“Shellfish” sounds like “selfish,” prompting the pun:
 
Q: Why do crabs/lobsters/oysters never give to charity?
A: Because they’re shellfish.
 
“You know why clams keep everything to themselves? They’re shellfish” was cited in 1963. “Why don’t lobsters share? They’re shellfish” was cited in 1999. “Why don’t oysters give to charity? Because they’re shellfish” was cited in 2004.
 
     
Wikipedia: Shellfish
Shellfish is a culinary and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environments, some kinds are found in freshwater. In addition a few species of land crabs are eaten, for example Cardisoma guanhumi in the Caribbean.
 
Despite the name, shellfish are not a kind of fish, but are simply water-dwelling animals. Many varieties of shellfish (crustaceans in particular) are actually closely related to insects and arachnids, making up one of the main classes of the phylum Arthropoda. Cephalopods (squid, octopus, cuttlefish) and bivalves (clams, oysters) are molluscs, as are snails and slugs.
 
Familiar marine molluscs enjoyed as a food source by humans include many species of clams, mussels, oysters, winkles, and scallops. Some crustaceans commonly eaten are shrimp, lobster, crayfish, and crabs.
       
Google News Archive
11 September 1963, Sarasota (FL) Journal, “Wed. Whittlin’s” with Bill Copeland, pg. 7, col. 3:
YOU KNOW WHY clams keep everything to themselves? They’re shellfish.
 
Google Books
January 1975, Boys’ Life, “Think & Grin,” pg. 80, col. 2:
Dimm: Why are crawdads so greedy?
Whit: Because they’re shellfish! — Whitney White, Arlington, Tex.
 
4 October 1989, Farmington (NM) Daily Times, “Mighty Funny’s Mini Jokes,” The Mini Page by Betty Debnam, pg. C3, col. 4:
Q: Why don’t clams have many friends?
Mighty Funny: Because they’re shellfish!
(Sent in by Bert O’Connor)
 
4 February 1999, Waunakee (WI) Tribune, “Pause a Moment,” pg. 10, col. 6:
Why don’t lobsters share?
They’re shellfish.
 
Google Groups: alt.wisdom
at work today
Dan
12/20/04
Why don’t oysters give to charity?
Because they’re shellfish.
 
Google Groups: alt.fan.pratchett
News Flash!! Hell Freezes Over!!
Aggie
3/20/06
(...)
Q. Why don’t oysters give to charity?
A. Because they’re shellfish.
   
Google Books
Man Walks into a Bar:
Over 6,000 of the Most Hilarious Jokes, Funniest Insults and Gut-Busting One-Liners
By Stephen Arnott and Mike Haskins
Berkeley, CA; Ulysses Press
2007
Pg. ?:
“Why don’t oysters give to charity? Because they’re shellfish.” Jay Leno
     
Twitter
smile
‏@jokeaday
Why don’t lobsters share…: Why don’t lobsters share They’re shellfish http://tinyurl.com/2mvs3m
4:22 AM - 14 May 2007
 
Google Groups: neworleans.general
Why don’t lobsterss donate to charity?
Wordy
6/2/07
they’re SHELLFISH !
 
Twitter
Bill Kidd
‏@wvk
Why will spanner crabs and lobsters refuse to contribute to Haiti charity appeal?
Because they are shellfish.
2:14 AM - 17 Jan 2010
   
Google Books
51 JOKES!
By Peter Crumpton
PeteyRF Creative
2015
Pg. ?:
Why do crabs never give to charity? Because they’re shellfish.
 
Twitter
Sean Says Relax
‏@seansaysrelax
Me: Why do crabs never give to charity?
Son: I don’t know
Me: Because they are shellfish
Son: Dad I hate you
#DadJokes
4:25 PM - 19 Jan 2015