Rhinogobius candidianus (REGAN, 1908)

SynonymsTop ↑

Ctenogobius candidianus Regan, 1908; Rhinogobius taiwanus Oshima, 1919

Etymology

Rhinogobius: from the Greek rhinos, meaning ‘nose’, and the generic name Gobius.

candidianus: named for Lake Candidius in Taiwan, type locality of the species.

Classification

Order: Perciformes Family: Gobiidae

Distribution

Type locality is ‘Lake Candidius, Formosa’, which corresponds to a body of water better known as ‘Sun Moon Lake’ located in Nantou County, central Taiwan, but R. candidanus is a strictly fluvial species and has never been recorded from the lake itself.

It’s widely distributed throughout much of northern and western Taiwan, from Taipei County at the island’s northeastern tip to Yunlin County  in the west, and has been recorded in numerous river drainages.

Habitat

Western Taiwan has a subtropical monsoon climate characterised by high temperatures and humidity with massive rainfall and tropical cyclones in summer with average annual air temperatutes of 22-24°C/71.6-75.2°F, though in winter it typically drops below 20°C/68°F.

In major drainage basins R. candidianus tends to be found in smaller tributaries and hill streams rather than main river channels though in smaller systems it may inhabit them.

All its habitats are seasonally variable with water flow rate, depth and turbidity typically increasing during the monsoon season.

Substrates tend to be composed of gravel, rocks, boulders, and exposed bedrock.

Maximum Standard Length

75 – 80 mm.

Aquarium SizeTop ↑

An aquarium with base dimensions of 80 ∗ 30 cm is large enough to house a small group.

Maintenance

Not difficult to maintain under the correct conditions; we strongly recommend keeping it in a tank designed to simulate a flowing stream with a substrate of variably-sized rocks, sand, fine gravel, and some water-worn boulders.

This can be further furnished with driftwood branches, terracotta pipes, plant pots, etc., arranged to form a network of nooks, crannies, and shaded spots, thus providing broken lines of sight.

While the majority of aquatic plants will fail to thrive in such surroundings hardy types such as Microsorum, Bolbitis, or Anubias spp. can be grown attached to the décor.

Like many fishes that naturally inhabit running water it’s intolerant to accumulation of organic pollutants and requires spotless water in order to thrive, thus weekly water changes of 30-50% tank volume should also be considered routine.

Though torrent-like conditions are unnecessary it does best if there is a high proportion of dissolved oxygen and some water movement in the tank meaning power filter(s), additional powerhead(s), or airstone(s) should be employed as necessary.

Water Conditions

Temperature: 18 – 28 °C

pH: 6.0 – 8.0

Hardness: 90 – 215 ppm

Diet

Rhinogobius spp. tend to be opportunistic carnivores feeding on a range of small invertebrates, crustaceans and similar in nature.

In the aquarium they should be offered small live or frozen foods such as chironomid (bloodworm) or mosquito larvae, Artemia, Daphnia, Mysis, etc.

Dried foods may be accepted following a period of acclimatisation but should not be used regularly.

Behaviour and CompatibilityTop ↑

This is a robust species and can be aggressive at times but it can be kept in a community provided tankmates are chosen with care.

Peaceful, pelagic species which inhabit similar environments in nature such as Tanichthys or Danio species are perhaps the best choices for the upper levels, but we’ve also seen Rhinogobius spp. being maintained alongside barbs, small characins, poeciliid livebearers, etc.

Less suitable are freshwater shrimp from genera such as Caridina and Neocaridina as these are likely to be predated upon.

Balitorid and nemacheilid loaches may prove suitable in larger tanks but in smaller set-ups R. candidianus should ideally be the only benthic species included.

It should not be mixed with congeners, especially closely-related ones such as R. nantaiensis, since it’s not yet clear if they’re capable of hybridisation.

Larger fishes are best omitted although in very large set-ups it may be possible to add a few non-predatory, surface-dwelling ones, while the majority of cichlids and other territorial fishes inhabiting the lower reaches should be avoided entirely.

While males are territorial with one another to an extent serious damage is unlikely provided the tank contains sufficient cover, and in fact they appear to actively require the presence of conspecifics.

Aim to purchase at least two males and as many or more females or the fish may become listless and inactive.

Sexual Dimorphism

Adult males develop an extended primary ray in the first dorsal fin and possess a slightly longer jaw than females.

In gravid females the eggs are visible as a bluish mass through the abdomen.

Reproduction

As is typical for the genus eggs are deposited on the ceiling of a cave or crevice and guarded by the male until hatching.

Several potential sites should be offered in the form of rocks (flat slate tends to be easiest to handle, see below), terracotta pipes, plant pots, etc.

A nuptial male will select a site and defend it against other males, but if  one is not available may excavate a cave beneath a rock or other piece of décor.

Spawning occurs secretively within the cave, and is usually signified by the pair disappearing from sight for an extended period of time.

They may remain inside for 2-3 days with the female ejected once spawning is complete, and should not be disturbed during this period.

Once the female is seen outside the cave, you can check for eggs by carefully lifting and tilting it without removing from the water.

Inexperienced pairs or males may eat the eggs, attach them poorly, or only partially fertilise them, but after a few attempts normally get things right.

At this point it’s advisable to remove all other fish except the male, or remove male and eggs elsewhere ( a small plastic container can be used to do so without harming either).

Brood size is normally between 20-100 depending on the age and experience of the fish and the eggs are relatively large as with other fluvial members of the genus.

Incubation is normally 9-11 days days, and the fry undergo an initial pelagic stage during which they live and feed up in the water column before dropping to the substrate.

They can be offered Artemia nauplii, microworm, and suchlike immediately.

NotesTop ↑

This species is normally imported alongside a smaller-growing, near-identical congener currently listed as R. nantaiensis here on SF, although normally both are traded as R. candidianus.

They can be separated by snout length, which is noticeably longer in R. candidianus, and adult size with  R. nantaiensis reaching just 50-55 mm.

R. candidianus can be distinguished from other congeners by a combination of characters including: 15-17 (16) pectoral-fin rays; snout length usually less than 29 % of head length; postorbital around 52 % of head length; 27-28 vertebrae; 34-38 (35-37) longitudinal scale rows; 11-15 (12-13) scales between origin of first dorsal-fin and pectoral-fin; absence of spot-like markings in the second dorsal or anal-fin; absence of spots on cheek in females and most males; lateral body normally a uniform dark brown colour.

It’s included in the R. brunneus group of closely-related species within the genus of which members share the following combination of characters: predorsal naked, or with small cycloid scales, the scaled area never extending beyond the vertical of the posterior margin of the preoperculum; cheek with 2 mainly horizontal rows of sensory papillae.

In addition it belongs to an assemblage of fluvial, land-locked, non-diadromous Rhinogobius spp. which have relatively large eggs and 27-29 vertebrae.

The Gobiidae is the most speciose vertebrate family and notoriously problematic in terms of identifying fishes down to species level.

Within this sizeable assemblage Rhinogobius is often included in the subfamily Gobionellinae alongside genera such as Brachygobius, Chlamydogobius, Mugilogobius, Pseudogobiopsis, Schismatogobius, and Stigmatogobius.

Members can be told apart from these and all other gobiid genera by the following combination of characters: head with four simple, longitudinal infraorbital sensory papilla rows a, b, c, and d, single cp papilla, and paired papillae in mental row f; head canal variable from complete loss to normal development of anterior and posterior oculoscapular canals, and preopercular canals, and always with double interorbital pores λ if the pore is present; body mostly covered with ctenoid scales; longitudinal scale series 25–42; head including cheek, snout, opercle, anterior part of nape as well as pre-pectoral region all naked; D1 usually VI; D2 I, 6–11; A I, 5–11; P 14–23; and V I, 5 + I, 5, forming a rounded disc with frenum present, performing two pointed spinous lobes, the spinous ray usually longer than the first branched ray; dorsal pterygiophore formulae modally 3–22 1 101; vertebrae 25–29, usually 26 for most landlocked species.

The genus is widely-distributed throughout much of continental Asia in Russia, Korea, China, Vietnam, Laos, Cambodia, and Thailand, plus numerous islands of the Western Pacific including Japan, Taiwan, Hainan, and the Philippines.

There currently exist over 60 recognised species with many more awaiting formal description, and a number of the described ones are only considered nominal taxa pending additional study.

Those exhibiting similarities in appearance, morphology and behaviour are therefore often aggregated in nominal species groups, e.g., the R. brunneus group, R. duospilus group, etc., for ease of reference.

The fused pelvic fins form a structure normally referred to as the ‘pelvic disc’, a common feature among gobiids which is used to adhere to rocks and other submerged surfaces.

Rhinogobius spp. also exhibit different reproductive strategies depending on environment, with those inhabiting rivers connected directly to the sea typically amphidromous, and those landlocked in upper reaches of rivers or lakes non-diadromous.

Many of those appearing in the aquarium trade have proven difficult to identify for a number of reasons including:

– taxonomic confusion.
– lack of aquarium literature.
– incorrect labelling by exporters and subsequently shops.
– historical over-use of some names, e.g., ‘Rhinogobius wui‘ which is itself an invalid synonym of R. duospilus.
– likely trade of undescribed species without locality data.
– mixing of species at export facilities.

Thanks to Jutta Bauer.

References

1. Regan, C. T., 1908 - Annals and Magazine of Natural History (Series 8) v. 1 (no. 2): 149-153
   Descriptions of new freshwater fishes from China and Japan.
2. Chen, I-S. and K.-T. Shao, 1996 - Zoological Studies 35(3): 200-214
   A taxonomic review of the gobiid fish genus Rhinogobius Gill, 1859, from Taiwan, with description of three new species.
3. Ho, H.-C. and K.-T. Shao, 2011 - Zootaxa 2957: 1-74
   Annotated checklist and type catalog of fish genera and species described from Taiwan.
4. Oshima, M., 1919 - Annals of the Carnegie Museum v. 12 (nos 2-4): 169-328
   Contributions to the study of the fresh water fishes of the island of Formosa.