Hydromedusae

Sea Wasp

Although superficially similar to jellyfishes, hydromedusae are more closely related to sessile hydroids. They are much firmer and sturdier in body than jellyfishes. Most are small ( less than 1 inch in diameter ) and do not sting. Others, such as the large and very dangerous Sea Wasp ( 9", right ) do.

hydromedusae
White Cross Hydromedusa
White Cross Hydromedusa - Staurophora mertensi

The White Cross Hydromedusa is found along the coast from the Arctic to Rhode Island. At night it rises to just below the surface. It feeds on other medusae and crustaceans. In the northern part of its range, it is seen from spring through late summer. In the southern range, it is normally found from spring to early summer. It can reach 12" wide and 2" high and is not dangerous to humans.

Many-ribbed Hydromedusa
Many-ribbed Hydromedusa of the Aequorea species

The "many-ribbed" genus of hydromedusae are similar in appearance to clear jellyfishes, but lack obvious tentacles and do not sting when touched. These are among the largest hydromedusae known, up to 12" across, and the identity of their corresponding polyp stage is still a mystery. Hydromedusae are mostly water, and when they dry up they lose half of their diameter and almost all of their thickness, becoming sand dollar-sized thin, brittle discs.

Beached hydromedusae
Beached hydromedusae. By the time they get here, the surf has stripped away all the tentacles, leaving just a harmless dying blob of goo.
Herb Segars Photography

8-D-cell light with lantern grip
8-D-cell light with lantern grip
now extinct

In the murky waters off New Jersey, you are going to need a powerful dive light if it is to be of any real usefulness. There are many different types of underwater lights to choose from.

Bulb Type

Nowadays, LEDs are the only way to go. They are cheap, bright, and efficient. They throw a white light that is much better than the dingy yellow of the old incandescent technology. Incandescent lights are completely obsolete. HID lights were never reliable, and terribly expensive.

Beam pattern is probably more important than brightness, Your main light should throw a wide beam, for area illumination. Many lights throw a narrow pencil beam that appears to be brighter but is actually less useful. These are best used as backups and in special situations, such as camera strobe aimers. Some lights are adjustable.

Printed from njscuba.net