Sally Lightfoot Crab (Percnon gibbesi)

A marine crab for an established saltwater aquarium

The Sally Lightfoot crab, Percnon gibbesi, is a marine crab for keepers who already run a settled saltwater aquarium. Each purchase is one live crab. The question that matters most before you order is whether your tank is ready for it: fully marine water that has been stable for some time, ammonia and nitrite at zero, and plenty of live or artificial rock for grazing and hiding. If that describes your aquarium, this guide takes you through water, feeding, moulting, health and tank mates. If the tank is freshwater, or a marine set-up that is still maturing, plan the crab for later.

Names and identity

Wikipedia's entry for this crab explains that Percnon gibbesi is one of at least two crab species commonly called Sally Lightfoot, and that it is also known as the nimble spray crab or the urchin crab. That shared name matters whenever you read about care or compare crabs: advice written for the other Sally Lightfoot does not automatically apply to this one, so check the scientific name before you rely on what you read.

The World Register of Marine Species lists Percnon gibbesi (H. Milne Edwards, 1853) as an accepted species in the family Percnidae, within the order Decapoda, which also contains shrimps and lobsters. You will also see it offered simply as the spray crab.

What you receive

• One live Sally Lightfoot crab per purchase.
• Hand-bagged in pure oxygen by our own team and packed in an insulated poly box, with a heat pack when the weather calls for it.
• Carried on a tracked service by a licensed live-animal courier, with the tracking number and your confirmed delivery date sent by email.
• Covered by our Live Arrival Guarantee, subject to the conditions set out in the arrival section of this guide.

Sex is guaranteed only when the selected option explicitly states male or female. This Sally lightfoot or spray crab listing has no sex-guaranteed option.

Is it right for your aquarium?

Before you order

1. Test ammonia and nitrite; both should read zero.
2. Check that salinity and temperature sit inside the ranges given in the water section.
3. Look at the rockwork: is there surface to graze and a cave or overhang where a moulting crab can hide?
4. Go through your fish one by one and make sure every one is reef-safe.
5. Make sure someone will be at the address to receive the parcel and can open it the same day.

A crab to keep, never to release

Wikipedia also notes that Percnon gibbesi has been described as the most invasive decapod species to enter the Mediterranean. Treat every crab you buy as a permanent aquarium resident. Never release one into the sea, a pond or any other open water; if you ever need to rehome it, pass it on to another marine keeper.

What to expect once it settles

Rock gives the crab its grazing surfaces and its hiding places, so the rockwork is where to look for it. Like every crustacean it grows by shedding its exoskeleton, so from time to time you may find an empty shell in the tank while the crab itself stays out of sight for a couple of days as its new shell hardens. That is normal, and the moulting section explains how to tell a cast shell from a crab in trouble. Between moults, a healthy crab moves with co-ordination, keeps all its legs and shows an interest in food.

About the photographs

Gallery images have been digitally enhanced to improve image quality. Natural colour, pattern, size and maturity can vary between live animals.

Several gallery scenes show the crab beside green plants or driftwood. Whatever the setting in a picture, this is a marine crab that needs full-strength salt water and plenty of rock, as the care sections below explain.

Preparing the aquarium

A mature filter comes first

Every marine crab depends on a biological filter that is already working when it arrives. Animals release ammonia, which is toxic. In a working filter, beneficial bacteria break it down to nitrite, which is also toxic, and then to nitrate, which is safer. The RSPCA's guidance on aquarium filtration explains that cycle and why no aquarium should run without a well set-up, well-maintained filter.

OATA's advice before adding any crustacean is to check that ammonia and nitrite are both at zero, and then to build up the livestock slowly. The bacterial population has to grow each time animals are added and feeding increases. Stock too quickly and the result is new tank syndrome: too few nitrifying bacteria for the extra waste, unhealthy ammonia and nitrite levels, and invertebrates that may become ill or die. A Sally Lightfoot crab belongs among the later additions to a settled tank, not the first.

How big an aquarium?

OATA's general rule for marine crustaceans is to buy, within reason, an aquarium as large as possible. It recommends at least 40 litres for the smaller species of crabs and shrimp and says larger crustaceans need a larger aquarium, with the size varying by species. The group covers an enormous range, from sexy shrimp that grow only to 2 cm to blue spiny lobsters of over 30 cm, so read the 40-litre figure as the floor for the smallest animals rather than a target for this crab.

In practice, size the tank for everything you plan to keep, give the crab room to move over the rockwork without crowding other invertebrates, and choose the larger aquarium whenever you are torn between two.

Filtration and skimming

Whatever its size, the aquarium needs a filter. For marine set-ups OATA describes three routes:

• Live rock with sufficient water flow through and around it.
• An internal or external filter.
• A sump-based filter.

A protein skimmer can also help, because it removes dissolved organic waste before that waste breaks down into more harmful substances. If you run one, it needs regular cleaning to keep working properly.

Equipment checklist

• One of the filter set-ups above, running and matured before the crab arrives.
• A protein skimmer, if you choose to use one.
• A heater and a reliable thermometer to hold the tank at 24–26°C.
• A hydrometer or refractometer for checking salinity.
• Test kits for ammonia, nitrite, nitrate and pH, plus calcium, magnesium and carbonate hardness if you keep a reef.
• Live or artificial rock arranged into caves and overhangs.
• A well-fitting cover for the aquarium.
• A siphon, buckets kept only for aquarium use, and marine salt for water changes.

Rock is essential

Marine crustaceans need very little décor, but OATA calls live or artificial rock essential. Rock gives a crab surfaces to graze and, just as importantly, places for a freshly moulted animal to hide while its new exoskeleton hardens. Build caves, overhangs and gaps at the base of the rockwork, and check that every piece sits firmly so that a climbing crab cannot shift it.

Keep the tank covered

Among the warning signs OATA lists for marine crustaceans is repeated attempts to leave the water. A well-fitting cover means that a crab trying to climb out stays safely inside the aquarium while you work out why, and it keeps the crab away from pipework and equipment above the waterline.

Choosing the right marine set-up

Most keepers thinking about this crab have one of a few kinds of marine aquarium in mind. The same OATA principles decide each case: reef-safe fish only, water set for the most sensitive animal in the tank, and careful thought about slow-moving invertebrates.

Reef aquarium

A reef can suit the crab, with two provisions. OATA notes that some of its suggested water values need altering in a reef aquarium to suit more sensitive species, so the reef's own residents set the targets. A reef also tends to hold slow-moving invertebrates, the animals OATA singles out as being at risk from opportunistic crabs, so think about each of them before you decide.

Fish-only aquarium with live rock

A marine tank of reef-safe fish and plenty of live rock gives the crab what OATA considers essential: rock to graze and hide in, and filtration that can include the live rock itself when the water flow through it is sufficient. If the tank holds few other invertebrates, there is also less to weigh up on compatibility.

Tanks with fish that are not reef-safe

If your fish are not reef-safe, this crab is the wrong choice. OATA's guidance is that such fish are likely to prey on marine crustaceans and should not be kept with them.

Smaller aquariums

OATA's figure of at least 40 litres is written for the smaller crabs and shrimp, alongside its advice to buy as large an aquarium as you reasonably can. Plan for a larger system rather than the minimum, especially when the crab will share the tank with fish and other invertebrates.

Which set-up fits?

Water chemistry for a marine crab

The OATA care sheet for marine crustaceans describes the group as usually undemanding of water chemistry and gives the suggested parameters below. They are broad guidance for marine crustaceans as a whole rather than a species-specific optimum for Percnon gibbesi, so the aim is to hold your water steadily within them.

Reef aquariums

OATA points out that some of these values will need adjusting in a reef aquarium to suit its more sensitive species. If this crab is joining a reef, set the water for the most demanding animals in the tank, keep it steady, and make sure ammonia and nitrite stay at zero.

A testing routine

• Test ammonia, nitrite, nitrate and pH before each weekly water change.
• Check salinity at the same time, and again after any large top-up.
• If you keep corals or other animals that depend on them, test calcium, magnesium and carbonate hardness as those animals require.
• Write the results down. A slow drift is far easier to spot in a log than from memory.

Salinity and evaporation

Water evaporates from a marine aquarium but the salt stays behind, so salinity creeps upwards between water changes. Replace evaporated water with fresh water rather than more salt water, and mix every batch of replacement salt water to the same salinity and temperature as the tank before it goes in. Keeping the reading inside 1.023–1.026 without sudden swings is the goal.

When a reading is off

Routine maintenance

For marine crustaceans, OATA strongly recommends a partial water change of 25% at least once every week. Good husbandry is essential, because crustaceans can be stressed by even the smallest amounts of ammonia and nitrite, and that stress may lead them to develop various diseases.

The weekly change, step by step

1. Test the water so you know the starting point.
2. Prepare the replacement salt water in advance and bring it to tank salinity and temperature.
3. Siphon out a quarter of the water, lifting solid waste from the substrate as you go.
4. Set aside a bucket of the removed tank water in case any filter media needs a rinse.
5. Refill slowly, then check salinity and temperature again.

Filters and skimmers

Check and clean filters regularly so they do not block. When media needs cleaning, do not run it under the tap: any chlorine or chloramine present may kill the beneficial bacteria that have built up in it. Rinse it lightly in the tank water you removed during the water change instead, which loses far fewer bacteria. The protein skimmer needs regular cleaning too, or it quickly stops pulling its weight.

Working around the rock

If you need to move rock during maintenance, look before you lift. A crab that has just moulted may be sheltering underneath, soft and vulnerable, and its cast shell may be lying close by.

Feeding

A varied diet

OATA's feeding advice for marine crustaceans is that their needs vary, but most species are omnivorous and will take algae, detritus and small meaty items. A varied menu covers those needs:

• Nori algae sheets.
• Small frozen foods, such as brine shrimp.
• Pellets.

OATA reserves larger meaty foods, such as pieces of lancefish, cockles or mussels, for bigger crustaceans like lobsters; they are not the everyday diet for a crab of this kind.

How often to feed

Many crustaceans graze throughout the day, and OATA explains that for this reason it is not possible to say how many times they should be fed. Much depends on how much natural food your aquarium already produces: a tank with established rock and some algae growth offers far more between meals than one that has just been scrubbed clean. When you do give extra food, offer small portions and take care not to overfeed, because uneaten food breaks down and releases toxic waste into the water.

Algae and grazing

Some crustaceans are bought to graze unwanted algae. OATA's advice for herbivorous species is to supply extra algae as nori sheets once they have eaten the natural supply, so a grazer is never left short when the rock is clean. Whatever your crab chooses to pick at, plan to feed it rather than expecting it to live on the rockwork alone.

Making sure food reaches the crab

In a busy tank the fish often reach food first. If the crab seems to miss out, place a small portion close to the rockwork where it forages once the fish have been fed. A piece of nori clipped or wedged against the rock stays put long enough for a crab to find it.

Watching how it eats

Keep an eye on where the food goes. A crab that picks over the rock, comes out when food lands and leaves nothing behind to rot is feeding well. Reduced intake or a lack of interest in food is on OATA's list of warning signs, so if the crab stops eating outside a moult, test the water first.

Moulting and growth

Why crabs shed their shells

Crustaceans have a hard exterior, the exoskeleton, and a body divided into segments. To grow, a crab sheds its exoskeleton, and OATA notes that it is common to find empty shells in the aquarium as a result. After shedding, the new exoskeleton takes a couple of days to harden, and the animal will often hide during this time.

Is that a dead crab?

Very often, no. An empty cast shell lying in the open, with the living crab out of sight in the rockwork, is the normal result of a moult. Look closely before you remove anything: a cast shell is light and hollow and usually looks complete, legs and all. Leave the crab in peace until it comes back out on its own.

Helping a moult go well

• Keep the water steady; stable conditions are the best support you can give.
• Provide plenty of rock, so there is always a sheltered spot where the new shell can harden.
• Avoid moving rock or netting the crab while it is soft.

Moulting problems to watch for

OATA lists difficulty in moulting, incomplete shedding of the exoskeleton, a soft-shell condition that goes on for too long, and changes in the usual moulting frequency among the warning signs in marine crustaceans. If you notice any of them, test the water and send us the readings.

Arrival and settling in

Every order is hand-packed by our team. The crab is hand-bagged in pure oxygen and packed in an insulated poly box, with a heat pack when the weather calls for it, and a licensed live-animal courier carries the parcel on a tracked service. We email you the tracking number and your confirmed delivery date. Someone must be at the address to receive the parcel, because live animals cannot be left unattended. The full terms are on our delivery and Live Arrival Guarantee page.

From doorstep to aquarium

1. Start a continuous video with the parcel still sealed and the outer box clearly in view.
2. Keep filming in the same take as you open the box and show the crab still inside its sealed bag.
3. Float the sealed bag in the aquarium for 20–30 minutes before releasing the crab.
4. Release the crab without pouring any of the bag water into your aquarium.

The parcel needs to be opened and inspected on the day it arrives. If anything has gone wrong, contact us that same day, WhatsApp being the fastest route, with your order number and the unboxing video, and we will arrange a replacement or refund.

The first weeks

Before the delivery day, confirm that ammonia and nitrite read zero and that salinity and temperature are inside the ranges above. After release, let the crab find its own way into the rockwork. Because the filter has to adjust to every new animal, test more often than usual for the first couple of weeks and keep extra feeding light while it catches up.

Health and warning signs

What a settled crab looks like

A settled Sally Lightfoot crab walks and climbs with co-ordination, keeps all its legs and claws, shows an intact exoskeleton and takes an interest in food. Learn how your own crab normally behaves, because a change from that everyday pattern is what tells you something is wrong.

Warning signs

OATA lists these changes as early signs of trouble in marine crustaceans:

• Movement: reduced activity, long spells of inactivity, difficulty walking, loss of co-ordination, repeated attempts to leave the water or unusual movement patterns.
• Colour: turning darker or paler than normal.
• Temperament: a change in aggression, or hiding more than usual outside a moult.
• Condition: damage to the exoskeleton, limbs, claws, antennae or eyes; lesions, discolouration, white or fluffy growths; or the loss of limbs.
• Moulting: the problems described in the moulting section.
• Feeding: eating less, or losing interest in food.

Keeping a simple record

A few lines in a notebook make changes far easier to spot: the date of each moult, how the crab is eating, and your water readings. OATA counts a change in the usual moulting frequency among its warning signs, and you can only notice that once you know what is usual for your own crab.

What to do first

Test the water before anything else. Crustaceans are so sensitive to ammonia and nitrite that the readings are usually the quickest clue. Share the results with us along with a description of what you have seen, and hold off adding any treatment to the aquarium until you know what you are dealing with.

Tank mates and stocking

Fish

OATA's guidance is that marine crustaceans will generally mix with fish considered reef-safe, while fish that are not reef-safe are likely to prey on them and should not be kept with them. Check every fish on your list against that line before adding a Sally Lightfoot crab, including any fish you plan to buy later.

Other invertebrates

Invertebrate companions need more thought. OATA notes that some scavenger species, crabs and lobsters among them, are opportunistic and may prey on slow-moving aquarium inhabitants, so they should be mixed carefully. Think about every slow-moving animal already in the tank before the crab goes in, and watch closely during its first weeks.

If the aquarium also holds hermit crabs, OATA's advice is to give them ample food and plenty of spare shells, so they are not tempted to attack snails for their shells.

Crustaceans with different roles

OATA's care sheet is a good reminder of how varied marine crustaceans are. Mithrax crabs and hermit crabs graze algae, peppermint shrimp eat unwanted Aiptasia anemones, and cleaner shrimp may pick parasites from larger fish. Because size, habits and diet differ so much across the group, no single rule covers every combination, and each pairing deserves its own check.

Reef-safe fish in practice

Reef-safe is a description that belongs to individual fish species, so check it for every species on your list rather than assuming it for a whole family. When you are unsure about a fish, settle the question before buying either animal.

Mixing invertebrates

Marine crustaceans differ widely in size, habits and compatibility, and OATA advises checking compatibility before any crustacean goes into an aquarium. Our marine invertebrate range covers animals with very different needs, so ask us about a specific combination before you buy rather than assuming two invertebrates will share a tank happily.

Stocking order

Add the crab to a settled aquarium rather than alongside a batch of other new arrivals, see how it uses the rockwork and behaves around your other animals, and add further livestock only once the filter has caught up and there is room. That slow approach follows OATA's general rule on stocking and gives you time to spot trouble early.

Quick compatibility checklist

Sally Lightfoot Crab questions

What water does a Sally Lightfoot crab need?

Stable, fully marine water. OATA's broad guidance for marine crustaceans is salinity 1.023–1.026, 24–26°C, pH 7.9–8.3, carbonate hardness 8–12°dKH, calcium 380–450 ppm and magnesium 1250–1350 ppm, with ammonia and nitrite at zero and nitrate no higher than 50 mg per litre.

What size aquarium does Percnon gibbesi need?

Buy as large an aquarium as you reasonably can. OATA's minimum of 40 litres applies to the smaller crabs and shrimp and larger crustaceans need more room, so treat 40 litres as a floor rather than a target, and furnish the tank with plenty of live or artificial rock.

Is a Sally Lightfoot crab a good choice for a beginner?

Only once your marine aquarium is established and stable. Crustaceans can be stressed by even the smallest amounts of ammonia and nitrite, so a new or still-cycling tank is the wrong place to start, while a keeper who tests the water and changes 25% of it every week is well placed.

What does it eat?

Offer a varied diet. OATA's advice for marine crustaceans, most of which are omnivores, is nori algae sheets, small frozen foods such as brine shrimp, and pellets. Many graze through the day, so give small extra portions and take care not to overfeed.

Is Percnon gibbesi aggressive?

Treat it as an opportunist rather than a harmless ornament. OATA notes that scavengers such as crabs may prey on slow-moving tank mates, so introduce it carefully and watch closely during the first weeks.

Which tank mates suit a Sally Lightfoot crab?

Reef-safe fish are the usual companions, since marine crustaceans generally mix with them. Fish that are not reef-safe are likely to prey on crustaceans and should not share the tank, and slow-moving invertebrates need careful thought.

How does a Sally Lightfoot crab arrive alive in the UK?

Our team hand-bags it in pure oxygen and packs it in an insulated poly box, with a heat pack when the weather calls for it, and a licensed live-animal courier carries the tracked parcel. Film one continuous unboxing video from the sealed parcel to the crab in its bag, then float the bag for 20–30 minutes before releasing it.

References

These public sources support the identity and care information on this page.

• OATA: how to look after marine crustaceans. The UK trade body's care sheet for marine crabs, shrimps and lobsters as a group, covering water values, aquarium size, filtration, rock, maintenance, feeding, moulting, health and compatibility.
• Wikipedia: Percnon gibbesi. Common names, including Sally Lightfoot, nimble spray crab and urchin crab, and its description as an invasive species in the Mediterranean.
• World Register of Marine Species: Percnon gibbesi (H. Milne Edwards, 1853). The accepted scientific name and its family, Percnidae.
• RSPCA: fish environment. General welfare guidance on aquarium filtration and the nitrogen cycle.

Packing, courier and Live Arrival Guarantee terms are set out on the Tropical Fish Co delivery page, linked in the arrival section.