Discover Top Posts Tagged with #hemichordata

Glossobalanus spp.

A genus of acorn worms found in oceans worldwide.

image by Philippe Bourjon

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Round 1 - Phylum Hemichordata

(Sources - 1, 2, 3, 4)

Hemichordata is a phylum of bilaterally symmetrical marine deuterostome animals. They include two main classes: Enteropneusta (Acorn Worms) and Pterobranchia.

Acorn Worms are detritus or filter-feeders. They live solitary lives on the seabed. They have an acorn-shapes proboscis. Behind their proboscis they have a short, fleshy collar that is sometimes colorful, which hides their mouth. Those that feed on detritus crawl or burrow very slowly, taking in large amounts of sand or mud through their mouth, processing the organic edible bits within their guts and pooping out long tubes of filtered sediment (casts). They live in U-shaped burrows. Those that filter-feed stick their heads out of their burrows and draw organic particles into their mouth using the cilia on the gill bars. They have gills and breathe in much the same way as fish, filtering oxygen from the water. They have a heart that also functions as a kidney. They have no eyes, ears, or other sense organs other than for taste, though there are numerous nerve endings throughout their skin. They have a Y-shaped nuchal skeleton.

Meanwhile, Pterobranchs are all filter-feeders, and they live within tubes that they secrete. They are colonial, with several zooids living together in a cluster of tubes, connected by stolons. The collars of pterobranchs have one to nine pairs of large arms, each of which includes a row of tentacles along one side. The tentacles are covered in cilia and aid in filtering food from the water.

Hemichordates have 2 seperate biological sexes, though some species reproduce by budding. Females lay a large number of eggs embedded in a gelatinous mass of mucus, or discharge them into the water column. Males discharge sperm which externally fertilize the eggs before they drift away. Their larvae are free-living plankton, some of which do not feed and instead drift until they hit the sea floor and metamorphize into their adult form. Others skip the larval stage, instead developing straight from egg to juvenile. Hemichordates arose in the Lower or Middle Cambrian.

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Voting ended onSep 23, 2024

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Phylum Round 1

🦠 XENACOELOMORPHA vs. HEMICHORDATA 🪱

Xenacoelomorpha

Hemichordata

Voting ended onJan 28, 2024

Xenacoelomorpha: Small, flat, worm-like animals that live in marine and brackish sediments, and sometimes around hydrothermal vents. They can be free-living, symbiotic, or parasitic. A unique feature is that their stomachs lack nerve cells, unlike many other animals. Their evolutionary relationship to the rest of Animalia has been highly debated, and very little is known about their lifestyles.

Hemichordata: Acorn Worms. These marine burrowing worms are both deposit feeders (eating sediment) and occasionally filter-feeders or detritivores. The acorn-like proboscis on their head helps them with burrowing and may help funnel sediment to the mouth, which is between the "acorn" and the "collar". Due to their unexpected relation to Echinodermata, these animals are used as a comparison to study the evolution of early Chordate zygote growth.

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Cambrian Explosion Month #08: Phylum Hemichordata – Pterobranchia

Now we get to the bilaterians, all the animals with bilateral symmetry as an ancestral feature.

The hypothetical common ancestor of all bilaterians (the "urbilateria") was probably a tiny worm-like species, and likely originated sometime in the early Ediacaran Period. The earliest definite body fossils of bilaterians come from about 558 million years ago, and possible burrow traces are a little older, from about 585 million years ago – but it was during the Cambrian Explosion that this group rapidly diverged into a wide variety of forms and ecological niches.

The deuterostomes are one of the two major evolutionary branches of the bilaterians, made up of modern hemichordates, echinoderms, and chordates (including the vertebrates). They split from their last common ancestor with the protostomes sometime during the Ediacaran, but the earliest probable deuterostome fossils are weird tiny potato-like blobs from the very start of the Cambrian.

(…We'll talk about those later in the month.)

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Hemichordates are the closest living relatives to echinoderms, and are a small phylum with only around 130 known modern species (most of which are acorn worms). But in the distant past they were much more numerous, with graptolites being major components of Paleozoic planktonic ecosystems.

All living members of this lineage have a three-part body plan, but they're otherwise very different in both appearance and ecology. Acorn worms are solitary worm-like animals living on in the sediment of the sea bed, and are mostly detritivores, while pterobranchs are tiny colonial filter-feeders that build protective tubular structures.

But Gyaltsenglossus senis seems to be a transitional form between both of these different lifestyles. Discovered in the Canadian Burgess Shale fossil deposits, this 2cm long (0.8") hemichordate lived about 508 million years ago and shows a mixture of anatomical features – it had a body and proboscis like an acorn worm, and six tentacled feeding arms like a pterobranch. A bulbous structure at the end of its body may have been used as a temporary holdfast, allowing it to attach itself to the seafloor and raise its arms higher up into the water column.

It wasn't a direct ancestor to either modern lineage of hemichordates, since actual acorn worms and pterobranchs already existed alongside it, but it instead seems to represent an evolutionary "cousin" stem lineage giving us a glimpse at what the ancestors of the whole group might have looked like. And it suggests that early hemichordates were versatile opportunists obtaining their food from both the water and the sea floor, and during their evolution the acorn worm and pterobranch lineages each became much more specialized for one mode of life or the other.

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Yuknessia simplex is one of the earliest known definite pterobranchs, found in both the Burgess Shale in Canada and the similarly-aged Wheeler Shale in Utah, USA (~507 million years ago). It was a colonial species, with multiple cloned zooids living in a cluster of tubes up to about 1.3cm long (0.5"), sometimes attaching to hard surfaces on the seafloor like empty shells.

And Chaunograptus scandens, also from the Burgess Shale, is one of the earliest known graptolites. Its colonies were up to at least 15cm long (6"), but the connecting stem was only a fraction of a millimeter in width.

Although early graptolites like Chaunograptus lived attached to the seafloor, many later forms evolved a free-floating planktonic lifestyle. They became a highly successful group during the Paleozoic, so diverse and distinctive that their fossil remains are often used to date rock layers.

And while they disappeared from the fossil record in the mid-Carboniferous, we now know the group as a whole didn't go completely extinct – graptolites were actually a type of pterobranch, and modern rhabdopleurids have turned out to be living representatives descended from the early seafloor-attaching forms.

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Cambrian Explosion Month #09: Phylum Hemichordata – Enteropneusta

Enteropneusts, commonly known as acorn worms, are the most numerous group of modern hemichordates with over 100 known species. Most of them burrow in sediment eating organic detritus, but a few are filter-feeders and some deep-sea species crawl and drift around over the sea floor.

Their fossil record is poor due to their soft bodies, but the transitional form Gyaltsenglossus has recently given us a glimpse at acorn worms' ancestral links with their cousins the tube-dwelling pterobranchs.

But that's not the only fossil hemichordate with surprising traits from both lineages. It turns out the characteristic tubes of pterobranchs may actually have been ancestral to all modern hemichordates – with the acorn worms later secondarily losing the ability to make them.

Oesia disjuncta is known from the Canadian Burgess Shale (~508 million years ago). It was first discovered in the early 1900s, but at the time all known specimens were poorly preserved and it couldn't be easily identified, being tentatively compared to annelid worms, larvacean tunicates, and arrow worms.

More recent studies and more fossil discoveries have now shown it was actually an early acorn worm – but one that lived inside large hollow perforated tubes that had originally been mistaken for fronds of algae.

The worms themselves were up to about 9cm long (3.5"), with a forked "claw-like" grasping appendage at the end of their bodies, and their tubes could be built as much as 50cm tall (1'8"). This was at least as tall as the very largest Burgess sponges, suggesting that Oesia used them to climb higher up into the water column, filter-feeding on plankton with less competition while also being well-protected from predators.

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Spartobranchus tenuis, also from the Burgess Shale, seems to have been closer related to modern acorn worms than Oesia – in fact, its anatomy was very similar to living harrimaniids.

About 10cm long (4"), the rear half of its body was long and "spaghetti-like" with a bulbous anchoring structure at the end, and it sheltered inside corrugated tubes buried in the seafloor sediment. It was probably also a filter-feeder and it may represent another transitional stage in acorn worm evolution, shortly before they gave up tube-building entirely and became dedicated burrowers.

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