A few weeks ago I described a site just off Sydney where I’ve been watching a pair of fish build what looks like a system of nests. The fish are Black-Lined Sleeper Gobies (Valenciennea helsdingenii). They’re known engineers. After building one tower to about 17 cm high, along with a series of den openings at various distances, they moved on to build another tower about 6 meters away. Some of the holes on the sea floor are shaped with small clumps of seaweed, and have some shells laid around them. Holes seem to be opened and closed on different days.
The city looks like this:
The den openings marked are the ones visible on the days the chart was done (December 28 and 30), and don’t include at least two others that came and went. (Click on the map for a bit more detail.) All the structures marked are ones I’ve observed being built or tended at some stage by the fish. I’ve no idea what the structure of tunnels looks like below.
Here they are at work on the second tower, which is next to a large rock.
Up until January 2, both partners were up top, adding to their complex. They seemed to lose interest in the first tower and spent most of their time in the suburbs. Then on a couple of days when I was around in the afternoon, one fish went below while the other stayed on deck. Eventually I was only seeing one fish when I arrived, not two, and this fish continued to tend various parts of the complex.
A week or so ago, one day the water was so clear that I was able to find the site and visit on snorkel rather than scuba. I was watching from up top when I saw two divers slowly approaching the city below. I hoped they wouldn’t notice it, but the city was too conspicuous for that. So I watched closely as they came in. When one of the divers reached the second tower, he immediately began rummaging and digging at it with his hands. How stupid can a person be? I swam straight down and stopped him, indicating what I thought of him as clearly as I could, with gestures. Off he went, leaving the second tower a mess, though not destroyed.
Over the next few days the lone fish was at work, not rebuilding the second tower, but tending the southern and eastern dens. Then, three days ago, both fish were there again, just as when I’d found them a month earlier. Over the last couple of days they’ve been at work on the first tower, which had been neglected, and also the southern den; they may be working on the first stages of a third tower there.
I was pleased to see them together once more, as they’re inseparable when both up top, but I wonder why the second fish reappeared. This might well be part of the normal course of events. A Japanese study of a related mound-building species notes that it’s quite common for pairs to stay together for several rounds of egg-laying and brooding, and in that species, it takes less than a week for the eggs to hatch.* Another possibility is that some of this is due to The Idiot’s efforts. Perhaps the male was established below the Second Tower, tending a clutch of eggs, and the water flow was destroyed by the intruder. Either way, now they’re starting again.
* The period when only one fish was present was January 3 to around January 7 (no observation on the 8th, no fish on the 9th, two fish on the 10th). On January 1 and 2, both fish were there initially but one disappeared – into the burrow, I assume – while I wasn’t looking. Full disclosure: the final picture of the two fish above was taken before their reunion, when the water was clearer.
** In the first version of my previous post I said that the pattern of behavior here differed from that in a related species described in a Japanese study, as both fish were building the first tower. An earlier paper by the same authors (as pointed out by David Scheel) makes it clearer that in that species both fish build for a while, and then one continues while one is below. So there’s no difference on that point after all.
*** UPDATE – May 26, 2014. I went back over 4 months later, and the fish (or a very similar-looking pair of fish) were still at work. Here is a shot of the Main Tower in May, and here is a shot of the Second Tower.