An amoeboid with a spherical floating form changes shape as it spreads slender pseudopods. Ferry Siemensma (www.arcella.nl) tells me it is “undoubtedly a vampyrellid”, and possibly a Lateromyxa. Evidently, some forms penetrate their algal prey, and others engulf the victim. This one seems to trying to engulf an unappetizing bit of debris. Unfortunately, the organism was swept away by an air bubble before I could see what happened next.
Diameter of floating form: 23 μm
