William A. Herdman, marine zoologist, was educated at the University of Edinburgh, where he was awarded the degree Master of Arts (MA). Later he obtained the degree Doctor of Science (DSc). He became an assistant to Sir Charles W. Thomson*, working with him on the collections of HMS Challenger and making a special study of the Tunicates (a group of primitive chordates including the sea-squirts), on which he became the leading authority. Later he compiled a Descriptive catalogue of the Tunicata in the Australian Museum, Sydney, New South Wales (1899).
In 1881 Herdman was appointed the first Derby professor of natural history at the University of Liverpool, retaining the post to 1919. In that year he was appointed professor of oceanography for one year, until his retirement in 1920. He organised the marine research laboratory at the university in 1891. His work included attempts to coordinate the fishing industry with the results of scientific research and assisting with the plankton survey of the Irish Sea. He was chairman of the Liverpool Marine Biology Committee, which issued its first annual report in 1886 on the fauna of Liverpool Bay. Its 22nd to 24th annual reports, in 1908-1910, dealt with work of the marine biological station on the Isle of Man, where Herdman had a summer home. With P.M.C. Kermode he carried out archaeological investigations on the island, which were described in Manx antiquities (1904). Over the years he produced a series of publications on the fauna of the Firth of Forth, Liverpool Bay, and the Irish Sea. In 1901 he visited Ceylon to report on its pearl oyster fisheries.
He was elected a Fellow of the Royal Society of London in 1892, serving on its council from 1898 to 1910, and was also a Fellow of the Royal Society of Edinburgh. He became a member of the British Association for the Advancement of Science in 1887, served as general secretary from 1903 to 1919, and was president of the association in 1920. He was a Fellow of the Linnean Society and served as its president from 1904 to 1908.
In 1905 Herdman attended the joint meetings of the British and South African Associations for the Advancement of Science in South Africa, delivering a paper on "Pearl oysters and pearls" in Johannesburg on 29 August. By that time he had already delivered an evening lecture on "Marine biology" during the delegates' visit to Durban. As chairman of one of the British Association's committees he reported on its ongoing investigation of the fauna and flora of the Trias of the British Isles. He was also a member of a committee reporting on the organisation and registration of British zoologists that year.
In addition to his many scientific reports and papers, Herdman wrote a book, The founders of oceanography, which was published in 1923. He was honoured as a Commander of the Order of the British Empire (CBE) in 1920, and knighted in 1922.