Trilobites: Cockroaches of the Paleozoic Seas

Trilobites were the sea cockroaches of the paleoworld. They were marine arthropods, highly adapted to multiple types of ocean ecosystems and diets that included all sorts things from the ocean bottom. This versatility and diversity helped them dominated the oceans for over 320 million years. Yes! That’s millions of years! To put in context, dinosaurs ruled the world for nearly 200 million years, whereas us humans, as a recognizable biological species, have existed for only a mere 300 thousand years. That’s less than ten percent the time trilobites existed on the planet.

Asaphus, a trilobite genus common in the Ordovician

Trilobites belong to the animal phylum Arthropoda. Arthropod means jointed legs, one of the characteristics that define the group of segmented, jointed-leggedness, hard-bodied animals that include the insects, spiders, scorpions, crabs, and lobsters. 

 

Horseshoe crab Limulus polyphemus from the Gulf of Mexico.
Note the segmented body, carapace, and jointed legs.
These are characteristics of the Arthropods.

 

Trilobites appeared on the Earth’s fossil record since the early Cambrian, around 540 million years ago, and went extinct during the massive extinction event that took place in the Permian-Triassic, around 250 million years ago. Trilobites had suffered previous minor extinctions before. The first occurring soon after their origination, during the late Cambrian, around 485 million years ago. 

 

Yet, their acme or time of greatest diversity and distribution, occurred during the Ordovician, and lasted for millions of years until the Devonian period, when fish-like predators began to keep them in check. Their large eyes, capable of all-around vision, and hard segmented bodies (carapace) likely arose out of the necessity to look out and scape from such predators. Their interesting and unique eye characteristics were reminiscent of frogs. For that reason, scientists gave them names such as Phacobs rana, or “frog-like eyes”, best known from fossils commonly found in northeastern North America. 

 

Phacops rana: trilobite species common during the north American Silurian-Devonian.
Note its segmented, frog-like eyes with minute lenses.

Trilobites inhabited shallow, shelf, marine environments. By having eyes and eyespots on the top of their heads, trilobites could watch the world above their backs. The hard, segmented body, allowed for multiple forms of locomotion, swimming, and flexibility. Some species were more swimmers, others more bottom crawlers. Their tracks are known from rocks made from ocean bottom muds. They fed mostly on anything, from small organisms to organic matter and sea bottom detritus. This included dead organisms that fell down to the ocean floor. This made them, sort of, ocean bottom vultures or cockroaches. That goes besides their obvious physical similarity.

An analogous species today would be the horseshoe crab or lobsters.

 

Horseshoe crab Limulus polyphemus from the Gulf of Mexico.

 

The living cave floor: intro to cave faunas

E. A. Martell, one of the first professional cave explorers, expressed in 1894 that "caverns and abysses are natural laboratories ready for numerous and curious researchers".

Those who visit caves frequently notice that some caves are alive! Walk into a cave room and with a swipe of the lantern you see the cave floor move! - Seems like something out of a horror film, but caves are truly alive.

Cave roaches of the family Polyphagidae, scavengers of the guano cave floors. This is an example of a guanophile troglobites. 

Caves are alive because they are teeming with life. From the microscopic viruses and bacteria to the much more complex bats, and even humans, caves provide a subterranean environment that is both beautiful and mysterious, resulting in the specialization of extraordinary organisms.

American roach Periplaneta americana (Blattidae) another common cave scavenger. 

The subterranean world is a complex environment. In caves, light enters unevenly. The deepest rooms, those farthest away from the entrance are the darkest if no other source of light exists. But even in such absence of light, organisms proliferate and evolve. These cave organisms can be defined or grouped depending on their cave habitat and preferences. Each has its own niche.

Troglobites are organisms that are strict cave dwellers, and complete their life cycles deep inside caves. Troglophiles are occasional cave dwellers. They do not complete their life cycle inside the cave, but they could if needed or preferred. They can be facultative cave inhabitants.

Trogloxenes, on the other hand, are similar to troglophiles in that they too are facultative cave dwellers, but their life cycles always require they leave the cave, surfacing to eat or reproduce. In this case, most organisms that people associate with cave dwelling are readily trogloxenes. Bats, birds like oil birds, owls, swallows, some reptiles, amphibians, and several species of insects, are all trogloxenes.

Cavernicolan glowworm in a cave in the Dominican Republic. Photographs courtesy of Adrian Tejedor. This is the larval state of a trogloxene arthropod. It is bioluminescent and truly interesting to observe. 

Other cave organisms are strictly aquatic cave dwellers. These are called stygobites, for they can only live in cave ponds or lakes. The blind fish and arthropods that live in vadose water lakes found within deep parts of a cave system represent this group. These organisms are very specialized. They have adapted to the dark aquatic environments and have lost their eyesights, color pigmentation, or eyes altogether.

There are organisms that can be considered accidentals because they enter caves by falling in, or because they are momentarily attracted by food or shelter inside. These include several mammals such as rodents and carnivores, birds, reptiles, amphibians, and many arthropods. Troglodyte is a term reserved for humans who use caves as a home or shelter. Humans have been using caves since the paleolithic, nearly a million years ago!

A carcass of a Cuban fruit-eating bat Artibeus jamaicensis eaten by
cave roaches of the family Polyphagidae: scavengers or the cave floor.
Cueva de Los Nesofontes, Mayabeque, Cuba 2003.

Caves not only provide secluded microclimates but microhabitats that are richly ecologically stratified and interconnected. An example is illustrated by numerous species of different bacteria and arthropods, that as an adaptation, live off the bat droppings (or guano) deposited in cave floors. These are known as guanophiles and are often troglobitic arthropods.

Plants growing from seeds brought in the excrement
of fruit bats A. jamaicensis. The Dominican Republic, 2004.

Trogloxenes often serve as the food of troglobites. Many are scavengers that feed on decomposing matter brought in by other organisms, such as seeds brought in by fruit bats, or the dying bats themselves, as is the case of the Cuban fruit bat Artibeus jamaicensis carcass devoured by cave-floor roaches in a cave in Cuba shown above.
Many of these organisms are agents of bone accumulations and dispersion within caves, a micro-field within taphonomy that I am most interested in. See my previous ruminations on this previous post.

Decomposed and guano-buried funnel-eared bat Natalus major eaten by small cave-floor ticks genus Pantricola (?). Los Haitises, the Dominican Republic, 2004.

A wide variety of organisms has evolved in caves. These are characterized by intricate specializations such as staged reproduction cycles, feeding mechanisms, or sensitivities to light that allows them to exist only in the intricate microclimates-ecosystems found in the subterranean world. Such diversity and these special characteristics have been naturally selected over millennia depending on the distribution of these organisms and the niches they inhabit withing the geography of a cave.

Cave lake in the interior of Feather's Cave, Matanzas, Cuba.This lake used to have blind fish of the genus Lucifuga. 

Most of troglobites and trogloxenes are relics of the past. Some of the aquatic stygofauna are descendants of freshwater, intertidal, or marine organisms that became trapped in a cave pools and lakes, becoming isolated there. Most of them evolved from exterior faunas that adapted to the cave conditions after isolation.

Cuban boa Chilabrothus angulifer on the hunt for fruit bats. Nesofonte's Cave, Mayabeque, Cuba. This species is a trogloxene that uses caves as a refuge, for feeding and reproduction. These reptiles have adapted to hang from the walls and catch flying bats. 

Exterior faunas enter caves with inflowing rivers, floodwaters, and groundwaters that invade, actively or passively, cave environments. Others become trapped or fall in accidentally as mentioned before. More so, streams and rivers can carry inside sediment and plant debris often carrying organisms from the exterior into the cave. These organisms then become underground colonizers, evolving in isolation from their exterior populations. The cave itself being the barrier to their gene flow.

Cave gours or pools from Bellamar's Cave, in Matanzas. Some of the green and red films that can be seen represent cyanobacteria that live off the chemicals of the rocks and the dim light that is artificially provided by the tour lamps (they are photosynthetic, thus needing light). 

But caves are sensitive environments and even the most minute changes can alter their microhabitats. Alterations to these delicate subterranean environments include hoards of people using caves as a tourist attraction, or "collecting" their endemic faunas or crystal formations, often downgrading their natural beauty. Or affecting their natural flora and fauna, thus affecting the natural cycles that occur within caves. Cave microenvironments are also sensitive to overall climate change, and rising sea levels. The former have the potential to drown existing habitats, and thus pushing many cave endemics to extinction or habitat fragmentation.

The isolation of subterranean environments makes cave faunas interesting subjects to the study of evolutionary pathways, genetics, biogeography, and even virology (-the Ebola and Zaire viruses seem to have evolved in African cave environments-). Some biologists consider caves even as islands, isolated natural laboratories for evolution and natural selection. 

I think Martell was right. And like him, and many other generations of cave scientists, also known as speleologists, we continue to be attracted to caves and their intricacies. We study and divulge the beauty and complexity of caves and cave systems with the hope of understanding such environments and ecosystems better, hopefully helping us protect and understand their faunas more effectively. 

I hope this post has been informative of other interesting aspects of caves since I have already covered bats, hot caves, and cave formation in my previous posts. This post is in honor of those who dedicate their time, and often health, to study caves and unraveling the mysteries of the subterranean world.

Stay tuned for more future ruminations on caves and cave faunas!