TOKYO (AP) -- Sweden's King Carl XVI Gustaf and Japan's Emperor Akihito attended a special exhibit on Swedish natural sciences in Tokyo on Monday to mark 150 years of diplomacy between the countries.

The king and Queen Silvia accompanied Akihito and Empress Michiko at the exhibition featuring three key Swedish scientists from the 18th century who have influenced Japan, including Carl Linnaeus, who set the basis of a systematic scientific classification.

Akihito, a marine biologist specializing in gobies, was awarded an honorary degree in 2007 for his research from the university where Linnaeus studied.

Japan's NHK television said Akihito was fixated by Linnaeus' original book of classification and stayed at the exhibition longer than planned. Television footage showed Akihito intensely looking at the book through a glass showcase, while a smiling Michiko stood by his side.

The Swedish royals were also to attend a state banquet hosted by Akihito and have lunch with the Japanese royals' oldest son, Crown Prince Naruhito. The king and queen will also meet with Japanese Prime Minister Shinzo Abe and attend a business seminar and other events before leaving Tokyo on Thursday.

Akihito is set to abdicate at the end of April 2019 after reigning for 30 years. Naruhito is to succeed the Chrysanthemum Throne the next day.