additional Liocarcinus

additional Ensis leei

additional Diogenes sp.

Why there are so many species at the equator and so few at the poles?

Earth is teeming with strange life forms—crabs with 12-foot-long legs scuttling off the Japanese coast, mushrooms that glow at night in eastern North America, butterflies that drink the tears of Amazonian turtles. Among all of the world’s natural kingdoms, however, one rule reigns supreme: There are lots of different species at the tropics, but their numbers drop off sharply as you move toward the poles. “This holds true across virtually all kinds of life and in all kinds of environments, but the reasons why are still hotly contested,” said Prof. David Jablonski, a leading University of Chicago scientist of extinction and biodiversity. “This is a fundamental question that goes back before Darwin.” Jablonski’s goal is to understand biodiversity, and the stakes are big—including how species will adapt to climate change. Scientists like Jablonski—whose research on mollusks has shaped the field—have traditionally researched either a species’ form (the shape of an organism’s body) or its function (the way it makes a living). For example, a clam’s shell can be spiny or smooth, and it can make a living eating sunken logs off a coast or by filtering plankton in tidal flats. Each way of looking at the animal tells you something different about evolution, niches and the patterns of biodiversity; but each is so complex on its own that they’re rarely studied in unison. Jablonski believes integrating the two could yield important insights. Two recent studies from his lab take this approach, combining forces with other specialists to investigate the diversity shift from topics to poles in an innovative way. In the first study, they worked with bird biologist Prof. Trevor Price to compare Jablonski’s mollusk data with Price’s bird insights on how species across the world live in different environments. In tropical birds, some ways to make a living are packed with species—lots that eat insects on tree branches, for example—and others support just a few. This pattern persists midway into the latitudes, but then there’s a distinct tipping point and the number of ways to make a living, and the uneven distribution among them, drop off. The exact same is true for mollusks. “That result knocked our collective socks off,” said Jablonski, who is the William R. Kenan Jr. Distinguished Service Professor of Geophysical Sciences. “For animals, you don’t get much different than a bird and a bivalve, but you see this strikingly similar pattern,” said Stewart Edie, a postdoctoral researcher and the joint first author of both papers. “That usually suggests we’re looking at a higher-order control that’s operating on a large scale around the planet, both on land and sea.” The theory is that in the tropics, there’s more room for fine-scale specialization among species—not just birds that eat seeds, but birds that eat only one kind of seed, in one part of the environment (branches high in the forest canopy, for example). But with increasing latitude, the climate becomes more seasonal and more difficult to survive in, and it’s less viable to be a specialist. “A lot of the ways of life are still present; you just have to eat every kind of seed, or live anywhere in the canopy,” Jablonski said. That’s something new about how biodiversity works, he said, and it may have implications for how things will play out as climate change progresses. “For example, what’s going to happen to parasites that attack crops or plants we care about—species will arrive that can focus on specific hosts as it gets warmer,” he said. The second paper, headed by postdoctoral researcher Katie Collins, turned to technology to analyze how the forms of mollusks change from tropics to poles. They used a micro CT scanner to scan samples of 95 percent of all the species found in the Florida Keys and the Gulf of Maine, yielding a treasure trove of 3-D images. Most studies just work with shell length and height, “but that’s not the whole picture,” Collins said. “A third dimension adds a new layer of understanding.” “Now we can put hard numbers on shell form where we just had general impressions before,” added co-author Rüdiger Bieler, curator of invertebrate zoology at Chicago’s Field Museum and member of UChicago’s Committee on Evolutionary Biology. When you go to a beach in the Florida Keys, your eye is drawn to large, brightly colored shells with spines and ridges and knobs. But north of Cape Cod, you don’t find those. The shells are all smooth, plain and often small, which at first blush seems to indicate a massive shift in species form. That’s not what scientists found, though. “It turns out there are plain shells everywhere,” Collins said.“The tropical shells are actually a mix of plain and fancy species; the fancy shells just drop out of the mix as you go north. So the individual species aren’t transforming to adapt to new conditions; whole lineages of fancy things drop out.” “It looks like the high-latitude climate narrows down the range of viable shell forms, and some lineages are just frozen out, so to speak,” said Jablonski. This too has implications for humans as they continue to alter the climate and environment. “What this suggests is that most animals are more likely to drop out than to adapt to the tougher conditions at high latitude,” Collins said. “This could affect humans in a very real way. Bivalves serve a huge role in fisheries, and we don’t know if the ones we like to eat most, like oysters or scallops or mussels, are going to disappear, or move out of their accessible fishing grounds, as the climate changes around them.” This study was done on modern species; the next step will be to add fossils. (Scientists love mollusks because their shells are easily fossilized.) “For example, we want to know whether the fancy lineages turn over more rapidly over time are more extinction-prone or if they’re stable,” Collins said. “Taken together, these new studies show that the different dimensions of biodiversity don’t all change together on a global scale, and the mismatches tell us something new about the forces shaping life on earth,” said Jablonski. Provided by: University of Chicago More information: M. Schumm et al. Common latitudinal gradients in functional richness and functional evenness across marine and terrestrial systems. Proceedings of the Royal Society B: Biological Sciences (2019). DOI: 10.1098/rspb.2019.0745 K. S. Collins et al. Spatial filters of function and phylogeny determine morphological disparity with latitude. PLOS ONE (2019). DOI: 10.1371/journal.pone.0221490 Image: A Spondylus americanus (left) from the Florida Keys, a tropical species with long spines that act as defense against predation; and Arctica islandica, sometimes known as the Methuselah clam for its long life, which is found only in cold waters like the Gulf of Maine. Credit: Katie Collins Read the full article

Pagurus longicarpus long-wristed hermit crab

Langarm-Einsiedler

image from the texan ID guide to marine species website here

Up to 2.5cm. Right claw slender and extremely elongated, highly variable colouring. Typically tan with a hint of green, top of legs and hands often described as iridescent, lighter claw tips.

Introduced from the coast of North America, likely through ballast water, and first noted in the north sea in 2020 when it seemed to have already established a small population. Going off beachexplorer data, so far they look to be confined to the south eastern north sea, though their range is clearly expanding. They seem to be coexisting with P. bernhardus just fine, according to Neumann et al. (2022).

Comparison photo from the (otherwise lackluster) Schutzstation Wattenmeer pdf.

Only present in some parts of the wadden sea at time of writing (march 2023), and thus not on Juist. Only time will tell when or whether they make it here at all, though there is a credible report of one from Langeoog from August of 2022 (and two false ones from Juist which clearly show the red claw marking of a P. bernhardus).

Shoutout to Beachexplorer user Rainer Schulz with his reliable and frequent reports.

Diogenes curvimanus. south claw hermit crab

Diogenes Einsiedler

Up to ~2.5cm. The left claw is noticably larger than the other, yellowish or tan, often with reddish markings or a green tint, the underside lighter.

Recently common in most parts of the southern north sea; migrating in from the Atlantic with rising temperatures. Often found congregating in great numbers in tidepools in the wadden sea. Able to bury themselves very quickly when disturbed.

In 2022 a paper proposed that D. pugilator, previously regarded as one species with high local variation ranging all the way from the north sea down to Angola (incl. the mediterranian), should actually be split up based on their genetic analysis which lead to them resurrecting D. curvimanus and coining their own new name. While they suggest there are likely more species in this genus previously lumped together, it appears that North Sea Diogenes are now called D. curvimanus. Beachexplorer has moved their entire species profile for D. pugilator over, in any case.

The only left handed crab here, doesn't use anything larger than Littorina snail shells.

Pagurus bernhardus common hermit crab, soldier crab

Einsiedlerkrebs

Up to 8cm. The right claw is noticably larger than the other, older animals are covered in small prickles while young ones are smooth and light in colour. Reddish-orange with a characteristic stripe along the right claw.

Common in the entire north sea. Will bury themselves in the sand when disturbed, so watch your step!

Young individuals are predominantly found, older animals are rare on beaches. P. bernhardus are the only hermit crabs in the north sea large enough to live in whelk shells (Buccinum undatum). Dead crabs typically lack their softer abdomen.

The only hermitcrab here that gets this big.

Additional Liocarcinus

additional Chamelea striatula

Additional Portumnes latipes

Austrominius modestus modest barnacle, australasian barnacle, new zealand barnacle

Australische Seepocke

Up to 10mm in diameter. 4 symmetrical shell plates, the bottom margin of each with a very characteristic large indentation, the upper margin smooth.

A non native species first identified in british waters during ww2. Has been known to outcompete native barnacles in areas suited to it, sometimes aided by man-made structures (Bracewell et al. 2012).

In October of 2023 I saw one on Juist the very first time, stuck on the carapace of a shore crab.

Comparisons: Carcinus maenas / Liocarcinus sp.

C. maenas to the left, undetermined member of the genus Liocarcinus to the right. Note the differently shaped lobes between the eyes.

Comparisons: Portumnus latipes / Carcinus maenas

P. latipes to the left, C. maenas to the right.

Comparisons: Carcinus maenas / Liocarcinus sp. / Portumnus latipes

C. maenas to the left, P. latipes to the right, an unidentified member of the genus Liocarcinus in the middle.

I am by no means an expert on crabs, but even knowing that I am staggered by how much I struggle to find any identifying criteria to tell Liocarcinus species from each other. Maybe best to take a DNA analysis kit and hope there's some tissue left stuck to the carapace :/

Best approximation below.

Liocarcinus holsatus flying crab

Gemeine Schwimmkrabbe

Up to 4cm wide. Last pair of legs flattened into paddles, typically reddish brown.

3 Pointy lobes between the eyes; according to DORIS the bump in the middle is the longest. Though I can't say how plausible that claim is, I believe they get it from Ingle & Clark (1998). Can't confirm it without a sample of confirmed L. holsatus, of course.

Common in the entire north sea, typically found bleached and almost orange or salmon in colour.

Liocarcinus depurator harbour crab, sandy swimming crab

Blaufußkrabbe, Rauhe Schwimmkrabbe, Ruderkrabbe

Up to 5cm wide. Last pair of legs flattened into paddles, 5 teeth to either side of the face and 3 pointy lobes in between the eyes. Carapace often rougher than that of L. holsatus.

Common in the entire north sea, though according to the beachexplorer (and only the beachexplorer) they live too far out to be washed up on shore with any reasonable frequency. I have no reason to believe them, but I do wonder where they got it from.

There are many more of these little beasts. I've mainly seen these two mentioned in ID guides however, so let's just hope for the best. The ID criteria listed here are of course my best estimation of accurate - many of the available sources disagree.

The only thing I can say for sure is how to tell them apart from shore crabs (Carcinus maenas): the three lobes between the eyes that both have are noticeably pointier in Liocarcinus, and shore crabs have much rougher texture and more diverse patterns over all.

[Liocarcinus left, shore crab right]

My mother insists you can also tell them apart by imagining a line between the two lowest teeth and seeing how close it is to the top of that X marking they have, like this:

Use with caution, I don't know.

I'll be honest with you, I can't figure out how to reliably tell Semibalanus balanoides and Balanus crenatus apart. Let me know if you can, in the mean time I've pulled together from all sources I could find what seems most likely to be accurate to me:

Semibalanus balanoides common rock barnacle, common acorn barnacle

Gewöhnliche Seepocke

Up to 15mm in diameter. Made up of 6 asymmetrical shell plates in the centre with a smooth upper margin. Base of the shell membranous.

Oftentimes slightly rough looking with lightly crenulated bottom edges.

Balanus crenatus wrinkled barnacle

Kerbseepocke

Up to 20mm in diameter. Made up of 6 relatively smooth shell plates, the upper margin jagged. Calcerous base of the shell, closing plates with slight ribs.

The inside is allegedly striped. Similar to Amphibalanus improvisus which is common in the baltic sea.

Maybe easiest to tell apart when only the base of the shell is left!