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Priapulida - Blog Posts

phoronopsis

3 months ago

The first image linked is not actually a priapulid but a sea cucumber in its spawning posture! It was misidentified on iNaturalist and went viral before it was corrected— see the original observation here. (It gets kinda heated which I think is kinda funny. Penis worms are serious business!) I have always said before that I want internet fame specifically for two reasons: to make PSAs about Anomalocaris’s head carapace which everybody always leaves out of drawings because of that one inaccurate museum model, and about the incorrectly identified sea cucumber photo about which is now like the first image result you get when searching for penis worms and is my NEMESIS 😠 (the misinformation, not the photo or the sea cucumber, those are great)

For all the worm fans— priapulids are super easy to identify; there are as of the time of writing only 22 recognized species, and for many of them the only photos of them are from articles in scientific journals. Over half the species are microscopic, and the macroscopic ones are mainly found in polar regions, often in the deep sea, where they are usually burrowed in sediment and thus are little-encountered by people. The only one of them that is commonly photographed (and studied) is Priapulus caudatus, which is broadly found across the northern northern hemisphere even in shallow waters and I think probably has to be the most accessible species in general. They look like this:

A live specimen of the priapulid worm Priapulus caudatus, pictured in a petri dish. It is a pinkish-tan worm with a bulbous, spined proboscis and a branching, feathery tail.

image by Thomas Trott

This species accounts for probably 99% of the images of priapulids out there, and its relatives look rather similar, such as its southern hemisphere counterpart Priapulus tuberculatospinosus or the two-tailed species Priapulopsis bicaudatus. The intricate, feathery tails (referred to in the literature as “caudal appendages”) are probably the most distinctive feature of this group; they are believed to be involved in respiration, though as with many things about the phylum it is not known for certain. (See this recent paper for a review of macroscopic priapulid morphology.) In the zoomed-out photos of that sea cucumber you can see on the iNat page, it lacks a tail which is a dead giveaway that it is not any of these; also note that while it has some longitudinal striations along what sorta looks like a proboscis, they don’t actually bear any teeth! The spined, toothed proboscides of priapulids are indeed super cool and are their most distinctive feature setting them apart from other proboscis-bearing worms like peanut worms or spoon worms, which are often also misidentified online as priapulids. A fun fact is that the shape of their teeth varies across species in a way that appears to be closely correlated with their diet, see this paper for a neat study that uses tooth shapes to examine the different ecological niches occupied by extant priapulids and their Cambrian relatives!

The only other macroscopic priapulids that don’t look much like Priapulus are the two species Halicryptus spinulosus and Halicryptus higginsi, the latter of which I believe there are literally like two full-body photos in existence of it, one of which is from a login-walled journal article from 1999 and the other of which is one of the specimens from that 1999 article photographed after 25 years preserved in a museum. There’s a decent number of photos floating around of H. spinulosus (though still not as many as P. caudatus); they look like this:

A group of specimens of the priapulid worm Halicryptus spinulosus, photographed in a petri dish. They are small, cylindrical yellowish worms with visible spines on their short proboscis.

image by Claude Nozères

As you can see, Halicryptus lack tails and have a much less prominent proboscis than Priapulus and its relatives, which you can only see the spines of on the very tip; H. spinulosus in particular has a rather short and small body that distinguishes it a lot, while H. higginsi is the largest known species of priapulid in the world (see this paper for a review of both of them). They’re maybe less distinctive-looking but idk, I don’t know off the top of my head if there’s super anything else you would mistake them for, and images of them are pretty uncommon anyway. In any case as far as macroscopic priapulids go, these are the only ones you have to look out for; if you’ve got those down you’re all set! As stated before, most priapulid species are actually microscopic; just for fun here’s the tropical meiobenthic species Tubiluchus corallicola:

A preserved specimen of the priapulid worm Tubiluchus corallicola. It is a microscopic worm with translucent skin revealing yellowish internal organs and a long, slender tail.

image by Museum of Comparative Zoology, Harvard University

look at that squiggly tail!

And yeah in conclusion priapulids are super cool and underrated and I wish there were more people paying attention to them; there’s soooo many neglected taxa that we’re still only just discovering basic aspects of their biology and priapulids are one of them! If you want to see their amazing extensible proboscis in action, linked below is by far the best priapulid video out there, I highly recommend it. And most of all remember everybody THAT PHOTO IS A FRICKING SEA CUCUMBER, NOT EVERY WORM THAT LOOKS LIKE A PENIS IS A PENIS WORM AAAAAAA 😭😭😭

Can't believe any real animal has teeth as awesome as penis worms have.

They are meat eaters :)