K.N.I. Bell: Tropical Anadromous Gobies -- Sicydium & others

Introduction to Sicydiine gobies

      Bell main site page --- --- main goby page --- How it started -- Intro -- Larvae

I have been keen on gobies for a very long time. My work interests me for its own sake, but I always try, whenever there's a choice, to follow (a) what might be useful for conservation, and (b) what others haven't noticed or what seemed too risky.


A brilliant blue-phase (territorial colouring) male Sicydium punctatum from Dominica, West Indies, perched on a rock.

This (above) was undoubtedly the first witnessed nest of a sicydiine goby from the Caribbean (before, only one instance of unfertilised eggs was seen, deposited by a lone female on the wall of a concrete water tank in Puerto Rico, reported by Donald S. Erdman). For scale, the filter slats are about 7mm. This is the male guarding the nest.
    When the larvae hatched and started swimming vertically, avoiding settling, it told me immediately that these were anadromous.
   These fishes have a life cycle very much like salmon, i.e. breeding in rivers, migrating to the sea, then returning some time later; they can be called anadromous (though based on some finer features an evolving term "amphidromous" is often applied). The gobies however do it much faster, returning in about 3 months, and they live a very long time (probably decades*) and continue breeding once they are back in rivers. (* That is why I beg fellow scientists to avoid sampling adults, and certainly not to do so without a very good reason.)


Einar Wide and I collecting gobies just above the Hillsborough bridge on Layou River, long ago. Multi-use of rivers continues to this day. Some years later I sampled plankton a little way above here, and below.
    The ladies doing laundry were very interested in my plankton sampling, though at first they couldn't imagine catching anything useful with that little plankton net; surely I must be a fool. But like everyone else, they'd ask me "where do the tri-tri come from?" so when I showed them larvae they were very excited. They were nice people, and picked up information quickly--they never needed to ask the same question twice.

Life cycle. Adults live in rivers. Eggs laid on the roof of little caves that the male digs out under rocks. Larvae hatch and then commence swimming up, sinking down, swimming up, drifting down, etc., and all the while they are carried by the stream to the sea. Larvae grow in the sea for a few months (varies worldwide, and varies across species; we aren't sure why; but in Dominica the range seen for Sicydium punctatum is 50 to about 150 days). Then they return to fresh waters (recruitment).

Note the diagram marks "Fishery". "Tri-tri" is the local name for the postlarvae, and they are so different from the adults that virtually everybody would ask me "Oh, you're working on tri-tri; tell me, where do they come from?" Or, some wanted to tell their theory; those ranged from a big mass of foam in the ocean, to a big mama tritri in the river (the closest but it was still not connected with the Loche, the local name for the adult Sicydium), to origins in a big bag, like a plastic bag. The latter seemed the wackiest, but after one very smart lady said the same I realised it might refer to postlarvae seen inside jellyfish (but that again is a theory, the story might mean something different again).

I call these taxa (the gobies, shrimps and gastropods) anadromous. Some use the word amphidromous; the differences are subtle and all classifications that are not mutually exclusive are descriptors, not categories. But few enough know what 'anadromous' is; I see no point in using a word that fails to communicate with the intended reader. The effective meaning of "amphidromous" to those working on these groups, is "this group", so it communicates little to them*; few others workers know it, so, ironically, ditto. (*like writing in J. Ferrous Metals "iron, a ferrous metal").

Geological processes determine biological habitats:
MAP of the research location in the Caribbean, Lesser Antilles. Dominica is just to the right of the "D". All this chain of islands are volcanic, mostly steep, many of them now or once covered with lush rainforest (rapidly being eliminated). Volcanic islands often form "island arcs", indicating an origin in the subduction of crustal material of one tectonic plate below another. This process gives rise to volcanoes, of a type often called 'andesitic', that are characterised by very explosive eruptions (e.g. Mount St. Helens). In contrast, basaltic volcanoes are like those in Hawaii, which are less explosive.
  Volcanic processes characteristically create landforms that reach very high, and the height in turn creates local weather patterns of high rainfall, technically called orogenic rainfall. This is why volcanic regions often have high rainfall, often predominantly on one side of the structure*. This creates full, fast, steep rivers with spectacular waterfalls. The high rainfall promotes the erosion of the original volcanic mount and over the long term (millions of years) balances the contribution of new volcanic material, while in the shorter term (not as many millions of years) generating a more complex coastline with embayments.
 (* if the landform is high enough, most of the rainfall occurs on the windward side, with the leeward side experiencing what is called "rain shadow". The same physics result in allied phenomena like 'chinook' winds, dry warm winds that descend from high mountain ranges.)
  Volcanic or frequently disturbed coastal terrain seems essential for anadromous gobies -- such habitats, while having the problem of frequent disturbance, also lack many competitor species, thus creating a niche for species (fish, shrimps and snails) that have a marine stage (so they can get from river to river, and island to island; it helps if they can climb waterfalls.
   (BTW, the rum will do as fish preservative ... but there are some nice rums for sipping)