Horseshoe crab

Shell damage and erosion

SignsChipped shell edge, dark patches or holes, white cotton-like growth, fluid seeping from leg joints

PreventionPrevent collisions with tank corners and rocks. Keep water clean; lift with both hands supporting the shell

Incomplete molt (juveniles)

SignsFront edge split but cannot crawl out; after molting, stays still and the body stays soft without hardening

PreventionKeep salinity and temperature stable; never touch during a molt. Avoid malnutrition

Oxygen deprivation and gill damage

SignsRapid book-gill movement, floating belly-up near the surface, gills dark red or fouled with algae or mud

PreventionConstant aeration. Do not let sand get stirred up or sludge build up; never let the gills dry out

Ammonia / nitrite poisoning

SignsSuddenly stops moving, beats gills violently, legs drawn in with dull response, several animals weakening at once

PreventionFully cycle the filter before housing. Test water weekly; remove leftover food the same day

Low-salinity / temperature shock

SignsThrashes right after a water change, legs go rigid, does not move or eat below 15°C

PreventionMatch specific gravity and temperature before changing water. Avoid sudden freshwater inflow; keep 20-28°C

Food refusal and weakness

SignsNo eating for several weeks, weak leg movement, legs hang limp when lifted

PreventionOffer near-live food such as clams or lugworms at night; provide sand and hiding places and reduce disturbance

Broken tail spine

PreventionNever lift by the tail spine. Design the tank with room so it does not get caught on fixtures or the lid

First aidDo not disturb the break; leave it in clean seawater. Without the spine it cannot right itself, so check daily for flipping

Cracked or chipped shell

PreventionDo not drop it. Support the shell with both hands when moving; use containers with ample depth and width

First aidDo not force chipped fragments back. Keep the water clean; if fluid keeps leaking, consult a vet the same day

Lost legs or appendages

PreventionAvoid crowding with other animals; carry by hand, not with a net. Cover the filter intake

First aidDo not disturb until the fluid stops. Legs may regenerate at the next molt, so maintain nutrition and water quality

Stranding and drying out on tidal flats

PreventionDo not approach or touch animals that come onto the beach during spawning season. Do not collect or take them home

First aidIf alive, move it to shade or back into seawater; if flipped, turn it upright. If tagged, record the number and report it

Vibrio infection

RouteCuts on hands that touched seawater or the animal; puncture wounds from the tail spine

PreventionWear gloves if you have hand wounds. Wash with soap after work; see a doctor if a wound swells or feels hot

Nontuberculous mycobacterial infection

RouteScrapes on hands that touched tank water or equipment

PreventionWear gloves for tank work. For a skin lump that will not heal, see a doctor and mention that you handle aquariums

Tetanus and wound infection

RoutePunctures and cuts from the tail spine or shell edge

PreventionIf stabbed, rinse under running water and disinfect. For deep wounds, see a doctor and check your vaccination history

Intestinal infection

RouteTidal-flat mud or seawater getting from hands into the mouth

PreventionWash hands after work and before eating or drinking. Do not let children put their hands in their mouths

Food

• Shucked clams (asari, shijimi), shrimp and lugworms 2-3 times a week at night
• Guide: 1-2 clams about 1/10 of carapace width. Remove leftovers the next morning
• Juveniles: finely chopped clams, mysid shrimp or frozen brine shrimp in small amounts daily

Water

• Artificial seawater. Keep specific gravity 1.020-1.025 and temperature 20-28°C
• Change 1/4-1/3 weekly with seawater of the same temperature and specific gravity

Toys & play

• Fine sand to burrow and rest in. No toys needed; do not overfill the tank with objects
• Gentle current. A wide floor to walk around on provides exercise
• Burying live clams in the sand brings out searching and feeding behavior

Nest, bed & hideout

• Fine sand at least 5 cm deep. A place to burrow serves as a hideout
• Gentle slopes with no sharp corners. Prevents getting stuck in a corner and flipping

Bedding & litter

• Fine silica sand or sea sand. Coarse gravel damages the shell and gills
• Clean the sand surface with a gravel siphon; do not let sludge build up
• No litter box needed. Siphon out waste and leftover food frequently

Cage / housing material

Acrylic or glass tank rated for seawater. No metal parts

Size & space

Adults: floor at least 1.5 m x 1 m; juveniles: floor at least 10 times the carapace width

Layout

• Fine sand over the whole floor. Keep rocks and decor to a minimum
• Strong aeration and an external filter. Cover the intake
• Water depth 2-3 times the carapace width. Leave clearance between the lid and the water surface

Placement notes

• Avoid direct sunlight and vibration. Choose a spot with stable water temperature
• Keeping several requires space. Crowding causes flipping and gill damage

Exercise, free-roam & walks

• No walks needed. A floor wide enough to roam provides exercise
• Do not take it outside. Even briefly, dry gills mean it cannot breathe
• Keep lifting for display or observation to a minimum. Support the shell with both hands

Temperature & humidity

TemperatureWater 20-28°C. Activity drops below 15°C; above 30°C is dangerous

HumidityLives in water, so humidity is irrelevant. Keep the gills wet when it is out of water

Summer & winter

• In summer, use a chiller to stay at or below 28°C. During spawning season, keep males and females calm
• In winter it goes dormant as the water cools. Avoid sudden heating; keep the temperature steady

Nail trimming

No nail trimming. Gently remove algae on the shell or gills with a soft brush under seawater

Brushing, skin & coat care

Inspect the shell weekly and photograph any chips, dark patches or cotton-like growth. Do not scrub

Other care

Juveniles molt about once a year. Do not discard shed shells; confirm whether it is a shed shell or a dead animal before recording

Protection from predators

• Fish and crabs in the tank nibble legs and gills. Do not house them together
• On tidal flats, crows and herons attack flipped animals. Turn any you find upright
• Do not let children or visitors hold the tail spine. Post handling instructions

Foreign-body ingestion

May swallow pebbles or foreign objects along with sand. Do not put coarse gravel, artificial objects or metal fragments on the floor. Remove food shells together with leftovers

Being stepped on

Can be stepped on by people on tidal flats. Watch your footing on spawning beaches and do not wander over flats where juveniles live

Getting pinched

Shell or legs can get caught in the lid, tank corners or gaps between rocks and cause flipping. Design without gaps and check daily that nothing is stuck

Falls

Dropping it from the tank cracks the shell and leaks fluid. Move it low, supporting the shell with both hands; never lift by the tail spine

Escape

It does not escape, but taking home a stranded animal can be illegal. Do not take it; return it to the sea and contact the municipal office, a museum or an aquarium

Safe release procedure

1. Horseshoe crabs do not bite. If a finger gets caught between legs, hold still and wait
2. Support both sides of the prosoma with both hands, lift, and wait in water for the legs to open
3. If the tail spine or legs snag on clothing or a net, return the animal to water before untangling
4. If the tail spine is stuck in you, do not yank it out; gently separate from the animal first, then treat

After the release

Rinse the wound under running water and disinfect. Seawater bacteria are present, so see a doctor if swelling or fever develops

What to tell the vet & what to bring

Bring water temperature, specific gravity and water-test readings, carapace width, collection site and tag number, molt history and photos

Checking consciousness, breathing & heartbeat

• Book-gill movement (opening and closing) means it is alive. Observe for 1 minute
• Touch a leg gently and check for a withdrawal response
• A shed shell is light, hollow and open at the front edge. A dead animal is heavy and smells

Stopping bleeding

• For leaking bluish-white fluid, do not disturb; leave it in clean seawater
• Do not replace or touch shell fragments. No pressure to stop bleeding
• Moisten dry areas with seawater; do not let it dry out

Transport

• Line a container of seawater with sand and place it in, supporting the shell with both hands
• Maintain temperature and aeration. For short trips, cover the gills with a wet cloth