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Cryptic caves and paleoecology of crustaceans in Cenozoic coral reefs

Just some months ago on a Saturday in July, I had the pleasure of snorkeling above the only coral reefs in the continental Unites States. These reefs in southern Florida still harbor many species of corals, fish, and other animals including crustaceans such as crabs, shrimps, and lobsters. These decapods are difficult to spot while snorkeling, but that does not mean they are not there. Their usually small size in this landscape of incredibly variable topography ensure they are able to hide effectively from predators. As for many other animals, coral reefs are a hotspot for decapod biodiversity. This was by no means different in the distant past. The rapid diversification of crabs and squat lobsters in sponge and shallow-water coral reefs during Late Jurassic is one of the best examples. When many reefs vanished in the earliest Cretaceous so did many of these crustaceans, highlighting the need to protect corals and, in doing so, also the associated, often cryptic animals.

One example of these cryptic animals are crabs from the Cryptochiridae family. Today, over 50 species are known of these tiny animals that have a carapace of less than a centimeter long. They do not hide in the rubble or between coral branches, but they create their own homes within the corals. Their home is either a true gall or a tunnel that is either circular/oval or crescentic in cross-section. Despite their high biodiversity, no convincing cryptochirid fossils were known until very recently.

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The modern cryptochirid crab Troglocarcinus corallicola sitting snugly in a crescentic home in the coral Manicina areolata. Scale bar width: 50 mm for a, 5.0 mm for b. Source: Klompmaker, Portell & Van der Meij, 2016, Scientific Reports

Earlier this year, an open access article together with Roger Portell and Sancia van der Meij was published showing superbly preserved crescentic-shaped holes in Plio- and Pleistocene corals from Florida and Cuba. No animals other than cryptochirids create such holes so the culprit of this trace fossil was easy to identify. Unfortunately, no crabs were found inside the holes because these relatively soft and tiny crabs do not preserve well. Such crescentic holes should be present in more fossil corals all over the world. Why? Cryptic crabs that make such holes are found in corals in nearly all (sub)tropical regions of the world today. Additional evidence would help tremendously in constraining the antiquity of this family and with getting a better sense about their past biodiversity. So check out your fossil corals at home or in a museum nearby! Some places in the world expose fossil coral reefs as a good third alternative.

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Pleistocene corals from Florida: Solenastrea bournoni (a, b) and Solenastrea hyades (c˗e) with close-ups of crescentic cryptochirid holes. Photo d shows the holotype of the trace fossil named after this particular shape: Galacticus duerri. The genus name is derived from Battlestar Galactica because of the similar cross-sectional shape of this battleship to these crescentic holes. Scale bar width = 50 mm for complete corals; 10 mm for close-ups. Source: Klompmaker, Portell & Van der Meij, 2016, Scientific Reports

That's exactly what I did in the summer of 2014, but for different reasons. I was lucky to receive funds from the Paleontological Society (Arthur James Boucot Research Grant) and a COCARDE Workshop Grant (European Science Foundation) to travel to Denmark to a very special fossil coral reef in the famous Faxe Quarry. This quarry is accessible to everybody and it certainly is a great place to visit when you are in Denmark as is the Geomuseum Faxe right next to it! My Danish colleagues Bodil Lauridsen and Sten Jakobsen helped to find the right places for collecting. The exposed coral and bryozoan mounds were living at 200-400 m depth in dim light conditions in the earliest Cenozoic (~63 million years ago). Such deep-water coral reefs can still be found all over the world up to depths of 1000+ meters by the way.


The Faxe Quarry at dusk after a long field day.


Author (right) with colleague Sten Jakobsen (left)

This complex reef at Faxe also contains decapods, primarily crabs and squat lobsters. After more than a century of collecting, as many as 25 species are known. That’s a lot right? However, well-sampled, shallow-water fossil coral reefs from elsewhere in Europe contain even more decapods.  The Cretaceous-Paleogene extinction event that wiped out the non-avian dinosaurs, ammonites, and severely affected many other groups has apparently nothing to do with the lower decapod diversity at Faxe. Our analyses show that decapod diversity is not affected by this event. Instead, less food and perhaps fewer hiding places have contributed to this lower diversity. A comparatively low decapod diversity is also seen in today’s deep-water coral reefs.

These critters may differ also in body and eye size compared to their shallow-water friends in corals reefs. The crabs at Faxe tend to be larger for half of the analyses, whereas other results show no difference. Some ideas about the reasons include a lower number of predators, a delayed maturity, and an increased life span of these crustaceans in deeper, colder waters. Quite spectacular evidence was found when we compared the eye socket size (true eyes are not preserved) for crabs of the same size and genus from Faxe to those from a shallow-water reef. While initial results did not seem to show much, a closer look at the data and additional measurements did show a distinct difference. The eye sockets of the crabs at Faxe are larger than those from a shallow-water reef! Thus, these crabs evolved larger eyes to see better in the dim light conditions in Faxe ~63 million years ago.


Leftover rocks from a number of days of field work at one of the sites in the Faxe Quarry.

Some crabs can be readily seen in the wall of the quarry. Here an example of a partially exposed carapace of Dromiopsis rugosus.

Some crabs can be readily seen in the wall of the quarry. Here an example of a partially exposed carapace of Dromiopsis rugosus.


Carapaces of crabs and some squat lobsters (c, d) from the Faxe Quarry in Denmark and some crabs from Spain (g, h). a. Dromiopsis rugosus; b, Dromiopsis elegans; c, Protomunida munidoides; d, Galathea strigifera; e & f, Caloxanthus ornatus; g & h, Caloxanthus paraornatus. The eye socket height of many specimens of the two species of Caloxanthus was compared. Scale bar width: 5.0 mm for a & b; 2.0 mm for rest. Source: Klompmaker, Jakobsen & Lauridsen, 2016, BMC Evolutionary Biology (open access)

The incredible biodiversity of fossil decapod crustaceans with ~3500 known species, many of them known from reefs, still results in the description of tens of new taxa each year by professionals and avocational paleontologists, often during collaborative efforts. With such data becoming more and more available, studies on diversity and paleoecology have become more common in recent years. The collection of the UCMP also does hold many, yet to be studied fossil decapods. Research on this exciting group of crustaceans continues!