- Kingdom: Animalia
- Phylum: Cnidaria
- Class: Anthozoa
- Subclass: Octocorallia
- Order: Pennatulacea
- Suborder: Subsessiliflorae
- Family: Virgulariidae
- Scientific name: Virgularia mirabilis
- Norwegian: liten piperenser
The slender sea pen may reach a length of 60 cm. It can be distinguished from the larger tall sea pen by the fact that the polyps are living on side branches rather than sitting attached directly to the main stem. It is however, more easily confused with V. tuberculata which has generally larger, but fewer polyps on each branch. The polyps of V. mirabilis sometimes form groups of as many as 16 individuals on each branch. The stem itself is only a few millimeter thick. The color of the colony is usually white, beige or bright orange.
The slender sea pen is common on locations with fine sediments, on shallow, sheltered locations or on deep water, down to 400 meters.
This is a common species along the coasts of Europe and the British Isles. It is registered from the Mediterranean in the south to northern Norway in the north. It may very well be widespread over the entire North Atlantic Ocean.