Pachygrapsus transversus
Found on hard surfaces in the Seagrass Meadow and Marine Shore, these flat, fast shore crabs graze algae, biofilm, and cyanobacteria-like growth from rock and tank walls, with two individuals confirmed active since July 2024.
The Mottled Shore Crab (Pachygrapsus transversus) is a flat, fast-moving intertidal shore crab with two individuals active in the miniBIOTA Seagrass Meadow and Marine Shore since their introduction on July 14, 2024. Both are confirmed female; one was observed carrying eggs in April 2025, which are treated as infertile in the absence of a male. The crabs have been repeatedly documented grazing algae, biofilm, and cyanobacteria-like surface growth from hard surfaces across the Seagrass Meadow, and a March 2026 observation with video evidence noted reduced cyanobacteria extent around active crab grazing. Population status is Vulnerable; two individuals with no confirmed recruitment.
Pachygrapsus transversus is a widely distributed intertidal crab found along the Atlantic and Gulf coasts of North America, through the Caribbean, and into South America. In Florida, it is common on hard-bottom shorelines, rocky and rubble intertidal habitats, mangrove piling edges, and seawalls, where it grazes biofilm and algae from exposed surfaces. It is categorized by OBIS under Grapsidae and carries WoRMS AphiaID 107457 as an accepted species. The species is well suited to the coastal conditions represented in miniBIOTA's saltwater system.
Pachygrapsus transversus is strongly associated with hard surfaces in the intertidal and subtidal zone: rock, rubble, concrete pilings, seawalls, mangrove roots, and similar structurally complex substrates. It uses surface irregularities and algal cover for shelter and forages by moving rapidly across these surfaces. In miniBIOTA, the crabs have been repeatedly observed on tank walls, rock surfaces, and macroalgal and cyanobacterial mats within the Seagrass Meadow. They anchor to the left tank wall where algae concentrations are highest and range through the far end of the Seagrass Meadow biome where macroalgae and surface growth accumulate.
Pachygrapsus transversus is primarily an algae grazer and biofilm scraper, using its chelipeds to tear and scrape material from hard surfaces. It is classified as an omnivore and can take plant material, detritus, small invertebrates, and animal remains opportunistically. In miniBIOTA, confirmed feeding items include algae from tank walls and rock surfaces (December 2024), macroalgae and cyanobacteria-like growth from the far end of the Seagrass Meadow (March 2026), and general plant and algal material across video observations. An April 2026 observation noted both crabs function largely as herbivores in this system.
Females of Pachygrapsus transversus brood fertilized eggs under the abdomen before releasing planktonic larvae into the water column. Larval development passes through multiple zoeal stages before settling. In miniBIOTA, one crab was observed carrying eggs on April 6, 2025; because both documented individuals are female, the eggs are treated as infertile and no larval release or juvenile settlement has been confirmed.
Pachygrapsus transversus is a marine intertidal species tolerant of typical subtropical and warm-temperate coastal temperatures and normal marine salinity and pH. It is adapted to tidal fluctuation and handles periods of emersion, wave action, and varying light. Formal tolerance measurements for temperature, pH, salinity drift, dissolved oxygen, and flow have not been taken in miniBIOTA; the crabs' nearly two-year persistence under system conditions is the best available evidence of their tolerance.
The Mottled Shore Crab functions as a hard-surface grazer and algae-pressure modifier in the Seagrass Meadow and Marine Shore. By scraping algae, biofilm, and cyanobacteria-like growth from tank walls, rock surfaces, and algal accumulation zones, the crabs transfer energy from producer-level surface growth into animal biomass. The December 2024 observation noted they may make a more significant difference in algae abundance than turbo snails, though no controlled comparison was made.
The March 2026 observation is the strongest single ecological event: one crab was filmed actively grazing through the macroalgae and cyanobacteria clump at the far end of the Seagrass Meadow, and the observer noted that cyanobacteria extent appeared greatly reduced at that time. The crab's grazing is a plausible contributor to this self-correction, but other factors including light shifts, nutrient changes, and other grazers cannot be ruled out. The crab did not resolve the separate Graceful redweed growth, which remained an unresolved light-layer pressure alongside this species.
No confirmed symbiotic relationships exist in miniBIOTA. The crabs share the Seagrass Meadow with Sea Anemone, Eelgrass Isopod, Marine Scud, and other saltwater residents without documented direct interactions.
Two Mottled Shore Crabs were introduced to the miniBIOTA Seagrass Meadow on July 14, 2024. No introduction observation note is preserved in the current Markdown archive for that date. Both individuals are confirmed female based on later observation. No introduction method or source origin is recorded in the database.
Two individuals introduced on July 14, 2024 remain documented in the Seagrass Meadow as of April 21, 2026. Their nearly two-year persistence is a strong survival signal for this system, but the population should still be treated as vulnerable because only two individuals are present and self-sustaining recruitment has not been confirmed.
In miniBIOTA, the Mottled Shore Crab is functioning primarily as a surface grazer in the Seagrass Meadow. It has been observed grazing algae from shoal grass, hard surfaces, tank walls, and the macroalgae/cyanobacteria clump at the far end of the biome. Published literature describes Pachygrapsus transversus as omnivorous, with algae as an important food source and animal material taken opportunistically.
Marine intertidal species associated with hard-bottom habitats such as rock, rubble, pilings, seawalls, mangroves, and sheltered marine shorelines. Exact miniBIOTA tolerance ranges for temperature, pH, salinity drift, and dissolved oxygen have not been measured.
Egg carrying has been observed in miniBIOTA, but viable reproduction is not confirmed. The current recorded population is two females, so eggs are treated as infertile unless a male is introduced and later evidence confirms successful fertilization, larval survival, and juvenile settlement. Published literature describes brooded eggs and planktonic larval release in this species, with reproductive timing varying by region.
The strongest miniBIOTA evidence shows the Mottled Shore Crab grazing through the macroalgae/cyanobacteria clump before sea urchins were added. Cyanobacteria later appeared reduced or checked, making the crab a likely contributor to that self-correction beat. This should not be treated as proof that the crab can resolve the full macroalgae problem; Graceful redweed remains a separate light-layer pressure.
Follow this species across the habitats where it currently appears in the miniBIOTA biosphere.
