Explanations for answers to quiz on the comparative advantages to a male goby in having a mate or being on its own:

Parental care is better for the offspring. No. Although male sharknose gobies will guard the eggs until they hatch, a female apparently will not. So, at least from the standpoint of a male, having a mate doesn't really help.

It makes it easier to defend the cleaning-station territory. Yes, this is possible. It has also been described for some butterflyfish species.

Males are actually unable to obtain more than a single female. Well, whether true or not, it doesn't address the question posed; namely, why a male sharknose goby would have a mate in the first place, let alone have more than one mate.

Good cleaning-station habitat is scarce, so it makes sense to double up. No. Good try, but in the Barbados study area and probably elsewhere, suitable habitat seems not to be scarce.

The population is sparse, so it's best to snag a permanent mate if and when you can. Yes, this makes sense. Why spend time and energy hunting down a mate for spawning, especially if spawning is an everyday (or several times daily) occurrence?

High-quality mates are rare, so hold on to a good one if you can. Yes, this goes along with the previous entry. For a male, a good mate is a large female with high fecundity. For a female, a good mate is a larger male who may attract more client-fishes and thus allow the female to get in more cleaning time. Where a male does court another female in the presence of his own mate, these new females will invariably be larger with desirable high fecundity. What happens if a male does find another bigger and better mate? Well, since the authors never observe polygamy in the field, then possibly the original, smaller female mate is driven off.

Client fishes are attracted to stations with more cleaners. Yes, this is likely. The more cleaners there are at a station, the better will be the service and the faster will be the turnover of client fishes. In this case, two can live better than one.

NOTE the potential to produce eggs or, more broadly, offspring