Babies of the Benthos – Crab edition

Eyes Under Puget Sound – Critter of the Month

Winter has been hanging on for dear life this year (I mean – snow and hail in April??) and it’s left some of us feeling a little crabby. Never fear, Critter of the Month is here, with some crustacean babies that are sure to lighten your mood.

Tiny brown larval crab with large eyes, on a white background. Scale bar reads, 1 mm.

Your baby is a little...shellfish? Dad jokes aside, this crab megalopa larva IS actually pretty cute. Photo by Eric A. Lazo-Wasem, from yale.edu (CC0 1.0).

Parental guidance

Many invertebrates allow their young to fend for themselves as larvae in the water column once they are born, and our beloved Puget Sound crabs, including red rock and Dungeness, are no exception. To a human parent, this approach to child-rearing might sound callous, but these crabs are anything but claws-off. Behind the scenes, they devote a ton of energy to setting their kids up for success.

Crab romance

Two crabs facing, with their claws wrapped tightly around each other, sit on the ocean floor amidst brown seaweed.

A male Dungeness crab holds a female in a “crab hug.” Photo by Scott Groth, from ODFW.

The biggest gift that Dungeness crab parents give their broods is the timing of their birth (more on that later). Male Dungeness crabs start hunting for eligible ladies in the early spring, and will “embrace” their crushes for weeks waiting for them to shed their shells. All crabs undergo this molting process in order to grow, and only after molting, when the female’s shell is still soft, can they successfully mate. Once mating is complete, females store the sperm away until the eggs are ready to be laid and fertilized in the fall.

 

And baby makes three…million?!

A female Dungeness crab can lay an astounding 2.5 million eggs, bound together in a mass on the underside of her body with a special cement secreted by the hairs on her abdomen. Her babies will be tiny and relatively helpless, so large numbers ensure that some of them will survive.

Pink baby announcement graphic with a larval crab illustration in a diaper, holding a rattle. Text reads, “Let’s shell-ebrate baby zoea!"

In winter, millions of pointy-headed zoea larvae break free from the eggs and drift off into the water column to feed and grow. Their strategically-placed spines help them stay afloat but also make them too big for some small-mouthed predatory fish to swallow. Zoea go through five growth stages before leveling up to become megalopae in the late spring (about a year after mating took place).

Current events

The spring timing of the megalopae stage is no accident; the crab parents have timed their mating perfectly to coincide with oceanic current patterns. Just as their kids morph into the more robust and mobile megalopa form, offshore currents begin to push water towards the land, carrying the little larvae to nearshore areas. Some will settle and grow into juvenile crabs in the safety of these intertidal nurseries, while the not-so-lucky ones will become an important food source for larger animals like salmon and gray whales.

Photo collage of crab larval development on a black background, with white arrows separating a zoea, a megalopa, and a juvenile crab.

A zoea (left) goes through five stages until it becomes a megalopa (middle). The long spines are replaced by telltale claws, making it look more like a juvenile Dungeness crab (right).

In hot water

Two scientists bend over the side of a marina floating dock to examine a piece of equipment.

Scientists deploy a light trap at Zittel's marina. The trap will monitor crab larvae in the South Sound region between April and September. Photo courtesy of Pacific Shellfish Institute.

With so many other species in the ecosystem depending on these amazing babies and the grown-up crabs they will become, it’s concerning that Dungies are on the decline. Possible causes include fishing pressure and climate change impacts like ocean warming and acidification. Recently, a group of scientists joined together to study the abundance of Dungeness larvae in the water, and what they can tell us about the populations of crabs making it to adulthood in Puget Sound. Here’s hoping that what they learn might one day give these iconic crusties – and their adorable offspring – a leg (or ten) up in life.

 

Critter of the Month

Dany is a benthic taxonomist, a scientist who identifies and counts the sediment-dwelling organisms in our samples as part of our Marine Sediment Monitoring Program. We track the numbers and types of species we see to detect changes over time and understand the health of Puget Sound.

A scientist wearing safety gear on the back deck of a boat holds up a sea star in a gloved hand.

Dany shares her discoveries by bringing us a benthic Critter of the Month. These posts will give you a peek into the life of Puget Sound’s least-known inhabitants. We’ll share details on identification, habitat, life history, and the role each critter plays in the sediment community. Can't get enough benthos? See photos from our Eyes Under Puget Sound collection on Flickr.