#pancrustacea

[herpsandbirds]

BUGS IS SHRIMPS.

[have-you-seen-this-animal]

Picasso bug?

Have you seen the Picasso bug (Sphaerocoris annulus)?

I have now

Yes, in photos/videos

Yes, irl

I'm not sure

Voting ended onJun 3

[brickytheprocrastinator]

[sina-man]

Teethy manyhorn crab (male)

Small crustacean found in reefs. Males have huge horns on their head that they use to fight with other males for the right to mate.

Blucaris

Deep in waters of southern hemisphere those derived ostracods hunt small arthropods with their long claws. Other ostracods are especially common on their menu.

Goat flea

Adults of this species are skittish grazers of seagrass and algae while juveniles live as parasites of huge marine reptiles.

[animalids]

Cup moth caterpillar (Limacodidae)

Photo by itchydogimages

[alphynix]

Cambrian Explosion #60: Crustacea – Larvae Larvae Everywhere

One of the characteristic features of the crustacean lineage are their larval forms, passing through various tiny larval stages. They often look nothing like their eventual adult forms and historically weren't even recognized as being the same species, with their complex lifecycles not being properly recognized until the late 1800s.

A lot of Cambrian crustaceans are only known from their larvae, preserved in exquisite microscopic detail in sites of "Orsten-type preservation". Only disarticulated fragments of larger-bodied forms have been found in a few places, and it isn't until much later in the Paleozoic that fossil crustaceans actually seem to become abundant in marine ecosystems.

It's not clear why there's such a bias in their early fossil record compared to most other arthropods, but possibly they were just very very rare animals early on. Adult forms may have mostly lived in places where they just didn't fossilize, while their tiny larvae sometimes dispersed into different environments with a better chance of preservation.

One of the earliest known of these crustacean larvae, and one of the oldest known true crustaceans, is Wujicaris muelleri.

Found in the Chinese Chengjiang fossil deposits (~518 million years ago), this microscopic larvae was just 270μm long (0.01"). It had a wide shallow head shield with a backwards-pointing spine, a pair of eyes, and another long spine projecting from its underside at the front of its body.

Like similarly-shaped modern larvae it probably lived on or just above the seafloor as meiofauna, using its developed head appendages for both locomotion and catching food particles.

For such an early example of a crustacean it's surprisingly similar to modern forms, resembling the "metanauplius" stages of some copepods and barnacles. Along with Yicaris, another larva from the same deposits, it was a probably a basal member of the major crustacean lineage that both those groups are part of: the altocrustaceans.

It suggests that some groups of crustaceans established their specific larval forms very early on in their evolution, and hit on a something that worked so well for them that they've barely needed to change it in over half a billion years.

———

The pentastomids, or "tongue worms", are a very unusual group. Small worm-like animals that almost exclusive parasitise the respiratory tracts of vertebrate hosts, outwardly they don't even look like they're arthropods – but their true affinities are revealed by their chitinous "skin" and arthropod-like nervous system.

Their evolutionary affinities have been controversial, but they're now generally considered to be highly specialized and modified crustaceans and very closely related to the parasitic fish lice. An alternate hypothesis proposes them instead as being surviving early panarthropods, related to tardigrades or lobopodians – but this is based purely on morphology, while the crustacean placement is also supported by genetic evidence.

While their fossil record is very poor, there are several different larvae known from the late Cambrian, including Heymonsicambria scandica.

Found in the Swedish Orsten Lagerstätte (~497 million years ago), this larva was about 0.5mm long (0.02"). Like other tongue worms it had four limbs on its head ending in hooked grasping claws, but unlike modern forms it also had two pairs of vestigial legs further down on its body.

It's unclear if these very early pentastomids were actually parasitic, or what their hosts would have been at the time if they were. Conodonts have been proposed as potential hosts, but they also may have initially externally parasitized other types of arthropods – a Silurian-aged fossil shows an ancient tongue worm "caught in the act" of clinging on to an ostracod.

———

Nix Illustration | Tumblr | Twitter | Patreon

[underthehedge]

Ok time to learn about Pancrustacea

Right so I've explained on that "don't eat cicadas if you're allergic to seafood" post a bit but I love a weird factoid so: Insects are actually a weird subgroup of crustaceans.

"But how?!" I hear you cry, "what forbidden knowledge is this? surely it cannot be true!" I imagine you say because I've decided you're all incredibly melodramatic about Arthropod taxonomy like I am. First things first, let's start with some key players.

Arthropoda - a group of animals with exoskeletons and jointed (arthro-) legs (poda), your bugs your crabs your spiders your millipedes etc. Nature's crunchy bois.

Hexapoda - insects, springtails and a couple of other small arthropods, all notable for having six (hexa-) legs (poda). Flies, cockroaches, ants, moths, get stickbugged etc.

Crustacea - a massive group of arthropods that includes crabs, lobsters, water fleas, copepods, woodlice, brine shrimp, seed shrimp, scuds, mantis shrimp, barnacles and also a bunch of secret ones you've likely never heard of. Nearly all of them live in the water.

Chelicerata - not so relevant here but that's your spiders, your scorpions, your ticks, your mites, also now found to apparently includes horseshoe crabs which is a bit of a mind fuck.

Myriapoda - Centipedes and millipedes and a couple of their less famous cousins, name means "ten thousand feet" which is a bit of an overstatement but these lads sure do have a lot of legs.

Ok so if you don't know how Cladistics works, well you can google that but briefly, taxonomists organise groups of animals by descent: who's closest related to who. Groups can be monophyletic, polyphyletic or paraphyletic, but the only valid taxonomic units are monophyletic ones. I'll put a short explanation under the cut, with pictures and alt text.

Aaaanyway, so there are many different groups in the Arthropoda and their relationships to each other is A Whole Thing, taxonomists have been puzzling and fighting over it for centuries at this point. Traditionally, morphological analyses have come up with various possible configurations for the trees, like grouping Hexapods with Myriapods into Tracheata because they both breath through tracheae as opposed to Crustaceans which breath through gills. Alternatively, based on some shared features of the simple eyes it was the Crustacea and Hexapoda that were in fact sister groups, which they called Pancrustacea; there were good arguments for both. There have been a lot of competing theories but genetic data is much easier to obtain now and has confirmed and refuted many of the well reasoned theories based on morphology alone, which is great.

It's also sometimes throwing a rather exciting hand grenade into the fruit salad of existing theories, as with the Pancrustacea hypothesis. In the 2000s people started to work on that, collecting genetic data for key genes and comparing them across groups, building trees and finding that...wait what? Pancrustacea was right, but also, it wasn't. Pancrustacea clearly formed a monophyletic clade but uh, not quite how people thought it would.

Genetic data showed that Hexapoda are definitely not the sister group to Myriapoda, but they were also not a sister group to Crustacea, because when you work out the trees they're sat firmly within the Crustacea as a sister group to either the Remipedia, Branchiopoda or Cephalocarida.

Remipedes are weird centipede looking swimming crustaceans that are found in coastal cave systems, they're small, blind and kinda elegant if you go look up a video.

Cephalocarida are weird tiny looking buggers that live buried in mud, grow no more than 4mm long and generally look a bit like a worm. Some phylogenies put these with the others and other's say they go with the crabs instead. Idk.

And then there's the Branchiopoda, a large group that you're probably familiar with if you've ever raised sea monkeys, triops, fairy shrimp etc. They live in a number of habitats but almost all prefer temporary pools and lay special eggs that can survive desiccation.

These are all undeniably Crustaceans but...they're all apparently closer related to Hexapods than they are to crabs and lobsters for instance. The exact relationship is kinda fucky to work out, even with genetic data but it's kinda like people looked at humans and great apes and said "ah, we are similar to them, they are a group and we are the sister group to that" except eventually it turned out that, despite superficial similarities between the chimps(and bonobos) and gorillas, actually, humans and chimps are closer related so we're not cousins to the great apes, we are literally great apes.

So either Hexapods are crustaceans or we have to evict like, half of all Crustacean classes from Crustacea.

Ok so in this imaginary tree, A, B, C and D form a monophyletic group all descended from (1). A and B also form a smaller monophyletic group descended from (2), and likewise for C and D from (3). Shit nests like Russian dolls.

If you decided to group A, B and D together though, that would be a paraphyletic group as it doesn't include all descendants of (1), excluding C for no good reason. This has happened a lot in the past before new information has come to light, it is not a valid taxonomic unit but the names are sometimes preserved out of habit, e.g. most people don't include birds when they say "reptile", but birds are totally in that group.

If you decided to make a group from B and D because you think they look kinda similar that's a polyphyletic group, like grouping birds and mammals because they're both warm blooded. This is not a valid taxonomic unit at all.

[ikimonogaku]

My classmate posted this pic of a huge moth and idk but I think I may have just used this meme in the most relevant context ever?