Cornish jack

Care at a glance

Overview

This is a metre-and-a-half-long electric predator, the giant of the elephantfish family, and it is really a public aquarium animal rather than a home fish. It is aggressive and must be kept alone, since it will eat tankmates it can swallow and does not share space peacefully. Housing it means thousands of litres, on the order of 4000 or more, with a deep soft floor, heavy filtration, and dim, stable water. Feed meaty foods such as whole fish, prawn, and earthworm. This entry exists mainly so the small, appealing juvenile in a shop tank is bought with its adult size understood.

In the wild

Mormyrops anguilloides occurs across much of tropical Africa, from the Nile through the great rivers and lakes, hunting smaller fish over open bottoms and among cover at night. No home tank can reproduce the scale it needs, but the water it wants is warm, 23-28 C, and neutral, around pH 6.5-7.5, over sand with wood and rock for structure. In the wild it patrols both flowing rivers and quiet lake margins.

Good to know

At up to 150 cm this is the largest member of the family Mormyridae. It was named by Linnaeus in 1758. Unusually for a fish, it hunts in coordinated groups at night, its members using their weak electric discharges both to locate prey and to keep in contact with one another. It is a weakly electric fish that is nonetheless a serious piscivore. Aquarium breeding is not documented.

Did you know

It is one of the few fishes known to hunt in packs, coordinating nighttime attacks on other fish through pulses of electricity in water far too dark to see in.

Common questions

Related species

• Nile bottlenose mormyrid

  Mormyrus kannume

  Freshwater

  35.4 in (90 cm)185 gal (700 L)75–82°F (24–28°C)
• West African bottlenose mormyrid

  Mormyrus rume

  Freshwater

  31.5 in (80 cm)185 gal (700 L)75–82°F (24–28°C)
• Eastern bottlenose mormyrid

  Mormyrus longirostris

  Freshwater

  29.5 in (75 cm)185 gal (700 L)75–82°F (24–28°C)

• 

  Bebe mormyrid

  Hyperopisus bebe

  Freshwater

  20.1 in (51 cm)264 gal (1,000 L)73–82°F (23–28°C)

• 

  Elephant trunk mormyrid

  Campylomormyrus elephas

  Freshwater

  15.7 in (40 cm)119 gal (450 L)75–82°F (24–28°C)
• Painted mormyrid

  Hippopotamyrus pictus

  Freshwater

  11.8 in (30 cm)159 gal (600 L)73–82°F (23–28°C)