Tropical vagrants - A sign of the times

Most marine animals are cold blooded, meaning the temperature of the surrounding water is vital for their existence. It maintains their metabolism, growth rates and eventually reproduction, such that species generally have an ideal temperature window in which each of these traits are ideal.

So why is it that we sometimes encounter individuals rarely, and usually outside their normal range?

Firstly, we need to consider oceanography. Here in northern New Zealand we are bathed in relatively warm water from currents originating in the tropics as part of the South Pacific subtropical gyre. Eastward-flowing currents driven by the trade winds in the South Pacific travel across the Tasman Sea in the form of the East Australian Current, or the EAC, made famous by the movie Finding Nemo. This conveyor of tropical water then splits upon reaching the tip of New Zealand, flowing south along both coasts. It is the north-eastern coast however, that gets the most of the warm current, and this is both getting stronger and extending further. With such a large-scale movement of water, it is not surprising that this serves as a pretty good vehicle for tropical animals.

It is hard to know whether some organisms arrive of their own destiny, or are helped along the way by human transport. For this new record of sap-sucking slug (Ercolania boodleae) in  New Zealand, the latter is suspected.

As divers, we terrestrial visitors may welcome slightly rosier water temperatures, but the question begs, are these tropical animals we see temporary vagrants, or is this the beginning of a wave of new permanent climate change-induced residents? It is typically the juveniles of species we see. These are likely brought down on the subtropical currents as larvae, and are spotted during their first year of life. As the water cools over the winter, often it is too cold for these animals to persist, and they don’t survive to become adults. It is likely this is the result of a warm influx of water (ie. a warm summer), rather than a range expansion.

On the other hand, if we spot these tropical species during the winter, or as adults, they have probably survived the winter temperatures, and their persistence could possibly be an indicator of a range expansion. Examples of this include the combfish (Coris picta) or painted moki (Cheilodactylus ephippium) – fishes that were hardly seen two or three decades ago, but now are regularly spotted, both as juveniles and adults.

This is not always the case, as pulses of larvae years apart could maintain adult populations, as is often thought of in the case of species such as Lord Howe Island coralfish (Amphichaetodon howensis).

Regularly seen, and often in resident pairs, there is yet no evidence to suggest species replenishment from local populations. Whilst they are surviving as adults, perhaps it is still too cold or conditions aren’t right for them to reproduce, or if they do, their offspring end up too far offshore or downstream where conditions are colder still.

There can be little doubt we are seeing an ever-increasing number of tropical vagrants appearing in our temperate waters, and that it is inextricably linked to a warming climate. So, what does this all mean? Well, it is fairly hard to say at this point. There is mounting evidence that the poleward shift in marine organisms is occurring up to five times faster than in terrestrial systems. This rate of climate-mediated global shifts has been coined ‘climate velocity’, and illustrates the potential of dispersal and the connectedness of the oceans.

It’s also hard to know how these new immigrants are competing with what is already there, and how they are restructuring these biological communities. It is likely many will not be adapted to cooler climes; though, as we see with some species in New Zealand, they have the potential to at least reproduce. And while Nemo hasn’t arrived here yet via the EAC, in the interest of the climate and the unknown, it might still pay to ride your bike to work tomorrow.