The Mysterious Marks of Marine Debris!

Did you know that some of the waste found on beaches is intact, while some is tattered? Intact waste can be identified by its shape and purpose; recycling labels reveal its material; manufacturing or expiry dates estimate its age; and brand logos suggest its manufacturer. Fragmented waste, however, is often unrecognizable,

though it occasionally bears “mysterious marks,” like animal bite marks, sparking curiosity about its journey.

The question first arose among volunteers tackling marine debris in Hawaii. They kept finding bitten plastic waste on shores, with irregular “holes” of various sizes—round, square, diamond-shaped—some with neat edges, others jagged. Initially, they thought these were dog bites, but bitten waste appeared even on remote beaches, and the marks didn’t match dog teeth, suggesting marine creatures were probably responsible.

At first, they pointed to sharks, dubbing the jagged or perforated plastics “SHARKastics” (shark-bitten plastics). Later, they realized sharks weren’t the only culprits. Volunteers issued a call for answers:

“Who’s biting the plastic?”

Research revealed that 16% of 6,000 randomly sampled marine debris items bore bite marks—triangular or hole-like, 1 to 20 mm long—indicating various species ingest plastic. Blue and yellow bottle-like objects were most frequently bitten. In Alabama’s Dauphin Island, plastic bottles with diamond-shaped bite marks from sea turtles often wash ashore.

Studies confirm bite marks on ocean plastic come from sharks, triggerfish, catfish, parrotfish, sea turtles, squid, and more.

Most Common Bite Marks

In Hong Kong, coastal cleanups often uncover bitten waste—a plastic bag with 20 bite marks or a straw bitten five times. Common marks are diamond-shaped, about 10 mm long, and typically punctured through. Given their size, these are unlikely to be from sea turtles, which are rare in Hong Kong and larger.

Later, it was found that squids cause the most common bite marks on Hong Kong’s marine plastic waste.

An online video filmed by fishermen shows squid easily piercing aluminium cans, leaving neat diamond-shaped holes. Squids have a hard, beak-like mouth, normally used to eat crabs and shrimp, with impressive biting strength. Commonly bitten plastics in Hong Kong include plastic bags, food packaging, straws, and soft plastic containers.

Reference video:
・Wolffish head bites through coke can
・Squid Bites Soda Can

Creatures with Stomachs Full of Plastic

Sea turtles are among the marine animals that frequently ingest plastic waste. Between 2019 and 2024, over 45% of live or dead sea turtles found in Hong Kong had consumed waste. In 2024, a young sea turtle’s stomach contained 81 pieces of marine debris, including food packaging, fishing line, foil, rope, and plastic bags, leading to intestinal blockage and rupture, causing its death. In 2020, another turtle’s autopsy revealed plastic gloves and other plastic waste in its stomach.

City University of Hong Kong 2024 Aquatic Animal Virtopsy Lab Annual Report

Why Do Marine Animals Ingest Plastic?

When plastic enters the ocean, algae and barnacles grow on it, emitting a scent similar to sea turtles’ food. Lacking taste buds, turtles can’t distinguish what they’re eating, making plastic debris enticing. Plastic bags resemble jellyfish, and 5 mm pearl-white plastic pellets look like fish eggs, increasing the risk of ingestion.

Impact of Ingesting Plastic Waste

Plastic waste absorbs persistent organic pollutants (POPs) from dyes, chemical additives, antimicrobials, plasticizers, pesticides, detergents, and flame retardants, threatening marine life. Indigestible by stomach acid, plastic clogs digestive systems, causing discomfort. Lacking nutrition, plastic-filled animals weaken and often strand, leading to death.

In June 2025, a 64-year-old Chinese man sought medical attention due to chest pain. Upon examination, doctors discovered a plastic children’s toothbrush lodged in his duodenum—it was something he had accidentally swallowed at the age of 12. He had assumed the toothbrush had long since been dissolved by stomach acid, but remarkably, even after 52 years, it remained completely intact. It ultimately had to be removed through minimally invasive surgery.

This incident highlights how persistent plastic is in digestive tracts.

Whether marine animals swallow entire plastic items or just take bites, it highlights the severity of ocean plastic pollution.

Reducing waste at the source and preventing waste from entering the ocean are critical.

Next time you’re at a beach cleanup, keep an eye out for the mysterious marks on marine debris.