The conservation status of “spider crabs” is complex because the term refers to a large, diverse group of marine crustaceans. Characterized by long, spindly legs, these animals inhabit diverse marine environments globally. Public concern often arises from intense commercial harvesting and reports of localized population crashes. The conservation status depends entirely on the specific species and the regional pressures it faces, ranging from stable populations to those facing local extinction. Official classifications require looking beyond the general name to the taxonomy and geography of the different crabs.
Identifying the Species Known as “Spider Crabs”
The name “spider crab” is a common term covering the scientific family Majidae, which contains hundreds of species varying greatly in size and habitat. These range from the colossal Giant Japanese spider crab (Macrocheira kaempferi) to smaller, commercially fished varieties. The Giant Japanese spider crab is the largest arthropod in the world, capable of a leg span up to 12.5 feet, and lives primarily in the deep, cold waters of the Pacific continental shelf off Japan.
The European spider crab (Maja squinado) is a medium-sized species found in the Mediterranean Sea and the eastern Atlantic. Along the Atlantic coast of North America, the common spider crab (Libinia emarginata) is a smaller, commercially relevant species inhabiting shallower waters. These distinct geographical ranges and depth preferences mean different species face unique threats and possess varied levels of vulnerability.
The Official Conservation Status
The conservation picture is incomplete for many well-known species on the International Union for Conservation of Nature (IUCN) Red List. The Giant Japanese spider crab (Macrocheira kaempferi) is classified as “Not Evaluated,” meaning there is insufficient data or no formal assessment of its extinction risk. This classification reflects the challenge of monitoring deep-sea populations.
The widely fished European spider crab (Maja squinado) also holds a global status of “Not Evaluated” by the IUCN. This global status often masks severe regional issues that drive public concern. For example, localized populations near the Balearic Islands are considered close to extinction due to sustained overexploitation. Conversely, some American species have been assessed as “Stable” in regional programs, illustrating the patchwork nature of their conservation health. The absence of a formal global threat category indicates a widespread data deficiency for this group.
Key Threats to Spider Crab Populations
The primary pressure on commercially relevant populations is intensive fishing, especially when targeting vulnerable aggregations. Species like the Australian giant spider crab gather in huge, dense groups in shallow waters for their annual molting. This behavior makes them extremely easy targets for large-scale harvesting, which removes a large portion of the breeding stock simultaneously. This practice can severely impact future recruitment.
Environmental factors linked to climate change pose an increasing threat to larval survival. Rising ocean temperatures negatively affect the successful development of larvae in the Japanese spider crab, reducing the number of juveniles that survive to adulthood. Warmer waters can also lengthen the reproductive season, sometimes leading to an explosion of spider crab numbers that threaten local aquaculture industries. Habitat degradation from coastal pollution and destructive fishing methods like bottom trawling also damages the seafloor, affecting the crabs and their food sources.
Monitoring and Regulatory Measures
Fisheries management programs implement specific regulatory measures tailored to regional needs to ensure sustainable harvesting. A common strategy involves establishing minimum landing sizes (MLSs), which protect smaller, immature crabs and allow them to reproduce before being caught. For example, European Union regulations mandate a minimum carapace size of 120 millimeters for the European spider crab.
Many jurisdictions also enforce seasonal closed periods when the crabs are most vulnerable to overfishing. Japan bans fishing the Giant Japanese spider crab during its mating season when it moves to shallower waters to spawn. Furthermore, regulations in some areas prohibit the landing of egg-bearing females to protect the stock’s reproductive capacity. In certain American fisheries, specialized traps with biodegradable escape mechanisms are required to minimize bycatch and reduce the impact on the seafloor habitat.