Spider Crab Profile – Detailed Guide

Neosarmatium meinerti (also known as the Red mangrove crab or Spider crab) is one of the rarest crab species in our hobby. If you by chance buy one you will be surprised to find out how little is known about them.

In short, Spider crabs are considered to be semi-terrestrial and mostly herbivorous. These crabs can make great pets in terrarium and paludarium setups.

In this article, I have gathered all information about Neosarmatium meinerti. It covers all aspects, from natural habitat conditions and how they should be cared for within your aquarium, to dietary requirements and how to breed them.

Quick Notes about Spider Crab

Name	Spider crab
Other Names	The red mangrove crab
Scientific Name	Neosarmatium meinerti
Tank size (minimum)	10 gallons (~40 liters)
Type	Semi-terrestrial
Keeping	Easy
Breeding	Very difficult
Size (carapace)	1 – 2 inches (2.5 – 5 cm)
Size across the leg span	3 – 4 inches (7 – 10 cm)
Optimal Temperature	73 – 82°F (23 – 28°C)
Aquarium type	Terrarium or paludarium
Moisture	60% and higher
Diet	Mostly herbivore/omnivore
Temperament	Semi-aggressive
Life span	up to 5 years
Color Form	Red-orange and yellow forms

Habitat of Spider Crab

Spider crabs are widely spread over the Indo-West Pacific Region (Madagascar, Seychelles, Republic of Mauritius, Kenya, Mozambique, Somalia, Australia, Sri Lanka, Taiwan, Thailand, Indonesia, Sulawesi, Fiji, Guam, Solomon Islands, and New Caledonia).

These crabs are the most abundant and widespread crab species of East African mangroves.

This species commonly occupies the dry and muddy area of the upper intertidal areas associated with mangroves and estuaries, where they dig deep burrows under the canopy and forage on sediment detritus and leaves.

Description of Spider Crab

Neosarmatium meinerti is a relatively large species. Fully grown Spider crabs can reach up to 3 inches (7 – 8 cm) long, with a leg span up to 5 – 6 inches (about 12 – 15 cm).

Note: However, generally, the average size (shell width) of the adult crabs is often near 1 – 2 inches (2.5 – 5 cm) long and 3 – 4 inches (7 – 10 cm) across the leg span.

These crabs have 2 different color morphs:

• Red-orange form (dominant). With red-orange claws, with the coloration extending to two-thirds of the proximal part of the outer margin of the palm and the fingers fading to yellow.
• Yellow form. With yellow claws and carapace.

Note: the Red-orange form is more frequently encountered whereas the yellow form can be found only in Australia, Taiwan, Guam, and Solomon Islands. There are even speculations on the
presence of two genetically differentiated groups.

Lifespan of Spider Crab

According to the study, Neosarmatium meinerti appears to live for about 5 years.

However, it can be assumed that if appropriately cared for, Spider crabs can live even longer.

Typical Behavior of Spider Crab

Neosarmatium meinerti is a semi-terrestrial species. In order to maintain an optimal moisture level, they construct deep burrows. Spider crabs have modified lungs and require high moisture to breathe.

Burrows provide a constantly available water resource. This is the main reason why they dig such deep burrows.

According to some studies, their burrows can reach up to 1 – 2 m (almost 3 – 7 ft.) below the soil surface, terminating in a chamber at the level of the water table.  

Interesting facts:

• Their burrows contain only one opening.
• Most of their burrows have a simple linear shape, and in some cases can be complicated by the presence of bends, rooms, and accessory branches.
• Juvenile Spider crabs (0.4 inches or 1 cm carapace long) do not dig their own burrows (they are too weak for that). Instead, they shelter in adult burrows during early life. Burrow casting revealed several narrow side branches to adult burrows.

Although Spider crabs are mostly nocturnal, they can be very active during the daytime as well.

Spider crabs are pretty destructive and may redecorate your paludarium to their liking. You should keep that in mind.

These crabs have very complex social behavior. On the one hand, this is a solitary and territorial species, they are not social. On the other hand, their burrows can be often found in close proximity to each other.

Anyway, it will be better to say that their aggression level is simply lower, compared to many other popular ornamentals crab species.

Features:

• Social: No
• Active: Yes
• Peaceful: No
• Burrowers: Yes 

Diet of Spider Crab

Initially, it was believed that this species was completely herbivorous.

However, recent studies showed that Neosarmatium meinerti should be classified as opportunistic omnivores (because they have adapted to eating both animal and plant material) with a primarily vegetarian diet.

Stomach content analysis showed that their diet mainly consisted of mangrove leaves, completed with little animal matter.

In captivity, for the best growth, Spider crabs need a good mix of meats and vegetation. These crabs are avid leaf and propagule eaters. Therefore, their feeds should contain protein at a level of about 10 – 15% of the diet (at most!).

Laboratory experiments showed that Spider crab can consume around 2% of its body weight per day.

Interesting fact: Leaf consumption per gram crab was higher in females than males.

Foods Spider crabs will enjoy (examples with links to check the price on Amazon), for example:

• Vegetables (like broccoli, cabbages, carrots, cucumbers, lettuce, corn, spinach, peas, squash, leafy greens, etc.).
• Fruits (Apple (only sweet), banana, pearl, melon, mango, etc.).
• Leaves.
• Shrimp pellets.
• Shrimp Granules.
• Fish food (TetraMin® flakes, etc.)
• Shrimp food (Hikari’s crustacean food like Hikari Shrimp Cuisine, Algae wafers, etc.)
• Frozen blood worms.
• Detritus worms.
• Brine shrimp.

Important: Leaves (for example, Almond leaves, dead beech, oak leaves, etc.) should always be on the menu, as they feed on these and require the detritus from the leaves. 

Features:

• Diet Type: Mostly herbivore/omnivore
• Food Preference: Leaves and plants
• Feeding Frequency: Daily 

You can also read my articles:

• • Indian Almond Leaves and Alder Cones in a Shrimp Tank.
  • How to Blanch Сucumbers and Zucchini for Shrimp, Snails and Fish the Right Way
  • What Do Crabs Eat and How to Feed Them?

How often should I Change the Food?

You can leave their food for 24 hours before removing it. Leaves can be left for weeks in the tank.

Make sure that whatever Spider crabs do not consume in one day is removed to prevent moles. Keep in mind that these crabs like to store food in their burrows.

How often should I Change the Menu?

Do not give them the same food for weeks. Ideally, you need to change their diet at least every week.

Are Spider Crabs Plant Safe?

Although Spider crabs prefer old or decaying over young leaf litter, they are still not safe to keep with plants.

Especially, if they are not saturated with food. In this case, they may consume anything, irrespective of the plant. They will eat, cut, and uproot everything that is possible in your tank.

Spider Crab: Calcium and Molting Cycle

In order to grow and/or restore lost limbs, Spider crabs must regularly molt (shed the old exoskeleton). Growing a new exoskeleton requires a high amount of Ca to facilitate calcification.

As with all crustaceans, calcium is a crucial component of a Spider crab’s exoskeleton and overall health.

Calcium can be found in kale, broccoli, spinach, cuttlebones, eggshells, figs, wonder shells, oyster shells, seaweed, seeds, insects, etc.

Always keep a small piece of Cuttlefish bone in their enclosure. Supplement their diet and make sure they get enough calcium (for the exoskeleton) by regularly feeding specialized invert foods.

In addition, the process of molting puts crabs in a vulnerable state. That is why it is crucial to have many hiding places in the tank.

Cuttlefish bones – link to check the price on Amazon.

Related articles:

• How to Supplement Shrimp and Snails with Calcium
• Crabs and Molting Process

Keeping and Housing Spider Crab

Neosarmatium meinerti is not an aquatic species of crabs. They are semi-terrestrial crabs and require a specific setup to live.

The good thing though is that these crabs are hardy and can survive in your learning curve.  

Tank Size:

Therefore, the minimum recommended tank size for one Spider crab is a 10-gallon (40-liter) tank. These crabs need a lot of surface area to roam.

As adults, one male and one female may be kept together in the same enclosure of 30 gallons (120 liters) or more, but two adult males would need far more space to reduce territorial fighting.

Important: Spider Crabs are very good escape artists. It is good to have your tanks covered as much as you can.

Land vs Water Ratio:

Ideally, the land area should take at least 90% of the space in your tank or even more.

Substrate:

The tank should be filled with a combination of moist sand or mud into which they can burrow.

In order to dig underground and form a cave, the substrate should always be kept moist enough. It means that it should hold its shape when you squeeze it. However, not so wet that it drips or pools water.

To get “sandcastle consistency”, a mixture of cocofiber and sand will be the best option for Spider crab setups.

Tip: There is a simple trick to test the consistency. Take a pencil and stick it all the way down and pull it back up. If the tunnel doesn’t collapse, your substrate is good enough.

The substrate may also require maintenance such as spraying with fresh dechlorinated water on a regular basis in order to keep it moist enough since the moisture in it will evaporate over time.

Important: Do not ever use saltwater to maintain moisture as salt does not evaporate and can lead to a harmful salt build-up.

How deep should be the substrate in the Spider crab setup?

Therefore, the deeper the better. In the wild, Spider crabs are known to burrow very deep (up to 2 m or 7 ft.).

Therefore, I’d recommend having at least 10 cm (4 inches) deep.

Substrate options (check the price on Amazon): 

• Zoo Med Hermit Crab Sand, 
• Zoo Med Eco Earth Loose Coconut Fiber Substrate, 
• Carib sea sand.

Water Bowl and Water Type:

Spider crabs do not need a lot of water. So, instead of creating a complex tank setup, we can use water bowls.

The only thing that you need to keep in mind – these bowls should be deep enough to completely submerge your crabs.

Spider crabs are mangrove crabs. So, it will be better to provide them with:

• a bowl of freshwater and
• a bowl of saltwater.

To prepare saltwater you can use Instant ocean marine salt (check the price on Amazon). This is a great choice. It is pretty cheap and will last very long. You only need about a half cup of Instant ocean marine salt per gallon.

Note: Do not ever use simple aquarium salt or table salt!

Regarding freshwater, ideally, you need to give them something like bottled spring or distilled water.

If you decide to use tap water, let it age for 24 hours before using it. Tap water contains chlorine and it is toxic to them. 

You can also use a water conditioner. For example, Seachem Prime (link to check the price on Amazon) will remove also toxic gases, bind to heavy metals, any ammonia, nitrites, or nitrates present for up to 48 hours.

Temperature:

The optimal temperature range for Spider crabs is 73 – 82°F (23 – 28°C). However, it does not mean that they are very sensitive.

On the contrary, these are very hardy animals and can tolerate a huge range of temperatures.

For example, in their burrows, the temperature can be 59 F (15 C) while above the ground it can be 97 F (36 C). 

Tip: If you decide to use a heat pad, it should cover 1/3 (or less) of the tank so that crabs could move from warmer to colder places anytime they want. This allows your crabs to regulate their body temperature to their needs.

Humidity:

Spider crabs are pretty resilient to humidity fluctuations. In their natural habitat, humidity can range between 50% and almost 100%.

However, for optimal conditions, it is still recommended to keep the humidity between 60 – 80 %.

Humidity and Thermometer (links to Amazon):

• Zoo Med^™Reptile Terrarium Digital Thermometer, 
• Zoo Med Labs Digital Thermometer Humidity Gauge.

Lighting:

No special requirements. You may not even need one.

Decorations and Hiding Spots:

Spider crabs will appreciate all types of leaves, rocks, wood, plants, PVC pipes, and other decorations to enrich the environment in your tank.

It is extremely important to minimize stress to the crabs by giving them a lot of places to hide. This is also critical for the molting process!

Related article:

• Do We Need Tank Decoration? Simple Ideas

Rules: How to Care and Handle Spider Crab:

• Spider crabs are not pets you can play with. Do not take them out just because you want to.
• They should never be lifted up by their legs or claws!
• Do not leave uneaten food for too long. It can cause smell and bacteria in their enclosure. Remember, these crabs like to store uneaten food in their burrows.
• Provide as many hiding places as you can.
• The substrate should be based on sand and Eco-earth to give them a homely feeling.
• Make sure the lid of the tank is closed tightly so that they cannot escape it.

Related article:

• How to Handle your Pet Crustaceans

Spider Crab – Male and Female Differences

There are a few indicators that give away the gender of the animal.

1. Size. Females are also generally smaller than males.
2. Abdomens. Different shapes of the abdomens. Males have a narrow and slimmer plate (triangular) while females have a broad plate on their belly.

Related article:

• How to Tell If a Crab is Male or Female

Breeding Spider Crabs

According to the study, in Neosarmatium meinerti, the estimated size (carapace length) at which males and females reached sexual maturity are 0.7 inches (17.3 mm) and 0.6 inches (15.6 mm) respectively.

Note: It is also known that sexual maturity can vary considerably when the different geographical locations are taken into account. For example, low temperatures lead to slow growth rates, with an increase in the time needed to reach sexual maturity.

Females release larvae into saltwater in synchrony with spring tides.

After the hatching, larvae have a supposed lasting period in the plankton of about 4 weeks.

In the open oceanic waters, larvae pass through a determined number of stages. The last larval stage, the megalopa, returns to the mangroves guided by chemical and physical cues.

In nature, Neosarmatium meinerti has a distinctive spawning peak every year during February, followed by offshore larval development from February to March, suggesting that the megalopae and first crab stages recruit back into the mangroves during March and April.
After that, they start living within burrows of adult crabs, in diverticula starting from the main branch of the burrow.

Spider Crab and Suitable Tankmates

Before putting any new tank mate in with your Spider crab, you should take some things into consideration:

• Although this species is not very aggressive, these crabs are still not peaceful and inoffensive. Remember, Spider crabs are not social. It can be risky to house multiple animals in the same tank as well.
• They are territorial (especially males). Males are extremely likely to fight when housed together.
• Do not make them hungry. It will increase their aggression and even the risk of cannibalization.
• Do not keep them with Hermit crabs. Spider crabs dig a lot. So, it can be very dangerous to any underground molting

Ideally, the Spider crab should be placed in a tank alone or with other crabs of another sex.

In this case, they should be well-fed at all times and there must be a lot of hiding places. Nonetheless, even having lots of hiding places in the tank and keeping them well-fed will not guarantee peaceful coexistence.

In a paludarium setup, it is possible to keep dwarf shrimp, snails, and fish with them. Spider crabs usually do not show much interest in going into the water.

In Conclusion

With their striking coloration, Spider crabs will definitely be an attractive addition to any paludarium setup.

Caring for these semi-terrestrial crabs is extremely easy and can be recommended even for beginners. The only problem is that this species is very rare in the pet trade.

References:

1. Dahdouh-Guebas, Farid, Marc Verneirt, J. F. Tack, and Nico Koedam. “Food preferences of Neosarmatium meinerti de Man (Decapoda: Sesarminae) and its possible effect on the regeneration of mangroves.” Hydrobiologia 347, no. 1 (1997): 83-89.
2. Ólafsson, Emil, Susanne Buchmayer, and Martin W. Skov. “The East African decapod crab Neosarmatium meinerti (de Man) sweeps mangrove floors clean of leaf litter.” AMBIO: A Journal of the Human Environment 31, no. 7 (2002): 569-573.
3. Ragionieri, Lapo, Sara Fratini, and Christoph D. Schubart. “Revision of the Neosarmatium meinerti species complex (Decapoda: Brachyura: Sesarmidae), with descriptions of three pseudocryptic Indo-West Pacific species.” Raffles Bulletin of Zoology 60, no. 1 (2012).
4. Berti, R., S. Cannicci, S. Fabbroni, and Gianna Innocenti. “Notes on the structure and the use of Neosarmatium meinerti and Cardisoma carnifex burrows in a Kenyan mangrove swamp (Decapoda Brachyura).” Ethology Ecology & Evolution 20, no. 2 (2008): 101-113.
5. Emmerson, W. D. “Aspects of the population dynamics of Neosarmatium meinerti at Mgazana, a warm temperate mangrove swamp in the East Cape, South Africa, investigated using an indirect method.” In Advances in Decapod Crustacean Research, pp. 221-229. Springer, Dordrecht, 2001.
6. Pedersen, Carsten, Bernadine I. Everett, Peter J. Fielding, Wendy D. Robertson, and Robert Kyle. “Subsistence utilization of the crab Neosarmatium meinerti in the Kosi Lakes ecosystem, KwaZulu-Natal, South Africa.” African Zoology 38, no. 1 (2003): 15-28.
7. Litulo, Carlos. “Size at sexual maturity in the red mangrove crab Neosarmatium meinerti (De Man, 1887)(Brachyura: Grapsidae).” Western Indian Ocean Journal of Marine Science 4, no. 2 (2005): 207-210.