Taxonomy

General appearance


Colour
Agalma elegans:

• is extremely transparent
• has groups of orange-red stinging batteries on its main stem. These distinguish it from other UK string jellies.

Size
Mature individuals typically measure 30-100cm long.

Morphology
A. elegans has a flexible body which is composed of 3 main areas:

1. the pneumatophore
2. the nectosome
3. the siphosome

Pneumatophore:The pneumatophore is a small gas-filled float at the extreme anterior end of the animal.In Agalma elegans, this float has:

• a red-pigmented tip
• a terminal pore which can be used to expel gas and help regulate buoyancy

Nectosome:Next to the pneumatophore at the anterior end, the animal has a series of swimming bells (nectophores). Together, these bells form the nectosome.The presence of a float and many swimming bells (and a long siphosome) make Agalma elegans a long-stemmed physonect siphonophore.The bells:

• are soft
• usually detach when the animal is captured
• each have a series of distinctive ridges on their upper surface, which can be seen under a binocular microscope (with staining)
• have an aperture known as the ostium at their distal end, which opens into a muscular bag called the nectosac

Contraction of the nectosac forces water out through the ostium. During swimming, all nectosacs contract together and the animal progresses forwards by jet propulsion. The proximal surface of each nectosac is flattened in Agalma elegans and lacks the bi-lobed extensions found in the nectosacs of most other UK physonects.Siphosome:Behind the nectosome is the main stem (siphosome), which is composed of up to 30 repeated units (cormidia). In Agalma elegans, each cormidium extends along the stem and comprises:

• a gastrozooid, a feeding polyp with:
  • a mouth
  • a single tentacle arising from the stem end, which bears 12 to 50 orange-red tentilla
• many bracts, which:
  • are leaf-shaped and transparent
  • help buoy up the stem
• many palpons - these are modified mouths which aid digestion of the prey
• some reproductive organs (gonophores)

In Agalma elegans, the tentilla comprise:

• an orange-red cnidoband of 3 coils, enclosed in a transparent sac (involucrum)
• 2 long terminal filaments
• a small ampulla

The reproductive organs are either male or female. Each cormidium contains both, but they will not become mature at the same time in any one individual.

Identification
Agalma elegans can be identified by the:

• pattern of ridges on the nectophores
• tentilla on the tentacles
• flexibility of the stem