Slime molds are remarkable organisms that blur the line between single-celled and multicellular life, often classified as protists. Their life cycle involves stages of dramatic transformation, requiring a highly effective means of locomotion. Slime molds are active foragers that employ distinct mechanical strategies to move across their environment. The difference in their fundamental cellular structure dictates two separate mechanisms for achieving mobility and directional migration.
The Fundamental Difference Between Slime Mold Types
The method a slime mold uses for movement is determined by which of the two major groups it belongs to: acellular or cellular. Acellular slime molds, such as Physarum polycephalum, exist primarily as a plasmodium—a single, gigantic cell containing millions of nuclei within a shared plasma membrane. This multinucleated mass moves as one cohesive unit, relying on internal fluid dynamics for propulsion.
Cellular slime molds, exemplified by Dictyostelium discoideum, maintain individuality as single amoeboid cells. When resources become scarce, these independent cells aggregate to form a temporary, pseudo-multicellular organism known as a slug.
Cytoplasmic Streaming in Acellular Slime Molds
The acellular plasmodium moves through cytoplasmic streaming, a form of pressure-driven flow that circulates the cell’s internal contents. This movement is powered by a rhythmic contraction and relaxation cycle within the plasmodium’s network of tubular veins. The motive force originates from actin and myosin proteins that form a contractile network in the cell’s outer layer, or cortex.
These proteins generate peristaltic waves of contraction that squeeze the veins, creating a pressure gradient across the organism. This gradient forces the internal cytoplasm to rush forward into areas of lower pressure, known as shuttle streaming. The flow direction reverses approximately every 100 seconds as the pressure gradient shifts, allowing the cytoplasm to shuttle back and forth while the organism slowly progresses forward. The timing of these contractions is regulated by periodic waves of intracellular calcium ions, which signal the actomyosin network to contract or relax. This results in efficient amoeboid motion.
Collective Migration of Cellular Slime Molds
Cellular slime molds begin collective migration when thousands of individual amoebas sense a shortage of their bacterial food source. In response to starvation, they secrete and respond to pulses of the chemical signal cyclic AMP (cAMP). This chemical attractant triggers chemotaxis, causing scattered cells to move toward the signaling center, forming streams that coalesce into a dense mound.
The mound transforms into an elongated slug, which can contain up to 100,000 cells moving as a single body. The slug moves through the coordinated effort of individual cells, which collectively secrete a cellulose-based slime sheath on the substrate. The cells move through this sheath, with the motive force generated primarily by the prestalk cells at the anterior tip. Within the slug, cells exhibit a dynamic circulation pattern described as a “reverse fountain flow.” Cells from the posterior continuously move forward toward the anterior tip, which acts as the main engine and steering mechanism.
Environmental Cues That Guide Slime Mold Migration
The direction of slime mold movement is precisely guided by external stimuli, a process known as tropism. Both acellular and cellular slime molds exhibit chemotaxis, moving toward chemical cues released by potential food sources like bacteria or yeast. They also show avoidance behavior, migrating away from areas containing toxins or excessive waste products. Acellular slime molds deposit an extracellular slime trail and avoid crossing it when exploring, using it as a spatial “memory” to prevent backtracking.
Temperature and light also serve as navigational aids. Cellular slime mold slugs exhibit thermotaxis, moving along shallow temperature gradients, often migrating away from temperatures slightly below their acclimation point. During the reproductive phase, slugs demonstrate strong phototaxis, moving toward light sources. This ensures they form their fruiting bodies in an elevated position, optimizing the dispersal of their spores by wind.