K.N.I. Bell: Tropical Anadromous Gobies -- Sicydium & others
Porfirio Manacop* was the first person to detect any goby larvae in
*Manacop, P. R. 1953. The life history and habits of the goby, Sicyopterus extraneus Herre (añga) Gobiidae with an account of the goby-fry fishery of Cagayan River, Oriental Misamis [Province, Mindanao, Philippines]. Philippine J. Fish. 2: 1-60.
Other workers in his area were reluctant to accept his findings and treated it as error. The next seen sicydiine larvae were from my aquarium specimens in 1970, when I was not long out of high school. Larvae were next found from more captive spawnings: Foster and Fuiman found larvae of Evorthodus lyricus, and Todd found larvae of an eleotrid.
On July 4 1989 I found the first Caribbean river goby larvae in situ, from plankton samples taken in the Check Hall river at about 400 m elevation, and the first larvae found in situ since Manacop's work. That showed that the often-held presumption of catadromy in these fishes was in error, and it began a sampling program that sampled rheoplankton for about 20 months in Dominica, with other work, e.g. on returning (recruiting) gobies. The rheoplankton work showed that most gobies spawn pan-seasonally, and also recruit pan-seasonally.
One spawning of a 40-50 mm female contains about 30,000 eggs.
<-- A very small sample of a nest; larvae are not all at the same stage, and typically do not hatch all at once, but over a few hours. Eggs are pyriform (shaped like a lightbulb, or like the Apollo capsule) are about 0.5 mm diameter, and, typically of gobies, are attached by filaments to a substrate. Some of these larvae could hatch if stimulated (e.g. by light), but would still require a few hours to attain the normal larval length of about 1.8mm. If very premature, hatching occurs before larvae can straighten out, and they swim helically (inefficient of course) for a while. Premature hatching could help save larvae following disturbance; at least larvae would be dispersed and predators would get fewer of them, and if the hatching had been stimulated by conditions related to a flood, useful transport downstream could still occur.
In aquarium conditions, females can spawn at intervals of only a few weeks
or less. As shown in a neat paper by Burt et al*, the annual reproductive
output of fishes that spawn only once per year is limited to about 30% of body
mass; but fish that spawn much more frequently can have annual reproductive
output >> body mass.
*Burt, A., Kramer, D. L., Nakatsuru, K. and Spry, C. 1988. The tempo of reproduction in Hyphessorbrycon pulchripinnis (Characidae), with a discussion on the biology of 'multiple spawning' in fishes. Env. Biol. Fish. 22: 15-27.
Sicydium punctatum larva about 3 d post-hatching. (incubation time is ~1-1.5 day); transmitted light, with 2 otoliths visible in sac behind eye; "fluor" pigments show as dark with transmitted light. Below: similar developmental stage, but with oblique lighting and dark field, showing the "fluor" yellow-green (the photo shows it as yellow, but it is a fluorescein-like colour) pigment pattern characteristic of Sicydium punctatum. I show it in horizontal orientation but it would at this stage still be alternating sinking (head down) and swimming (head up). Under anaesthesis with 2-phenoxyethanol the "fluor" pigment bodies become strand-like (anterior-posterior).
<-- plastic glass with halocline (seawater below, fresh above, mixture zone shows as an optical distortion), and Sicydium larvae avoiding full seawater (they are very small, just long dots, above the seawater).
Typically, fish larvae have been thought to sink to wherever they are neutrally bouyant. This is not necessarily the case, as shown (for the first time) by these fish; Sicydium punctatum larvae are the first fish larvae ever shown to actively choose a salinity range. [paper]
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.
The behaviour had profound implications for early life history, and related experiments showed that they could not survive in fresh water more than a few days, nor could they survive full seawater as long as they could in a halocline where they could occupy salinities less than about 15 ppt.
This was shown for Sicydium punctatum but applies for anadromous gobies generally.