Diphyllobothrium latum causes anemia by:

Diphyllobothrium latum, also known as the fish or broad tapeworm, is the biggest worm and can reach 10 m in size. Humans acquire the infection by eating raw fish containing the larvae of the tapeworm. The worm attaches to the small intestine and causes abdominal discomfo. Nausea, diarrhea, weight loss, and pernicious anemia can result. The anemia is induced by the tapeworm's tendency to compete with humans for vitamin B12, which it easily accumulates from the intestinal contents. Ref: Levinson W. (2012). Chapter 54. Cestodes. In W. Levinson (Ed), Review of Medical Microbiology & Immunology, 12e.