Relationships
The distinguishing characteristic of shrimpgobies is that they form a partnership with shrimps. As the name implies they are a type of goby, within the fish family Gobiidae. There are at least 2000 species of gobies with more still to be described. Half of them live on or around coral reefs.
Gobies are small fishes vulnerable to predation for which reason they form associations with numerous other lifeforms such as sponges and soft corals or, in the case of the shrimpgobies, with burrow-making shrimps. There are approximately 100 species that are associated with shrimps. Shrimpgobies (and all other types of gobies) have a short life span so they breed while still small and continue to grow throughout their lives.
Morphology
Shrimpgobies are small, thin, elongated fish that include several distinctive genera that look quite different, although the species within each genus can be very similar and hard to identify.
The small size of shrimpgobies has led to their use of shrimp burrows and the development of several morphological modifications.
Many species of shrimpgobies have the pelvic fins joined, useful for their custom of perching on a high spot to survey the surrounding area for food and predators. Other possibly relevant shrimpgoby modifications are a tendency to scalelessness as seen in Tomiyamichthys nudus, and a tolerance of low oxygen levels which may be important to species living in burrows in anaerobic silt.
Design and colouration
Usually, species can be distinguished by their colouration and patterns but there are certain characteristics common to all members of a genus, or even most sand gobies, shrimpgobies or otherwise.
The presence of 4 to 6 bars or patches along the side of the body is one of these. These may be replaced by fine vertical lines. The side of the face is usually marked with spots or lines, the arrangement of which can be useful in distinguishing species. The surrounds of the eyes often exhibit a characteristic alternating black and white pattern. This readily changes to all black for a minute or so in response to emotion.
One of the most beautiful aspects of shrimpgoby colouration is an overlay of vivid iridescent or reflective spots and lines that catch the light and outshines the base pattern.
The previous image shows how this iridescence depends on the exact incidence of the light striking the fish. It can be seen that the reflective patches may or may not match the base pattern. We do not know the purpose of this but presumably, it is visible to other fish.
The shrimpgobies are usually counter-shaded darker above and paler below, camouflage enhancing for a creature living in well-lit shallow water.
They can change colour dramatically when they display. The photo shows a Diagonal Shrimpgoby, Amblyeleotris diagonalis, changing from a bold pattern to a very pale pattern in just a few seconds.
They also show considerable variation in different habitats with individuals from dark sand being particularly colourful and those from white sand being very pale.
Growth rates
Figures are available for the Japanese Shrimpgoby, Amblyeleotris japonica (Yanagisawa 1982). Newly hatched larvae are 1.5 mm long and they settle on the sand when about 10 mm long. At this stage, they are transparent with blotches where the lateral bands of the adult will be. In 6 months they grew to 20 to 40 mm, and by 12 months they were 50 mm. Sexual maturity indicated by fully developed gonads occurred at 60 mm in females and 65 mm in males. Males are usually larger than females. Life span was calculated to be 2 years.
Pair formation
In general pair formation is usual in gobies that live in low densities or exposed conditions on sand and rubble. This is because of the risks of being killed in the course of a prolonged search in scant cover. Pair bonds formed in these conditions are typically monogamous and long-lasting. A problem with monogamous relationships is that it becomes difficult for unattached individuals to link up with suitably sexed and available potential partners. Some other goby genera are capable of bi-directional sex change though not shrimpgobies as far as we know. When they live in greater concentrations shrimpgobies have more opportunity to meet potential partners and consequently, pair bonds are more flexible with a certain amount of partner swapping going on.
Shrimpgobies breed throughout the year in tropical latitudes and pair formation is more consistent than in sub-tropical regions where breeding and pair formation are summertime activities. The pattern is not consistent and varies between different genera. Males are more active in initiating pair formation and may compete aggressively for females (the larger usually wins). The victor may develop a stable relationship or may suffer a reversal at the hands of a new, bigger contender. Pairs are maintained for several days and usually result in spawning but even when pair bonds are loose and temporary the male only mates with one female at a time as he is needed for care of the egg mass.
Reproduction
Mating and egg-laying take place in the burrow and cannot be observed in the field.
In sub-tropical waters breeding is seasonal but most species are tropical and breed continuously.
Several observations, notably by Karplus (1987) in the Red Sea, and Yanagisawa (1982) in Japan indicate that the gobies form pairs in readiness for mating in the summer. At this time males roam the breeding territory, displaying to other males. Although the larger males establish dominance there is no evidence of polygamy and once a male pairs off with a female they settle down together. Once the eggs are laid in the burrow the female leaves and the male stays inside the burrow guarding the eggs until they hatch in something like 4 to 10 days. These are aquarium observations and may differ a little from the wild. Yanagisawa had a shrimpgoby lay an egg mass in an evaporation dish in an aquarium. There were twenty thousand eggs, each ellipsoid and 1.1 mm in length, a modest number by marine standards. After they hatch they are wafted to the burrow entrance by the action of the male’s fins, and join the zooplankton.
Observations by Preston (1978) in Hawaii and Cummins (1979) in Australia show that there is no seasonality in the tropics and the gobies breed throughout the year. In sub-tropical climates, like the Red Sea (Karplus) and Japan (Yanagisawa) breeding is restricted to the summer months. We have observed possible seasonality in the sub-tropics in Australia at Hervey Bay.
Duerbaum (2012) studied Shrimpgoby reproduction in the burrow using an aquarium setup. What he found was that the pair indulge in a vigorous mating dance in a side chamber purpose-built by the shrimps. In the confined space this tends to bring the roof down which requires regular repair by the shrimps. Eggs are laid at night and the female leaves the egg chamber. The male remains in the chamber oxygenating the egg mass by fanning water past with his pectoral fins until the larvae hatch. This may be of additional benefit to species living in burrows constructed in anaerobic silt. During this time the male rarely leaves the chamber. The shrimps are excluded from the egg chamber possibly because they could mistake the egg mass for food. All males that successfully spawned were the larger, socially dominant ones and they remained in the burrow for 4 to 7 days after spawning to care for the clutch of eggs.
Shrimpgobies have a lifespan of about two years and a pelagic larval duration of about 5 to 10 weeks. Higher seasonal temperatures increase growth rates. In common with other gobies they may breed from an early age and continue to grow throughout their lives, characteristics of fish with a high mortality rate.
Daily Activity and Home Range
By day the shrimpgoby’s time is partitioned between that spent within the burrow, that spent at the burrow entrance and that spent away from the burrow, where they interact with other fish. Shrimpgobies spend 60 to 80% of the day around the entrance, 10 to 30% away from the burrow and the rest within the burrow. The proportions depend on local predation pressures, the fish spend more time inside the burrow and less time away when predation pressure is high.
They seldom spend long away from the burrow and as soon as they return insert their tails into the entrance while waving it gently to and fro. This usually results in the immediate appearance of the shrimp apparently eager to resume work.
Movements are restricted to an area of about 3 metres radius around the burrow, usually much less. Experiments in which they are relocated away from the burrow show that they know their surroundings well enough to get back within this range but get lost if taken more than 3 metres away.
Gobies, singly or in pairs may use more than one burrow entrance at a time. Home range areas consequently are randomly shaped and generally don’t overlap, being considerably smaller where population density is high.
At night the shrimpgobies and shrimps stay inside the burrow which is sealed by the shrimp. Soon after dawn the goby breaks through and opens the entrance. Burrows have several entrances but only one is opened at a time, the position changing from day to day.
At the end of the day, the shrimp collects a pile of rubble outside the entrance and uses this to seal the hole when they retire for the night
Diet and Foraging
Most shrimpgobies feed on benthic organisms spotted and pounced on near the burrow. These are mostly amphipods and other small crustaceans. Most of them are too small to see with the naked human eye though the fish makes vigorous chewing movements after snatching one from the sand. From time to time the goby will be seen to catch a shrimp as big as a small partner shrimp. This raises the interesting point that there must be a mechanism inhibiting them from eating their partner shrimps.
Diet Photos
Shrimpgobies sometimes feed on planktonic organisms they catch in the water column immediately above the burrow entrance. This is normal behaviour for species of Stonogobiops and Vanderhorstia but is also seen from time to time in bottom-feeding genera such as Amblyeleotris.
Like other sand gobies, the shrimpgobies repeatedly take mouthfuls of sand and filter it through their gills to obtain microscopic food matter.
Seasonal variation in population size
In places where there is a disparity between summer and winter water temperatures there seem to be fewer shrimp and goby pairs in the cold months. This has been investigated by Yanagisawa (1982) in southern Japan where water temperatures range from 12o C in winter to 28o C in summer and it seems that this is because they stay in their burrows in the cold conditions. More are seen out and about as the water warms and by early summer females were usually pregnant. Population numbers are boosted by the arrival of small juveniles in late summer. Some of these juveniles do not survive and the remainder balance the mortality of fish over 2 years of age, the usual life expectancy for these little fish.
Display and threat behaviour
This takes the form of adopting maximum visibility usually by hovering, and erecting all fins while displaying with a widely gaping mouth. This response is seen when a similar-sized fish, usually another shrimpgoby, approaches too close to the burrow.
The other shrimpgoby will display similarly and the encounter may end there. If not the pair line up head to tail and threaten each other. The encounter may then escalate into full-blown fighting with one of the fish vacating the scene much the worse for wear.
Fighting is mostly restricted to males and the outcome is usually that the larger fish wins. Aggressive encounters between adjacent gobies sometimes result in changes in the occupation of burrows.
Much of this aggression is directed against ‘floaters’, those smaller gobies who currently have no base and patrol the substrate keeping out of trouble while looking out for a vacated burrow.
Similar display behaviour is seen when a pair reunite after separation. This is not aggression.
Locomotion
The goby moves around by swimming chiefly with its pectoral fins. This enables it to swim forward or backward and generally manoeuvre into the most suitable position to keep contact with the shrimp. When it needs to swim powerfully to escape from danger it swims in a sinuous fashion using the side of the body and the tail for propulsion.
Normal activity with the shrimp involves simple movement to and fro depending on how far the shrimp needs to go to complete its construction work. This is carried out with the goby always facing away from the entrance, eyes directed towards any untoward activity nearby. In times of real danger, the goby spins around and dives headfirst down the burrow. The shrimp shoots in backwards.
Some goby species, notably the Scaleless Shrimpgoby, Tomiyamichthys nudus, do not swim to and fro from the burrow but crawl along using the pectoral fins as legs in a slow and debilitated-looking way. This may be more inconspicuous.
Some gobies such as the Blackchest Shrimpgoby, Amblyeleotris guttata, suddenly move away from the entrance in a bizarre series of sharp hops with the body almost upright and fins extended at the peak of each hop, This behaviour is possibly directed at other nearby gobies, After moving some distance they settle into an observation posture and wait. As does the shrimp left behind in the burrow entrance.
Posture at the burrow entrance
The pelvic fins in most species, are joined proximally forming a strong Y-shaped structure on which the goby can perch with its head high surveying the surrounding area for danger or food. The goby perches on this fin on the highest point at the burrow entrance that will allow the shrimp to have antenna contact from within the entrance.
From time to time the shrimpgoby leaves the lookout, either briefly to catch something in the sand, or simply wanders off over the sand often for a considerable distance for no obvious purpose. When this happens the shrimp is suddenly deprived of contact and will not emerge no matter how focussed it has been on the current project. When the goby returns it backs into the burrow tail-first. If it has swam back a long way it will turn around at the entrance before backing in. The goby inserts its tail into the burrow and with a broad swish, encourages the shrimp to resume work, a message that usually gets an immediate response.
Some goby species do not perch but hover just above the entrance within antenna’s reach. If they hover too high the shrimp can tell that they have not left and will reach up and grab at fins or tail to bring it closer. If the goby does go off out of reach the shrimp stays in its shelter.
Parasites of Shrimpgobies
(ADD PHOTOS HERE)
also a spiral parasite Great Barrier Reef late Dec 2013
The nudibranch Gymnodoris nigricolor attaches itself to the fins of shrimp gobies at times. We filmed this nudibranch moving along the sand until it encountered a burrow and entered it. Presumably, they climb onto the fish while it is sleeping. It is thought that the nudibranch feed on the fin membrane of the shrimpgoby and they usually attach themselves to the base of the dorsal or caudal fin.
Predators of Shrimpgobies
The primary predators appear to be fish. Two categories come to mind. The first is the opportunistic predators such as trevally cruising the sand flat, and then there are the creatures living permanently in the same habitat such as lizardfish, grubfish, flounders and snake eels.
Lizardfish and Sauries seem to be the major predators. They are consistently found in the vicinity of the burrows. They bury themselves in the sand, sometimes right in the sand channel that leads to the burrow entrance, with only their eyes showing, ready to ambush the goby. The image shows a lizardfish (species) eating a Yellowfoot Shrimpgoby, Vanderhorstia phaeosticta.
The gobies keep watch for roving predatory fish patrolling the sea bed. They seem to distinguish between more and less dangerous species, responding appropriately by partially or fully withdrawing into the burrow. It is difficult to assess their reaction to lurking lizardfish, but these are probably noted as the goby scans the neighbourhood sand.
The shrimps are probably vulnerable to the same predators as the shrimpgobies.
Dolphins
Dolphins are not thought of as predators of shrimpgobies but it is not unreasonable.
We formed this impression while diving at Bolt Bay, Percy Islands, Queensland. We had noticed dolphin fins at the surface before the dive and heard their calls during the dive. The sea bed there was flat sand with small amounts of seagrass. Although we saw many burrows the fish were excessively nervous and disappeared long before we could get close. We found several conical holes typical of dolphins foraging by tunnelling into the silt to capture fish or other creatures they have detected. The Giant Shrimpgoby, Amblyeleotris fontanesii was common in this area and would provide a tasty mouthful for a dolphin.
Sharing the burrow
Some fishes that hover in mid-water use the shrimpgoby burrow as a temporary refuge when threatened. Notable amongst these is the Threadfin Dart, Ptereleotris hanae. These beautiful blue fish seem to be accepted by the shrimpgoby and it has been suggested that in some instances they take up permanent residence. The image shows the Threadfin Dart emerging from a burrow accompanied by the Ventral-barred Shrimpgoby, Cryptocentrus sericus. (ADD PHOTO)
Werding et al. 2016, found the Porcellanid crab, Enosteoides lobatus, sharing a burrow with Amblyeleotris and Cryptocentrus shrimpgobies and their partner Tiger Shrimps. It is not known if the porcellanids are in pairs or move from burrow to burrow to find mates.
De Grave and Anker 2000, found a second species of shrimp (Orygmalpheus polites) possibly sharing the burrow with Alpheus rapicida and a Vanderhorstia shrimpgoby.
We have found a Periclimenes species (LOOK UP PHOTO) shrimp cleaning Amblyeleotris ???? at the burrow entrance. This was in an area of bare sand a considerable distance from any other shelter for the shrimp so presumably it was living in the burrow. (Sue’s photo) Capricorn, Bunker, Mooloolaba region