Yellow coral goby, Gobiodon okinawae

The Yellow coral goby is a street-wise fish which enjoys a bizarre sex life, yet is tough enough to look after itself when threatened, says Dave Wolfenden.

An endearing fish with bags of personality which can change sex to either gender, the remarkable Yellow coral goby can, for many reasons, be a great choice for the smaller reef aquarium.

Gobiodon okinawae hails from the western Pacific centred around Indonesia and ranging from the Great Barrier Reef north to southern Japan. It’s found in shallow coastal waters up to 15m/50’ and invariably seen perching among branches of Acropora SPS corals — which determines its distribution.

In fact, the species seems to be an obligate coral dweller in the wild, having to live among its host.

Growing to just over 3cm/1.2”, this goby’s bright yellow colour includes a patch of white on the cheek. They’re normally found in pairs or small groups, but up to around 15 may be found on the same patch of Acropora.

Make G. okinawae feel at home in the aquarium by replicating the conditions of the wild — so plenty of branching corals are ideal. Sparse furnishings will make this fish unhappy. The more hiding places you create the better, as these mean more opportunities for territory establishment.

Paradoxically, the more hiding places available the more likely the goby will display well!

Toxic shocker
This fish secretes a toxin from the skin as an anti-predator weapon, its bright yellow coloration possibly acting as an early warning to other species of its poisonous nature.

The toxin itself appears to have an unpalatable bitter taste, as well as being haemolytic — able to destroy red blood cells — and ichthyotoxic, meaning directly poisonous if consumed.

For this reason many other species leave them well alone, but G. okinawae may become stressed by the attentions of the more persistent species. Many damsels, for instance, do not tolerate them and the pugnacious, feisty damsels can leave these gobies highly stressed and emaciated.

Don’t mix this species with larger, predatory fish such as lionfish, triggers, puffers and eels, which seem to completely ignore any warning signals! Even some of the ‘quintessential’ reef inhabitants, such as hawkfish, can be risky. They may bully the goby and steal much of its available food.

Suitable tank mates include other less boisterous fish. Seahorses, pipefish, Mandarins and blennies are ideal, as are many other gobies, apart from other coral gobies. These more sedate species will allow the Yellow coral goby to display to its full potential.

G. okinawae can quickly become endearing aquarium inhabitants, and, in spite of their tiny stature, some exhibit plenty of personality! This is, in fact, one of the most extrovert of all the coral gobies and individuals can assume 'pet' status.

Best of both worlds?
By anyone’s standards, G. okinawae has an 'interesting' sex life. Some individuals hatch as females, and become fully functional at sexual maturity as females. Other members of the species, however, are more adaptable to mating. They are technically referred to as 'bi-directional protogynous hermaphrodites', which means they start as females and mature as such, but may, if necessary, change into breeding males.

Not only that, but males can, with equal ease, turn back into girls if the need arises!

Why would this species, and seemingly other coral gobies, have evolved such a bizarre strategy?

Scientists suggest that G. okinawae’s gender bending is an adaptation to the hazards of finding a partner in their natural habitat. If an individual’s partner dies, a coral goby will have to move to another area to seek a new one.

Moving from areas of Acropora into open water is pretty hazardous for such fish, despite their toxin, so the ability to change sex means that, for the hermaphrodites at least, any new member of their species they encounter is a potential partner!

G. okinawae individuals may be difficult to adapt to aquarium life and challenging to feed. They don’t appear to particularly like flake foods, but will accept a wide range of frozen feeds, including frozen Cyclops, Artemia, Mysis and other meaty foods. However, getting them to start in the first place can be tricky.

Artemia should preferably be enriched, as the un-enriched form has a generally poor nutritional profile. This can stimulate the first feeding, after which the fish can be weaned on to frozen fare. However, offer them as much variety as possible from day one.

Supplementing diet
It’s extremely important to get this goby in the mood for feeding as soon as possible. An established reef aquarium, with its natural planktonic populations, can help augment the ingredients of any artificial diet by supplementing frozen feeds with copepods and other tiny crustaceans.

Many specimens of G. okinawae enter the trade in poor condition because they refuse to feed and their small size. They can lose condition very rapidly, so when choosing a specimen select one which doesn’t seem in any way emaciated. Better still, try to watch a specimen feeding in the dealer’s holding tank — although sometimes it’s worth just taking a chance with an otherwise healthy-looking specimen.

Stress can mean disease
G. okinawae doesn’t seem to be particularly susceptible to disease, but the ‘usual suspects’ of Cryptocaryon (whitespot) and Oodinium can be a problem for stressed individuals, particularly following transport.

These fish need to be monitored closely, with optimal water conditions maintained.

Do these fish need live corals to perch on?
Not necessarily. Even though G. okinawae naturally associates with live corals, especially Acropora, they will take up residence on fake corals if that’s all that’s available.

They can associate with a variety of other stinging invertebrates, such as anemones, soft corals and other stony corals, so this doesn’t seem to be a particularly fussy little goby...

Are they expensive?
Far from it. This is one of the most affordable goby species, representing excellent value for money. Pay as little as £10 for a small specimen!

Is G. okinawae suitable for the nano aquarium?
This species is ideally suited to the smaller reef tank, with a minimum 50 l/11gal recommended for one specimen. Bear in mind that its territory in the wild is naturally extremely small, so it makes a great nano fish — provided the other inhabitants aren’t too boisterous. 

Can I keep more than one in the same aquarium?
Possibly. In really small aquariums they may fight as they can be rather territorial. Providing as many nooks and crannies for territory establishment increases the chances of more than one getting on.

The greatest success will come, however, if adding more than one to aquariums over 100 l/22 gal. You might even become a matchmaker, successfully forming a breeding pair!

Do I need to quarantine new specimens?
In an ideal world, yes you should! Yet this fish can easily become stressed by the process of quarantine itself, so this can do more harm than good.

It’s good practice to quarantine all new stock, but with any coral goby monitor specimens closely in quarantine. Typically, quarantine tanks may be sparsely furnished, but best provide plenty of hiding and perching spaces for this species. Some fake corals are ideal.

Can I breed them?
There are sporadic reports of hobbyist breeding and TMC produce them, albeit infrequently — but it’s a start.  

Being bidirectional hermaphrodites, it’s possible to obtain pairings in a large aquarium as they tend to squabble in very small environments. In fact, simply adding more than one fish to an aquarium can sometimes give rise to a viable pair.

Fertilisation is external. The female deposits eggs on the coral host, or whatever else they live on in the aquarium. They are then fertilised and guarded by the male, and hatching takes place less than a week after laying. The male plays a vital role in egg care, fanning them to maintain oxygenation and keeping them clear of fungus.

The real challenge is growing the fry. They’re extremely small and providing food of the correct size is not easy. The newly-hatched fry may number up to 500, so will need rotifers in sufficient quantities.

The fry are planktonic for the first 30 days or so before they metamorphose and settle on the branches of host corals.

By this time, they should be able to accept newly-hatched Artemia nauplii.

This item first appeared in the January 2010 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.