Most coastal cities are built at the mouths of rivers and in estuarine systems where soft sediment was the dominant marine habitat type historically. Though we’ve added all sorts of artificial structures to these landscapes and altered urban shorelines considerably, mud and sand habitats are sometimes still evident in some coastal cities, either at low tide, or if you look carefully through the water to the seafloor below from piers and boardwalks. Initially, this soft sediment substrate will probably look rather boring and featureless, not to mention “icky.” There couldn’t be much living in that stuff, right?
Believe it or not, soft sediment environments are incredibly diverse. In a single sediment core sample (a cylindrical area of sediment that’s only a few inches wide and maybe half a foot deep), I find more invertebrate species than I typically find on an entire artificial reef in Puget Sound. They’re modest creatures – small and unassuming, often cryptic, and typically not as colorful as their ostentatious, rock-dwelling counterparts. Regardless, soft sediment organisms, termed “infauna”, are an important part of urban marine landscapes.
Over the last few years, I’ve had the pleasure of becoming more closely acquainted with some of these critters as I’ve sorted through sediment samples from West Seattle. The samples are part of a larger experiment that I promise to report more about soon, but in the meantime, I want to share with you some of my favorite “infaunal” organisms. Specifically, they are the current winners of 3 categories for which I’ve had running lists over recent years:
| Category #1: Most frightening
WINNER: Glycerid worms If I were a marine organism about the size of an ant, there is little I can imagine that would be more frightening than a glycerid worm. These fierce predators construct complex networks of burrows in soft sediment, which they move through rapidly. Remember the movie Tremors from the early 90s? This is like Tremors the real version. When glycerids find their prey, they shoot out their pharynx complete with four terrifying fangs (pictured to here). They’re known as bloodworms. This is said to be because of the ceolomic fluid you can sometimes see through their body wall, which contains hemoglobin and is the color of blood. I wouldn’t be surprised is the true origin of their name is more morbid than that, though. At least they seem to want blood when I’m handling them in the field… Remind me not to re-watch Tremors anytime soon.  
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| Category #2: Most adorable / cutest
WINNER: Euphilomedes ostracods Since teddy bears weren’t in the running, we have the next best thing. Ostracods are tiny crustaceans that live inside a little circular house; sort of like a seed with legs. The genus I tend to encounter is Euphilomedes spp. and is relatively large, reaching well over a millimeter in diameter. It may not sound like much, but in the world of ostracods, these are giants. Life for Euphilomedes ostracods tends to consist of puttering around between sediment grains, presumably in search of food. 
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| Category #3: Most elegant
WINNER: Tellina clams Though clams in Puget Sound come in all shapes and sizes, one stands out with its elongated figure, smooth, shiny shell, and occasional radiating pink patterns. Though the Tellinid pictured here is from the Bahamas, it’s Puget Sound relatives are no less elegant. 
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These are just a small selection of the many critters that inhabit muddy and sandy marine habitats. The softs sediments we see in urban marine environments may look like a lonely place to call home, but these critters are by no means alone. It’s amazing to see all that lives in a handful of mud!