The sea has provided what could be an important chemotherapy drug for patients with hard-to-treat cancers.
The drug Kahalalide F is a protein produced when molluscs eat the sea slug Elysia rufescens.
A trial looking at its effectiveness in treating liver cancer is about to start, researchers told the European Cancer Conference in Copenhagen.
Liver cancer is one of the hardest cancers to treat because it is extremely aggressive.
Surgery is the main treatment available, but it is not suitable for all patients.
The cancer has also shown considerable resistance to conventional chemotherapies, so the development of new agents is a key aim of cancer doctors.
It is also one of the most common cancers worldwide, with two to five cases per 100,000 people in Europe and the US.
The number of cases is rising, largely because of the spread of hepatitis C infections.
Kahalalide F is extracted from the molluscs in the Pacific and was shown to be an effective chemotherapy agent in laboratory and animal tests.
Early trials on 60 patients with cancers of the prostate, breast, liver and colon which had not responded to other cancers showed promising results and demonstrated what the ideal dose was.
This means researchers know what dosage to give patients in the next stage of research, which has just started to recruit patients.
Ana Ruiz-Casado, a cancer specialist from Pharma Mar in Madrid, who is involved in the research, told BBC News Online: “There are some tumours, such as liver, melanomas and mesotheliomas that we do not have many active drugs that we can use.
“So if we have anything, it’s better than having nothing.”
Kahalalide F is not the only potential cancer drug which could come from molecules found in the sea.
Dr Ruiz-Casado said: “Many people are trying to identify molecules in the sea.
“It’s very difficult to find active molecules in the earth – now many people are interested in the sea.”