Occurrence and Habitat Characteristics of Aurelia sp. Polyps in a High-Latitude Fjord

Causes and consequences of jellyfish bloom formation are subject to controversial discussions worldwide. While medusae have been studied to a broader extent, the knowledge on polyp stages of scyphozoans is limited thus hampering reliable prediction of jellyfish bloom formation. This study describes the occurrence, abundance, habitat characteristics and interactions of scyphozoan Aurelia sp. polyp colonies with other fouling organisms in intertidal and subtidal sectors of Trondheimsfjorden (Norway). In total, 982 polyps were found on 70 substrata of varying material types during a field survey in spring-summer 2018 along a longitudinal gradient within and outside Trondheimsfjorden. The polyps were identified as Aurelia sp. based on molecular species identification. Most polyps were found in bays with macroalgae canopy on the down facing side of artificial and natural substrata (rocks, concrete, iron) and inside rock cracks. Polyp microhabitats included the surface of Ascidia mentula (solitary ascidian), increments of Pomatoceros triqueter (polychete) tubes and dead Balanus balanoides (barnacle) shells. Based on the deployment of settling plates, abundance of Aurelia sp. polyps ranged from 1.2 (± 0.7) to 0.12 (± 0.07) polyps cm–2. Settlement occurred either directly on the PVC settling plates or as epibionts on the ascidian Ascidia mentula and on barnacle shells of Balanus balanoides. This study provides insights into the potential of local Aurelia sp. polyps contributing to the seasonal occurrence and abundance of Aurelia sp. in fjord systems, where intensive blooms occur annually.

On the antennular secretion of the cyprid of Balanus amphitrite amphitrite, and its role as a settlement pheromone

In exploring a substratum, Balanus amphitrite amphitrite Darwin (Crustacea: Cirripedia) cyprids deposit ‘footprints’ of antennular secretion. The results of in vitro settlement assays suggest that in addition to serving as a temporary adhesive, the secretion acts as a pheromone, in that its presence induces the settlement of conspecific cyprids. This result is in accord with a previous study on Balanus balanoides (L.)(=Semibalanus balanoides). In settlement assays, the pheromone is likely to contribute to an observed positive linear relationship between settlement and cyprid density. The density effect should thus be an important consideration in the design of barnacle settlement assays. In the field, cyprid searching behaviour may render a surface more attractive to settlement by cypris larvae.

The relation between the dimensions of the cirral net, the beat frequency and the size and age of the animal in Balanus balanoides and Elminius modestus

The expected allometry between the linear cirral length and animal volume or weight for isometry should be 0·33; similarly the rate of beating, db/dt, might also be expected to increase as −0·33 with volume. The average allometry index of cirral length is less in both Balanus balanoides (0·22–0·23) and in Elminius modestus (0·27–0·32). The segment numbers also rise less than expected.The rate of beat in Balanus balanoides correlates well with size (-0·28) and with temperature with a correlation coefficient of 0·898, significant for both variables. For a group of 11 species the relation between maximum rate of beating and size over 5 decades gives an approximate allometry of −0·24 which is also less than the expected −0·33 for an isometric linear appendage moving at constant velocity. It is suggested that the approximately reciprocal allometry index between cirral beating rate (−0·28) and cirral length (+0·22) may be mutually compensating, resulting in the cirri moving through the water at a constant rate during the growth of the animal. This rate may maximise the chance of prey being captured.

Identification and hatching factor activity of monohydroxyeicosapentaenoic acid in homogenates of the barnacle Elminius modestus

A crude eicosanoid extract of the barnacle Elminius modestus caused hatching of isolated egg masses from both E. modestus and Balanus balanoides . On thin-layer chromatography (TLC) two bands containing hatching factor activity were identified, a monohydroxy fatty acid (MHFA) band with potent hatching factor activity and a less active trihydroxy fatty acid (THFA) band. Gas chromatography-mass spectrometry (GC-MS) of the MHFA band showed that it consisted mainly of positional isomers of monohydroxyeicosapentaenoic acid, predominantly 8-hydroxyeicosapentaenoic acid (8-HEPE). A range of positional isomers of monohydroxyeicosapent­aenoic acid was produced by autoxidation of the methyl ester and this mixture was highly active in hatching E. modestus eggs. The commer­cially available stereoisomer 8( S )-HEPE hatched E. modestus eggs at concentrations as low as 10 -8 M. The saturated derivative 8-hydroxyeicosanoic acid did not show hatching factor activity.