La concha está en la orilla del mar.

Spanish normally requires a definite article before a noun when you’re talking about a specific object.
• la concha = “the seashell” (it’s a particular shell you have in mind)
• una concha = “a seashell” (any shell, non-specific)
• Dropping the article (concha está…) sounds unnatural in this context.

Location and temporary states in Spanish always use the verb estar, not ser.
• está (from estar) = “is located” or “is in a place”
• es (from ser) = “is” for inherent qualities or permanent characteristics

Spanish grammar contracts de + el into del. You never separate them.
• de + el = del
• No valid alternative is de el.

• en expresses static location (“on/at the shoreline”).
• sobre implies “on top of” a surface (“La concha está sobre la arena”).
• a indicates motion toward a place (“Voy a la orilla”) rather than “being at” it.

• orilla = the precise edge where water meets land (“shoreline”).
• playa = the sandy area people relax on (“beach”).
• costa = the wider coastal region, often used for larger stretches of coastline (“coast”).

Yes—meaning changes by region and context:
• In Mexico, concha is also a sweet, shell-shaped bread roll.
• In some Southern Cone countries (e.g. Argentina), it’s vulgar slang for female genitalia.
Here, the beach context tells you it simply means “seashell.”