The Marine Life Information Network

Pachycordyle navis lives attached to algae, so would be removed with the algae upon substratum loss. There would be no recovery of the population because only two extant populations of Pachycordyle navis are known: Widewater lagoon, Sussex and Kiel Canal, Germany.

The species would be affected by smothering if the algae on which it lives is completely covered in the sediment. If the algae protrudes sufficiently above the sediment the hydroid may escape the effects of smothering.

Pachycordyle navis is likely to have some tolerance to siltation as it inhabits lagoons where siltation frequently occurs. The algae on which the species lives will also lift the hydroid above the accumulation of silt. However, the heath of the host algae may be adversely affected by siltation.

The species is vulnerable to desiccation because it is soft bodied and has no protection from the drying effects of sun and wind. Some of the population may be sheltered from desiccation if they are present on the underside of the algal frond. However, if the whole population is destroyed recoverability would be non-existent because only two populations of Thieliana navis occur worldwide.

The species is vulnerable to emergence because it is soft bodied and has no protection from desiccation. Some of the population may be sheltered from desiccation if they are present on the underside of the algal frond. However, if the whole population is destroyed recoverability would be very low because only two populations of Pachycordyle navis occur worldwide.

The species would probably not be affected by a change in water flow because it is permanently attached to the algae and may be able to withstand high water flow rates because they have been transported long distances on ships hulls.

The temperature resistance of the Pachycordyle navis is not known.

The species is unlikely to be affected by a change in turbidity as it is not dependant on light availability and it would not interfere with its feeding. However, the host algae may be adversely affected by a reduction in light availability.

A change in wave exposure is unlikely to occur in a lagoon unless one of the lagoon boundaries is breached. The species would probably not be affected by an increase in wave exposure because it does not present a large surface area to wave action. However, it's host algae may be intolerant of wave exposure and may be washed away.

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The species and its host algae are flexible so will 'give' under abrasion. However, they occur on top of the sediment and would probably be removed, along with surface substratum by a passing scallop dredge (or equivalent force). The impact is likely to be equivalent to substratum loss. Therefore, an intolerance of high has been recorded.
Hydroids are generally regarded as opportunistic species with good recruitment, the ability to reproduce asexually or sexually, colonize space rapidly and with good powers of recovery from damage (see Boero, 1984; Gili & Hughes, 1995). Hydroids can form highly resistant resting stages and recover or spread by fragmentation (Gili & Hughes 1995). Therefore, hydroids are likely to recover rapidly from physical disturbance from resting stages or pieces of hydrorhizae on the remaining substratum, or fragments. However, Pachycordyle navis releases planulae from its gonothecae that probably have limited dispersal capability (see Sommer, 1992; Gili & Hughes, 1995). It is an introduced species thought to have been transported by shipping, either on the hull or in the ballast water (Reise et al., 1999) but has a very limited distribution, which suggests either a limited recruitment capability and/or a narrow range of environmental preferences. Although it was recorded in the Widewater lagoon in 1973, its has not been recorded from any other sites in the UK since. It seems unlikely that it can recruit from other areas, or extremely slowly, save by the chance anthropogenic introductions, e.g. via shipping. If the population was completely destroyed by physical disturbance then recovery is unlikely. Nevertheless, the population may recover from resting stages or fragments. Therefore, a recoverability of low has been recorded.

Pachycordyle navis is permanently attached to algae and would be unable to re-attach itself if removed. If the whole population is destroyed recoverability would be very low because only two populations of Pachycordyle navis occur worldwide.

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Evidence suggests that the species is tolerant of fully saline conditions because it can survive on ships hulls. The species must be tolerant of reduced salinity because it occurs in lagoons but the tolerance of the species to very reduced salinities is not known.

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