The Slender Sunfish

A slim oddity of warm open seas

The Slender Sunfish is a strange open water fish of warm seas. It is 1 of the smallest members of its family. Its body is long and flattened from side to side. It has no proper tail fin.

A rounded rudder takes the place of a tail. The fish drifts in the open ocean and follows warm currents. It is rarely seen alive by most people.

Family tree

Tetraodontiformes, Molidae

The Slender Sunfish belongs to the phylum Chordata. Its class is the Actinopterygii, the ray finned fishes. Its order is the Tetraodontiformes. Its family is the Molidae, the sunfishes.

Our fish shares the family with the Ocean Sunfish. That fish grows to about 2 to 3 m across. It looks like a huge flat disc. Our fish is far slimmer and shorter.

Both belong to the same small family. The family holds only a handful of species.

100 cm and no tail fin

How to tell one

The fish reaches about 100 cm from the snout to the rudder. It is much longer than it is deep. The skin is smooth and has no spines. Fine plates of bone sit under the surface.

The colour is silver grey with pale mottling. Dark bands may run along the flanks. Two tall fins stand at the top and the bottom. Those fins are the main means of swimming.

The mouth is small and shaped like a short beak. The teeth are fused into a single plate.

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The open ocean far from land

Where it lives

The species lives in the open ocean far from land. It prefers warm and mild seas across the world. It swims near the surface on calm days. It may also drop to deeper water to feed.

The young drift among floating weed and jellyfish. Sick animals are sometimes carried onto beaches. It is found around the Mediterranean in some years. Many records come from strandings rather than from boats.

The fish needs clean open water to move freely. Cold currents set a limit to its range.

Jellyfish, salps and floating eggs

What it eats

The Slender Sunfish feeds on soft drifting animals. It takes jellyfish, salps and small free floating creatures. It also takes fish eggs and tiny larvae. It sucks its food in through the small beak.

Its teeth are fused into a plate, so it cannot chew. It swallows the prey whole. The fish may follow a bloom of jellyfish for days. It feeds slowly and often.

It needs a great deal of food to grow. Its flesh is soft and full of water.

A very large number of floating eggs

The breeding cycle

Very little is known about the breeding of this fish. The female is thought to release a very large number of eggs. Those eggs float in the open water. The male releases his cells into the same water.

The young hatch as tiny drifting larvae. Those larvae look nothing like the adults. They carry spines and look like small spiky balls. They drift with the plankton for many weeks.

Then they change into the adult shape. The grown fish lives alone in the open sea.

Drift nets and floating plastic

Conservation

The Slender Sunfish is listed as Least Concern by the IUCN. It is spread across most warm seas. It is caught by accident in drift nets.

Long line fishing also takes some animals. Floating plastic is a growing problem for it. The fish mistakes plastic for jellyfish and swallows it.

Boat strikes cause a few deaths each year. The species is hard to study, so its numbers are unclear. Clean open water is its main need.

Quick hits

Five things to remember

• It swims mainly with the two tall fins on its back and belly.
• Its teeth are joined into one plate, so it cannot chew.
• The young fish look like tiny spiky balls.
• It has no true tail fin at all.
• Most records come from fish found on beaches.