Weird things found inside the stomachs of deep-sea fish #1.

Figure 1. A basketwork eel.

This is post started out as a description of something that came out of the stomach of a basketwork eel (Diastobranchus capensis Barnard 1923), but expanded to be about the eels themselves and their potential role in the deep-sea ecosystem they inhabit. 

Basketwork eels are a deep-sea species that are mostly found at depths of between ~800–2500m, in oceanic waters of the southern hemisphere. They belong in the family Synaphobranchidae (cutthroat eels). We only really know about them because they are taken as bycatch in deep-water fisheries, particularly those for orange roughy (in New Zealand waters). Once caught, almost all basketwork eels are discarded (dead or dying) back into the water. As a they have no commercial value their ecosystem role is not very well known.

Their role in the deep-sea ecosystem

For reasons I'm not going to go into here, the deep-sea ecosystem I'm talking about is an abstraction with rather vague boundaries. There are many deep-sea ecosystems, so in some ways it would be more accurate if I talked about a fishery, rather than an ecosystem. Assume when I say ecosystem I'm talking about the depth range between about 800 and 1200 m. This is because even though basketwork eels can live deeper than this, not much commercial fishing for orange roughy occurs deeper than 1300 m and this is the source of most of the data.

Basketwork eels feed on (in decreasing order of importance) fishes, squids, crustaceans, and whatever else will fit in their mouths. They are likely to be opportunistic scavengers and could perform an important role in deep-sea ecosystems cleaning up carrion. This is a good thing, as there have been concerns that dead animals (particularly fisheries discards) left on the seabed, will then decompose, use up the available Oxygen, and possibly create dead zones in the deep-sea. However, the evidence for them being scavengers is largely circumstantial; previous reports have suggested that they might scavenge, and they frequently turn up to deep-sea baited camera deployments. However, they are probably also predators, as the line between scavenging and predation is a blurry one and it can be very difficult to discern the difference between the two modes of feeding.

Figure two. The stomach contents of a basketwork eel.

Discerning their mode of feeding matters on an ecosystem-scale, because if they are mainly scavengers, then they may benefit from influxes of discarded fisheries bycatch. However, if they don't scavenge, (like orange roughy - a mid-water predator which only seems to take live prey), then discarded bycatch may not be of benefit to them. The answer to this question has implications for the structure of deep-sea ecosystems in response to fishing activity. 

For example, if basketwork eels benefit from bycatch, then their population could increase at the expense of other bycatch species (which may occupy a similar niche), and in turn change the structure of the ecosystem (called a trophic cascade). Which could possibly lead to a break down in the normal functioning of this ecosystem. And when I say normal functioning of the ecosystem. I really mean 'it stops giving you anymore of what you were extracting from it'.

Figure three. A basketwork eel with a distended stomach, containing the remains of a lighthouse fish (Phosichthys argenteus).

As an aside, this ecosystem has already had signficant alteration through a, the selective removal of ~70-90 % of the orange roughy through fishing, and b, the destruction of bottom habitat (large, erect corals, sponges etc...), also through fishing. These impacts mean that regardless of what research is done now, we are not looking at a pristine, unaltered ecosystem.


However, trophic cascades are complex phenomena and not very well understood, especially in the deep-sea. For the eels to benefit from the fisheries discards the assumption would have to be made that they could reproduce faster than they themselves were being taken as bycatch. I would hasten to add that there is at the moment no evidence for trophic cascades in the deep-sea, but this may only reflect a lack of data.

Figure two shows the stomach contents of a basketwork eel from northeastern Chatham Rise, New Zealand (from between 1062 and 1117 m depth). At the time I called it seaweed, as I had found brown seaweed in the stomachs of eels I had previously dissected (see below). As for this particular identification, I'm not so sure anymore… However, if it's not seaweed, what is it? I'd be interested to hear suggestions.

Figure four. The remains of a lighthouse fish taken from the stomach of a basketwork eel.

Figure three and four are of half a lighthouse fish; what it looked like before I opened the eel's stomach and after I had removed it.

The question here is, did the eel predate the fish; bite though it and lose the other half. Or did it just pick up half a fish that something else had bitten through? Or perhaps there is an alternative explanation? It's an interesting question and not one easily answered.

Figure five shows seaweed taken from the stomach of another eel. This is seaweed in the stomach of a fish caught in 1117 metres of water. So either this fish swam to the surface (not very likely), or the seaweed sank over a kilometre to the seabed and was consumed once on or near the bottom. recent work in Mediterranean has shown that deep-sea fish do actually consume vegetable matter It is not known whether the eels selectively target vegetable matter, or if these are just small items that fit in their mouths.

Figure five. The green material below the eel is seaweed.

After looking at the stomach contents of about 135 eels I reasoned that scavenged material could be defined as: anything that the eel would not normally have access to e.g., fragmentary animals that lived further up in the water column (as far as we know this eel stays close to the bottom throughout its adult life*); plant matter as mentioned earlier; and fragments of animals larger than the eel itself. Anything else was probably/possibly prey. 

Using those conditions it turns out that about 30% by weight of the food of basketwork eels (from northeastern Chatham Rise, New Zealand) was scavenged material. This suggests that these eels play an active role in cleaning up the deep-sea through the consumption of dead material. Interestingly there was no evidence of any kind of change in diet as the eels got larger. Usually as fish get bigger they are able to handle larger prey, so their diet changes (called an ontogenetic shift in diet). This lack of an ontogenetic dietary shift could be interpreted as further circumstantial evidence for scavenging, since a smaller eel could tackle a large chunk of carrion without having to catch it as prey. 

To conclude, what I found suggests that any influx of carrion in the form of fisheries discards is likely to have some sort of effect on the basketwork eel population and in turn the ecosystem they inhabit. However, without further longer term monitoring the form those impacts could take are unknown. This is an area of deep-sea research which really interests me.


References
Drazen, J.C., Bailey, D.M., Ruhl, H.A., Smith, K.L., 2012. The role of carrion supply in the abundance of deep-water fish off California. PLoS ONE 7, e49332

Fujiwara, et al., 2007. Three-year investigations into whale-fall ecosystems in Japan. Mar. Biol. 28, 219–232.

Jefferies, R.M., Lavaleye, M.S.S., Bergman, M.J.N., Duineveld, G.C.A., Witbaard, R.,2011. Do abyssal scavengers use phytodetritus as a food resource? Video and biochemical evidence from the Atlantic and Mediterranean. Deep-Sea Res. I 58,415–428.

Jones M.R.L., Breen, B.B., 2014. Role of scavenging in a synaphobranchid eel (Diastobranchus capensis, Barnard, 1923), from northeastern Chatham Rise, New Zealand. Deep-Sea Res 1 85, 118–123.

Zintzen, V., Anderson, M.J., Roberts, C., Harvey, E.S., Stewart, A.L., Struthers, C.D., 2012. Diversity and composition of demersal fishes along a depth gradient assessed by baited remote underwater stereo-video. PLoS ONE 7, e48522

*we don't really know for certain, but this species doesn't turn up in shallower water fisheries as bycatch, so it's not a bad assumption.