They look like some sort of granulocyte.  Take a look in some histology textbooks, at the white blood cells, or pathology books, if your dolphin was ill.    Having this clump of WBCs suggests the cells of the immune system were proliferating in response to an infection or disease?  I do histology, but am not an expert on disease states of immune system.  But some further searching in Pubmed might get you definitive answers.

Try LPS (10-100 ng/ml).

Sorry for the earlier feed (was a mistake for some other query). Their big size suggests that they might be macrophages with some engulfed particles. Granulocytes are usually smaller in size and multinucleated. HnE staining may further help.