Remoras could be classified as freeloaders, having evolved sucker discs on the top of their heads, which they use to attach themselves to their hosts. They hitch a ride on sharks or other large marine animals (rays, whales, turtles), even though they are fine swimmers on their own. They also eat the leftovers or possibly the feces of their host animal. Living on a large animal also protects remoras from predators.

Remoras cause no damage to their shark host, who don’t get much back from remoras, unless sharks find amusement in their oddly upside-down disc-shaped heads. Remarkably, these suckers evolved over time from the fishes’ dorsal fins.

Two recent studies, looking at a fossil remora and remora larvae, have determined how these fins develop over time into a strong sucking device.

The first study, led by Oxford University’s Matt Friedman, examined a 30 million year-old early remora fossil with a fully functioning sucker on its back.

“The remora sucker is a truly amazing anatomical specialization but, strange as it may seem, it evolved from a spiny fin,” Friedman says. “In this fossil the fin is clearly modified as a disc but is found on the back of the fish. It enables us to say that first fin spines on the back broadened to form wide segments of a suction disc. After the disc evolved, it migrated to the skull, and it was there that individual segments became divided in two, the number of segments increased, and a row of spines were developed on the back of individual segments.”

The second study looked at the development of remora from the earliest larval stages and compared it to the larval development of white perch, fish that have typical dorsal fins.

The research team found that up to a certain stage in the fish’s development, the dorsal fin develops in the same way and looks very similar in both fishes. Then, through a series of small changes, the remora’s dorsal fin begins to expand and shift toward the head. By the time the remora has reached about 30 millimeters (1.18 inches) in length, the dorsal fin has become a fully-formed two-millimeter sucking disc. It still has the components found in the dorsal fin—the tiny fin spines, spine bases and supporting bones—but the spine bases have greatly expanded.

The study confirms that the specialized sucking disc is formed by a massive expansion of the dorsal fin through small changes while the fish is developing. This completely new structure is homologous to other fish with dorsal fins and is not an evolutionary offshoot.

Fins turning into suckers? It sounds like a super-power! Well done, remoras.

Image: Dave Johnson, Smithsonian Institution

Oysters, like many bivalves, are important for marine ecosystems. The organisms filter water through their feathered gills, removing impurities as they inhale and exhale. In fact, native and invasive bivalves might filter the entire volume of the San Francisco Bay every 3-4 days!

However, oysters around the world are threatened by ocean acidification. The acidity breaks down the calcium carbonate shells of the oysters, as we reported in a video several months ago.

Recently, researchers discovered other effects of acidification on oysters and what the breakdown of the oysters’ calcium carbonate shells could mean for the acidic balance. Science Today sat down with the Academy’s own oyster expert, Dr. Peter Roopnarine, curator and chair of Invertebrate Zoology and Geology, to get some perspective on these recent studies.

In the first study, published earlier this month, scientists reported that acidification has negative effects for oysters in the larval stage. The acidity in the water makes the larvae expend much more energy than in neutral waters to make their shells.

“As the oyster larvae struggle early on and expend that embryonic energy,” Roopnarine says, “they have difficulty cranking up their own feeding.”

According to the paper’s lead author, George Waldbusser, “It becomes a death race of sorts. Can the oyster build its shell quickly enough to allow its feeding mechanisms to develop before it runs out of energy from the egg? They must build their first shell quickly on a limited amount of energy—and along with the shell comes the organ to capture external food more effectively.”

Last month, headlines reported that “Oyster Shells are an Antacid to the Oceans,” based on a study of oyster reefs in Chesapeake Bay. Roopnarine explains how oyster reefs are built over time, “Oysters do best on hard ground. The first oysters in a soft bottom environment eventually become the hard substrate that future oysters build upon. As the reef grows, the presence of the shells promotes a healthy, low acidic environment.” Or as the study’s introduction states, “Active and dense populations of filter-feeding bivalves couple production of organic-rich waste with precipitation of calcium carbonate minerals, creating conditions favorable for alkalinity regeneration.”

On a micro-scale, like the Chesapeake Bay, Roopnarine agrees that this could work. Restoration of oyster reefs could contribute to the reduction of ocean acidification problems. On a macro-scale, over geological time and large ocean mass, however, it seems that these oyster reefs could do little to undo the large amounts of CO₂ humans have been pumping into air (that’s absorbed by the oceans) for over a hundred years.

I asked Roopnarine about the San Francisco Bay’s oyster population. We had native oysters before overharvesting, pollution and sedimentation from gold mining in the Sierras buried the oyster reefs, Roopnarine says. A few are still found around the bay, but their numbers are small.

The oysters farmed locally are Japanese oysters, which, until a few years ago, were only found in hatcheries. Wild populations are now establishing themselves in the bay, Roopnarine says, which could be due to warmer temperatures. He and colleagues wrote a study a few years ago that looks at the Japanese oyster population locally.

With the important work these marine organisms do, it’s important we learn more about them to restore oyster reefs.

A former Academy staff-member, Jill Bible, is doing just this near Bodega Bay. To learn more watch this great video by the UC Communications team.

Image: Oysters showing the effects of ocean acidification, OSU

According to the textbook, An Introduction to Marine Ecology, “A typical coral colony forms several thousand larvae per year to overcome the odds against formation of a new colony.” The larvae have to act quickly to find a safe place to land and establish a colony or they will die.

Recently, Dutch scientists discovered that, like baby reef fish, coral larvae use sound as a cue to find those safe places.

According to their abstract in PLoS One:

Free-swimming larvae of tropical corals go through a critical life-phase when they return from the open ocean to select a suitable settlement substrate…. Here, we show that coral larvae respond to acoustic cues that may facilitate detection of habitat from large distances and from upcurrent of preferred settlement locations.

The team designed a “choice chamber” (a device that  offers small test subjects two or more contrasting conditions and allows them to move freely towards the one they prefer), put coral larvae into it and played them recordings of a coral reef. The results clearly showed that the flea-sized larvae were strongly attracted to the noise.  This presumably influenced what they then perceived as a suitable habitat within the chamber.

How these creatures detect sound is unknown, but Dr. Steve Simpson, one of the authors of the study, offers, “At close range sound stirs up water molecules, and this could waggle tiny hair cells on the surface of the larvae, providing vital directional information for baby corals.”

Understanding how these vulnerable animals complete their lifecycle is essential to ensure appropriate management. Coral reefs around the world are already under threat from various conditions like global warming and ocean acidification. Now you can add noise pollution to the list.

Dr. Simpson states that, “Anthropogenic noise has increased dramatically in recent years, with small boats, shipping, drilling, pile driving and seismic testing now sometimes drowning out the natural sounds of fish and snapping shrimps.”

With this study (and according to it), “The alleviation of noise pollution in the marine environment may gain further urgency.” Here’s hoping we listen up…