Crab claws

Sand fiddler crabs are a common sight on the beaches of the eastern U.S. The small creatures (typically about an inch wide, not including their legs) are distinguished by the unusual asymmetry exhibited by the claws of the males – one claw grows to almost grotesque proportions, longer than the body of the crab itself.

The males use their claws to defend their breeding burrows from other males, and as an advertisement to females looking for mates – they stand at the opening to their burrows, waving their giant claws in circles, trying to entice female crabs to join them.

In areas with a semidiurnal tide regime, sand fiddler crab mating tends to peak during spring tides, which occur when the moon is full or new – when the gravitational forces between the sun, the moon, and the earth line up to create higher high tides and lower low tides, and the ‘tidal flux’ (the difference between the high tide and the low tide) is especially large. The sand fiddler crab larvae that result from mating during a spring tide are released during the next spring tide, when that greater tidal flux can help the larvae survive.

A group of researchers working on the western coast of Florida recently investigated whether or not male sand fiddler crabs’ claw-pinching strength follows the lunar cycle to peak during spring tides, too.

Their results, published in the Journal of Experimental Marine Biology and Ecology, suggest that male sand fiddler crab strength and behavior does correspond to the lunar cycle. The scientists found that crabs who were courting females and defending burrows (which were located in relatively high and dry areas without much food nearby) had more force behind their claw-pinches than crabs roaming around in large groups, or droves, looking for something to eat, and that both the number of courting males and the males’ claw strength peaked during new and full moons.

Courting females and defending burrows appear to be energetically costly activities, and male crabs periodically took a break from mating activities to restore their strength by eating; as the scientists note, “claw power declined during courtship and increased while droving.” In general, the strongest male sand fiddler crabs were able to synchronize their eating-and-mating cycle with the lunar and tidal cycles, allowing them to stand outside their burrows waving their claws during the times when they were most likely to successfully mate.

A male sand fiddler crab will try to attract females to its breeding burrow by waving its large claw in circles above its body.

(Image by Rachid H via Flickr/Creative Commons license)

Turtle footage

In several of the video clips, a loggerhead sea turtle, shell studded with barnacles and matted with an undulating crop of green algae, is flanked by an entourage of fish, apparently eating the bounty of food growing on the turtle’s shell or perhaps using the large turtles for cover (adult loggerhead sea turtles are typically about three feet long).

In another clip, a turtle, evidently in response to the shadowy presence of a shark, flips over so its shell is facing the threat and swims away.

These and other natural loggerhead sea turtle behaviors were captured on video by a remotely operated vehicle, or ROV, deployed off the coast of New Jersey, Delaware, and Maryland, by a group of researchers from the Coonamessett Farm Foundation and the Woods Hole Laboratory of the Northeast Fisheries Science Center, both based in Massachusetts. They recently published their findings in the Journal of Experimental Marine Biology and Ecology – and, as the scientists note in their paper, the “study represents the first example of an ROV for tracking sea turtles.”

The scientists spotted the turtles from a boat, focusing on areas where loggerhead sea turtles have been active in the past; when they found one, they launched their ROV, tethered to the boat, and followed the turtle wherever it went. Over their 10 research trips (during which they recorded footage of 70 turtles), the researchers found that they could maneuver the ROV to within about three to five yards of the turtle without disturbing it.

From that vantage point, the scientists were able to observe a number of apparently natural behaviors which would have been difficult to capture by other means, such as human divers, cameras attached to turtle shells, and tracking tags implanted into turtles, all of which have been used to study sea turtles in the past. “[T]he ROV add[s] a new technique that complements existing technologies while overcoming several of the limitations,” the researchers write.

ROVs appear to be a new and useful tool for scientists attempting to understand how loggerhead sea turtles behave in their natural environment, and how they interact with each other and other animals.

Loggerhead sea turtles are typically about three feet long and weigh about 250 pounds.

(Image by Richard Ling via Flickr/Creative Commons license)

Sandy spawning

Fish live underwater, so it seems like it would be safe to assume that they always spawn underwater, too – but, as it turns out, there are exceptions to that rule.

California grunion are small, silvery fish, typically about five or six inches long when full-grown, that live along the Pacific coast between Punta Abreojos, Baja California Sur, Mexico and Point Conception, Calif. (they’re occasionally found as far north as Monterey, Calif.).

For a few nights during full moons and new moons (when high tides are particularly high), between March and September, sandy beaches along the coast can be overrun with the flipping and flopping of California grunion, starlight reflecting off their iridescent scales, as the females release their eggs under the cover and relative safety of wet sand and the males circle around to fertilize them. Their tasks complete, the adults ride the tide back underwater, and the eggs incubate under the sand for about ten days before they hatch and the larvae are washed out into the ocean to join their parents. (The California Department of Fish and Wildlife has a nice description of California grunion spawning behavior on their website.)

Female California grunion bury themselves in wet sand as they release their eggs; males curl their bodies around the females to fertilize the eggs with their milt. 

(Image by arne heijenga via Flickr)

One spring, a team of researchers from Stony Brook University visited some of the sandy beaches where California grunion reproduce to collect fertilized eggs. The researchers were interested in investigating another unusual feature of the small fish – sex differentiation doesn’t occur until they are about two months old, and it appears to be determined, at least partially, by the environmental conditions the fish experience rather than completely by their genetics.

Those scientists recently reported the results of their study in the Journal of Experimental Marine Biology and Ecology. They raised larval California grunion at three different temperatures and under two different light regimes – one approximating longer day-lengths, such as would be experienced by the larval fish at mid-summer, and one approximating the shorter days late in the spawning season.

The researchers found that the environmental conditions that would indicate to the larval fish that they had been born earlier in the spawning season – colder temperatures and longer day-lengths – led to a higher proportion of female fish, presumably because the females born early in the spawning season gain an advantage over those born later. With more time to grow, the early-born females can maximize their size, and, eventually, their ability to reproduce – bigger females produce more eggs. (There doesn’t appear to be a similar size advantage for male California grunion.)

California grunion rely on at least two environmental cues – day-length as well as temperature – that indicate seasonality. As the scientists write, California grunion “appears to be the first documented case of a vertebrate with [environmental sex determination] that is cued by both temperature and photoperiod. Photoperiod may provide [California grunion] with a more reliable cue than temperature alone, given the small seasonal changes in temperature along the North American Pacific coast.”

On those summer nights when adult California grunion are racing up onto sandy beaches to spawn, the females laying the most eggs – the biggest females – may have been some of the earliest-born fish in previous years.

California grunion spawn on sandy beaches during the three or four nights surrounding full and new moons, when high tides are highest. 

(Image by arne heijenga via Flickr)