Sea urchins-Swiss army jaws

by Matthew Norton

Sea urchins belong to the phylum Echinodermata, which includes starfish and sea cucumbers, and have interested scientists and natural historians for centuries. This is best demonstrated by the structure of their mouths, which is still described as an ‘Aristotle’s lantern’ after the ancient Greek philosopher, and natural historian. Recently it has been debated whether Aristotle meant that the mouth itself resembles a lantern, or that the mouth’s position makes the whole internal body resembles a lantern.

Sea urchin lanterns consists of five symmetrically arranged ‘teeth’ around a mouth opening, demonstrating the definitive fivefold symmetry of all echinoderms, on the underside of the sea urchin body. Despite its apparent simplicity, the sea urchin’s lantern is a powerful and multipurpose tool used to exploit various prey, such as algae, mussels and sea cucumbers, and participate in various other activities such as burrowing into sand and boring into rock.

(Rev) urchin article image 1 — The jaws of a sea urchin as seen from below (left) and inside (right) are made of five symmetrically arranged teeth, operated by a series of calcareous plates and muscles around a mouth opening.

The teeth themselves have a self-sharpening mechanism built into its microstructure to prevent them from getting dull from intense use. The boundaries between the bony plates of each tooth act as fault lines so that when the outermost plates are chipped away the newly exposed plates remain in the same sharp arrangement. However, the continuous growth at the base of the teeth to replace the lost bony plates requires a high material and energy investment. This can be problematic when food is scarce, on the one hand they should be more frugal with the resources they use up, but on the other hand if they don’t properly maintain their lanterns they would struggle even more to feed themselves. The solution that many species seemed to have arrived at is keeping up the maintenance of their lanterns at the expense of other structures, such as their spines and gonads (reproductive organs), and their overall body size.

Their feeding activities can have a considerable impact on their surroundings, especially when they feed on, or damage habitat forming species. In particular I would like to draw attention to how seaweed eating urchin species, such as Strongylocentrotus droebachiensis, can maintain ‘urchin barrens’, now dominated by small coral like algae, in kelp forests. They don’t create these barren areas, normally they would feed on ‘drift algae’ rather than living kelp, but once they form the heavy urchin grazing on the seafloor makes it especially hard for kelp seedlings to settle in without being quickly consumed.

(Rev) urchin article image 2 — Given how tightly packed groups of sea urchins can be, it’s hardly surprising that they can do serious damage to some habitats if there is enough of them.

In other circumstances sea urchins can protect habitat forming species. For example many coral reefs would be overgrown by the faster growing seaweeds if the latter are not kept under control by grazing sea urchins. Admittedly sea urchins can also damage reefs, but the shift from coral to seaweed dominated reefs after mass mortalities of urchins suggests their influence, at least in some species (e.g. Diadema anrillarum) , is beneficial.

In either case it is clear that, in sufficient numbers, sea urchins have the potential to influence the structure of underwater habitats.

From a human perspective

Sea urchins, especially their gonads (otherwise called roe), are popular seafood in many regions, including Chile, Japan, Pacific North America and Mediterranean, and are sometimes kept in aquarium tanks to keep them clean from algae build up. As with effectively everything we take from the sea there is a risk of overfishing, a risk that has already been realised in many cases of local urchin fish stocks declining, or collapsing. For example in 1998 Chile dominated the urchin fishery, taking half the worldwide catch, but by 2002 a series of major depletions had occurred.

However, the Chilean fishery still held its position among the worldwide sea urchin fisheries, which suggests the other countries were not doing much better.

(Rev) urchin article image 3 — Fresh sea urchin gonads, commonly called roe (left), served with fish cake (right).

Along with the danger to the urchins themselves and the loss of income to the fishermen, the loss of the sea urchins’ influence on the surrounding habitat can cause ‘unnatural’ shifts in its structure. For example due to their previously mentioned suppression of seaweed growth on coral reefs, removing the vast majority of sea urchins could put them in danger of being overrun. On the flip side major increases in urchin populations, due to declines in their predators, which normally keep them at manageable levels, can be just as damaging to kelp forests and other seaweed based habitats.

Many of these predators are also in danger from human activities, for example the sea otter (Enhydra lutris) is currently classed as endangered, due to historic hunting and vulnerability to recent impacts (eg. oil spills and disease), and the American lobster (Homarus americanus), while not endangered, is under increasing fishing pressure.

All of this can be avoided by keeping sea urchin fishing sustainable. However, with many fisheries this is not easy as the short term effects on fishermen’s’ livelihoods can inspire some resistance to the regulations aimed at keeping fishing sustainable. It doesn’t help when the issues around fishery regulation are exploited for political gain. A recent and sobering example is the use of EU fishing regulations to demonise the EU during the UK Brexit referendum.

I want to make it clear I am not a fishing industry expert, nor am I attacking the men and women they employ, any concern they have is understandable, and there can be cases where regulations are poorly handled. However, without proper management it is the fisheries themselves that have the most to lose in the long term.