. But it’s tricky for scientists to count and track wild animal populations, especially when their targets spend a lot of time underwater.

Colorado State University associate professor Melvin Hooten and postdoctoral fellow Perry Williams turned to math to document sea otters’ return to Alaska’s Glacier Bay. Starting with a mathematical model that used partial differential equations to describe the growth and spread of sea otters, they used statistical methods to infer how quickly the Glacier Bay otter population was growing.

“Using our new approach, we discovered that the Glacier Bay sea otter population grew more than 21 percent per year between 1993 and 2012…That means that the Glacier Bay sea otter growth rate was near or at maximum, and greater than any recorded sea otter population in history,” they write.

Aquatic hitchhikers

Since there are no borders in the oceans, scientists need to know where sea creatures travel in order to make effective conservation plans. That’s especially true for sea turtles, which can migrate thousands of miles to reach their preferred nesting beaches.

Nathan Jack Robinson, a postdoctoral fellow at Indiana University, studies epibionts – tiny organisms that attach to sea turtles’ shells as they swim. Examining sea turtles that came ashore to nest in Costa Rica, Robinson and his colleagues found diverse epibionts, from barnacles to small remora fish. Different turtle species carried different colonies of tiny hitchhikers. Robinson believes these tagalongs provide valuable data.

“[I]t’s becoming clear that the creatures found on each sea turtle can tell a story about where that turtle has been and what it was doing there. The information encoded in each sea turtle’s unique set of hitchhikers can, in turn, help guide management decisions to protect these animals during their lives at sea.”

As a bonus, he points out that saving endangered sea turtles will also help conserve the tiny organisms that catch rides with them.

Going with the flow

Most marine organisms don’t travel via turtle. Numerous plankton species and fish larvae drift freely, steered by ocean currents. Intriguingly, however, some species show up in the same places every year, even though they seem to be at the mercy of winds and tides.

Researchers at North Carolina State University and the University of California, Davis teamed up to study where marine larvae go and how they get there. NCSU’s Tom and Donna Wolcott designed floating robots that could swim vertically like marine larvae and record data on their surroundings, such as depth and water temperature. UC Davis researchers released the bots (nicknamed ABLEs) in California’s Bodega Bay to see how larvae programmed with different swimming patterns would behave.

They found that vertical positioning in currents – whether larvae floated at the surface or sank to deeper water – strongly affected where the larvae moved:

“Larvae of many species occur near the surface in offshore-flowing currents early in their development and are carried away from shore. Older larvae descend into deep shoreward-flowing currents and return onshore to habitats in which they can metamorphose and become adults. In effect, they use currents like conveyor belts to travel out and back across the continental shelf.”

Next, the scientists want to tease out connections between larvae’s vertical migration patterns and whether they travel far offshore or stay near the coast. Knowing how larvae behave could inform many ocean policies, from designing marine protected areas to tracking the spread of invasive species.