What could possibly wash up on (be found live on) the shore of the Harlem River (right in the heart of New York City, one of the most urban areas in the world) that would be of considerable interest to naturalists?

I naively assumed there would be very little at all. But I was wrong!

First I should explain that the Harlem "River" is not actually a river -- it is a tidal inlet, and so is the East "River". They are both part of the complex geography of the estuary of the Hudson River. Thus the Harlem River is saltwater, not freshwater, but the water may almost never be at full seawater salinity, because it is an estuarine environment.

On Randall's Island Park during low tide, there is a very small sand beach on the Harlem River. There is usually some beach drift there, and it is quite interesting. So far I have found three crab species, a jellyfish species, a comb jelly, and various species of green, brown, and red algae.

With a lot of careful and thorough searching over a number of visits, I /we have also managed to find a surprising number of marine mollusk species, including four live species and quite a few fresh-dead shells in good condition.

The species I have found so far are listed here, in decreasing order of commoness.
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BIVALVES

Softshell Clam -- Mya arenaria, including small live individuals
Atlantic Jacknife -- Ensis directus, many fresh paired valves in good condition
Dwarf Mulinia -- Mulinia lateralis", many single valves
Baltic Macoma -- *Macoma petaluma , some paired valves
Eastern Oyster -- Crassostrea virginica, single valves, fairly common.live ones at low tide
Atlantic Ribbed Mussel -- Geukensia demissa, a few sets of paired valves, several single valves
Atlantic Rangia -- Rangia cuneata, single valves, AN INTERESTING ESTUARINE SPECIES!
Hard Clam -- Mercenaria mercenaria, one live juvenile, some broken pieces of adults
Blue Mussel -- Mytilus edulis, two valves

GASTROPODS
Eastern Mudsnail -- Tritia obsoleta including three or four live ones, and a lot of empty shells. Also eggs of this species laid on red algae.

.........................................................
June 3rd 2017, I found:

MORE GASTROPODS!
Shark Eye -- Neverita duplicata -- one broken shell
Eastern White Slippersnail -- Crepidula plana, live inside the Shark Eye shell

And a few days ago on June 1st I found these small, white, very fragile shells, each of them as a complete shell with paired valves:

MORE BIVALVES!
Northern Dwarf-tellin -- Ameritella agilis (formerly Tellina agilis)
Glassy Lyonsia -- Lyonsia hyalina (by June 18th a total of three intact shells)
Shining Macoma -- Macoma tenta (only one so far)

These three species are very surprising for such an urban locality, the Shining Macoma being the biggest surprise of all, as this species was thought to be very particular about where it lives, and it is usually only found far out on Long Island where there is very little pollution.

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Wherever people go, they take stuff with them, and most often they leave some of it behind, either deliberately or accidentally. Some of these abandoned objects are ugly, and some are quite beautiful.

Sea shells have been considered to be beautiful by humans for millennia. The oldest jewelry ever found, over 10,000 years old, was made from seashells that were strung together into a necklace.

Therefore, not surprisingly, when biologists including myself are searching to see what they can find in places like the intertidal zone on Wards Island and Randall's Island in Manhattan, part of the estuary of the Hudson River, we do occasionally find shells from far-distant parts of the world.

So far we have found:

Two valves of an exotic Anadara species (4 more since then!) (and another two on August 19th for a total of 8!)
http://www.inaturalist.org/observations/6484705

Two faded and worn shells of a Turritella species (one more since then for a total of 3!)
http://www.inaturalist.org/observations/6484760

Two beads made from the gold ring cowry (another two since then!) (and another one on August 19th for a total of 5!)
http://www.inaturalist.org/observations/5643978

One tropical top snail species (another one since then!) (and another completely different species on August 19th.)
http://www.inaturalist.org/observations/6771040

And one Indo-Pacific small conch off the southern end of Wards Island.
Shown in the image below

Who knows what I might find next?

UPDATE:
From the same little beach on Ward's Island on July 27th:

Two more valves of that same Anadara species

One more shell of a tropical top snail

Amazingly: One shell of Indothais lacera

And this kind of thing happens to some extent almost everywhere, even in the least likely, far-flung places.

A lot of people seem to think that the best places to throw away a shell is on a beach, no matter where that shell came from originally. If they even know where their shell was from. People tend to think the shells they find in a shell shop are local, even though most are from the international shell trade and were actually collected live in the Philippines.