Shrimps and Shrimpgobies

Fish and Crustaceans are not closely related. One is a vertebrate and the other an invertebrate. There is no good reason why animals of these two groups should be able to communicate with each other at all, never mind do so in a way that benefits them both. 

As we shall see, the relationship involves a remarkable degree of cooperation and awareness of each other’s behaviour. 

The partnership is restricted to only certain gobies among the fish and a small number of crustaceans, all from the group known as snapping shrimps. The gobies belong to several genera but in all confirmed associations the shrimps belong to only the one genus, Alpheus. They are often called partner shrimps to distinguish them from other Alpheus shrimps that do not form this type of association. 

Some gobies are able to dig their own burrows but the group we are concerned with are quite unable to. When placed in an aquarium they make a depression in the sand and lie half buried in it. They need a partner shrimp to dig a burrow. 

A single species of shrimpgoby may show a preference for only one species of partner shrimp but this is not usually the case. Mostly it will have a favoured shrimp species with which it will regularly form a bond and a number of alternate shrimps with which it forms less frequent bonds. Shrimp associations with shrimpgobies follow the same pattern. Even when the relationship involves a number of species it is selective, not random. 

Although we have so far talked of a single shrimp teamed up with a single individual goby the ideal combination is a pair of shrimps, male and female, with a pair of gobies, again male and female, so each pair can breed. In the early stages of the burrow there is always one goby. Later it will be joined by a partner. It is less easy to be sure whether the burrow is initiated by a singleton or a pair of shrimps. Among the gobies there is a certain amount of changeover of burrow partners but a shrimp pair tends to remain constant. 

The relationship is one of “mutualism”. That is to say both parties benefit and neither is disadvantaged. This contrasts with “commensalism” in which only one partner benefits though the other in not harmed and “parasitism” in which one partner benefits at the expense of the other. 

When the relationship is so well established that the shrimp and goby have a consistent stereotyped set of behavioural and communication patterns it is said to be obligatory. When it is still in the process of evolving and communication is inconsistent and behaviour unpredictable the relationship is called facultative. In this case the shrimp maintains less consistent antenna contact and the goby tends to spend time away from the burrow. All the shrimpgoby species but one studied in this book have obligate relationships. The exception is Acentrogobius cenderawasih a species recently described by Allen and Erdmann (2012) from Indonesia and observed by us in the Solomon Islands. Its behaviour and ecology are described in the species account. 

Benefits of the partnership

Predator protection for both

The obvious benefit is that the goby provides the shrimp with a lookout and the shrimp provides the goby with a home. This increases life expectancy for both partners. The ocean is full of predators at all levels and without each other life could be short and troubled.

Habitat adaptability

Many areas of the sea floor are left unoccupied by fishes because of lack of cover. The shrimp and goby partnership makes its own cover and so gets to use these areas. 

Improved reproductive success

Both shrimp and goby are able to reproduce in a secure underground tunnel away from predators during this distracting time. Eggs are produced and fertilised in the burrow. The fertilised eggs get parental care and protection in a place where they and the protecting parent are shielded from predation. 

Partitioning of food 

Harada (1969) studied stomach contents of the Tiger shrimp, Alpheus bellulus and its partner Amblyeleotris japonica and found that, although they were sourcing food from the same place they did not compete for food. The goby was eating mostly Corophiid amphipods, Harpacticoid copepods and forminiferans, while the shrimp was eating other some amphipods and nematodes, but mainly detritus. 

In the field it is clear that most of the diet of the shrimps is fresh algae. 

More food for the shrimp

The relationship allows the shrimp to forage in greater safety. Foraging involves collection of vegetable matter, fresh algae, seagrass and decaying detritus, often some distance away from the burrow. This would leave it very vulnerable but for the fact that the goby accompanies it. This seems to be learned behaviour driven by the shrimp, and the early stages can be entertaining as a goby that is slow on the uptake is chivied along by a nip on the tail from the shrimp. 

The goby finds food for the shrimp to eat

Jaafar (2014) has an image of a goby Amblyeleotris latifasciata handing over a small crab to its shrimp, a Tiger Shrimp. His interpretation is that this was food for the shrimp. He makes the point that the importance of gobies as food providers to shrimps has rarely been discussed. 

We have watched a goby bringing back a small non-partner shrimp and leaving it in the burrow entrance, possibly as food for the shrimp. 

We have also seen a Maude’s Shrimpgoby, Cryptocentrus maudae, dragging wood shavings alga (Padina species) into the burrow for a Pale Marbled Shrimp. 

The video below shows the Yellowface Shrimpgoby, Stonogobiops xanthorhinica, catching current-borne algal matter and bringing it to the burrow entrance for Randall’s Shrimp. 

The goby helps the shrimp in another way: if the shrimp loses its grip on a foraged fragment the goby will go after it, retrieve it and carry it back to the shrimp at the burrow entrance. (Duerbaum 2012). 

This is truly wonderful behaviour when you reflect on it. There is an altruistic relationship at work here. It goes a long way beyond a chance association between unrelated species that has persisted because it happened to increase their chances of survival. 

More food for the goby

Gobies can forage for themselves over undisturbed sand but more frequently rely on the tasty morsels that turn up in the sediment stirred up by the shrimp’s bulldozing activity. They also pick up mouthfuls of turned up sand and filter this through their gills. 

The shrimp cleans the goby

Cleaner Shrimps are not the only crustaceans that engage in cleaning fish. There are reportedly 40 species from 20 genera that do so. The role of the partner shrimp as a cleaner-of the shrimpgoby was first reported by Karplus in 1974 from a pair within an artificial burrow in an aquarium. The shrimp steadied the goby with its major chelipeds and cleaned it with the second pair, transferring the pickings to its mouth. 

In this study we only observed cleaning at the burrow entrance on a couple of occasions. 

More recently Hou, Liew and Jaafar (2013) have studied this behaviour in more detail and quantified its frequency and nature. They studied Myersina macrostoma and a partner shrimp identified as Alpheus rapax, one of each, in an artificial burrow in an aquarium. Behaviour was recorded by video camera and later analysed. They found that cleaning sessions involved the caudal region (60%), followed by the head (28%). Cleaning sessions were always initiated by the shrimp and always terminated by the goby. The goby repositions itself so the area it wants cleaned is brought close to the shrimp, even lying on its side to make sure its lateral and ventral aspects get attention. 

This has also been described in the wild by. Senou and Mori (2001) in Stonogobiops yasha. 

It is more likely that most cleaning only takes place within the burrow in the natural habitat. 

Jaafar (2014) provides beautiful photographic evidence of the Red Shrimp, working as a cleaner shrimp for its Ventral-barred Shrimpgoby, Cryptocentrus sericus. In another instance Jaafar observed and photographed Randall’s Shrimp feeding on the faecal matter of its associate goby, Stonogobiops nematodes. This may have as much to do with cleaning as diet. 

It is postulated that this cleaner behaviour is directed at removing parasites, making it beneficial to the health of the goby, but this has not been proved. It may be mostly a matter of removing dead skin, which the shrimp would still count as food. Certainly it is not fully effective as the black nudibranch Gymnodoris nigricolor, small spiral isopods and other parasites are commonly seen on shrimpgobies. 

How breeding pairs meet up 

There are two aspects to this question: how do the shrimps meet the gobies and how do each find a partner. Yanagisawa (1984) has studied how the shrimp finds a mate, using the Tiger shrimp, Alpheus bellulus, partnered with the Japanese Shrimpgoby, Amblyeleotris japonica as the example. 

The shrimps start to build a burrow from the time they leave the plankton. This usually has the effect of separating them and making pair formation more difficult. Juvenile shrimps remain solitary in their burrows, from the start of their benthic life until 4 to 6 months after settlement when they manage to start forming pairs. How they do this is unclear. Walking across the surface is too risky. It has been suggested that they sometimes encounter other burrows as they tunnel their shallow offshoots. If this leads to an encounter with a neighbour of the appropriate sex they form a bond. When two shrimps move into a burrow together some other burrow loses its shrimp and some goby or gobies must make other plans. . In the end only 20% of adults have no partner. paired off. 

For the shrimps finding a partner is critical, not only for breeding but because both are capable of excavating a burrow and two can do a better job than one. 

How the gobies find mates is less complicated. They can move away from the burrow and find one when the time comes to breed. In temperate waters breeding is seasonal, but not in the coral sea,though things seem to slow down in the southern part in winter. 

How the right Goby meets up with the right Shrimp

How the right goby meet up with the right shrimp in the first place is one of the most mysterious aspects of the relationship, particularly in the specialist associations where the pair bond is restricted to a single species of goby and a single species of shrimp. 

It is assumed that the goby, being more mobile, chooses the shrimp, which seems reasonable. It is generally stated that a solitary shrimp constructs a burrow, and a partnerless goby wandering randomly over the sea bed locates the burrow by vision and joins up with the shrimp. This is not a very satisfying explanation and it does not agree with the fact that shrimp and goby distribution is far from random. 

Studies of how the goby finds the right shrimp have not really been helpful. The studies are carried out in an aquarium setting using gobies caught in the sea after having habituated to living in burrows with a single species of shrimp. The gobies under study show a preference for burrows rather than rock or other shelters. They prefer them wide mouthed and dark. They recognise the appearance of the shrimp species with which they have been living and will choose it in preference to a shrimp with a different appearance. This is especially the case when the alternate choice is a species of shrimp that does not form partnerships with gobies and has a very different appearance. They show no size preference in the shrimp so long as it is the partner shrimp they have been living with. 

Once again we must look to field observations for explanations. Both shrimps and gobies have precise patterns of distribution based on depth, distance from the reef and particularly substrate characters. 

Both are small animals and very vulnerable to predation if they spend time unprotected. This would be particularly true if they start looking at the wrong depth or distance from the reef because these factors determine the best location for the burrows. 

It is tempting to speculate that the successful pairing of shrimp and goby may start before they settle on the reef and be facilitated by their reproductive biology. 

Shrimpgobies spawn in one of the chambers of the burrow and the fertilised eggs are attended by the male until they hatch. The shrimp spawn in a separate chamber of the same burrow and fertilised eggs are carried by the female until they hatch. In both cases hatchlings are released to begin their planktonic life from a localised spot, the burrow entrance. 

This would greatly increase the chances of appropriate pairs drifting in close association.We have encountered very juvenile shrimp and shrimpgoby pairs indicating that they can pair up at a very early stage of their life.

The effect would be enhanced if the release of hatchlings were synchronized. They could also be chemically attracted to each other while in the plankton or soon after settling. 

If there were a different chemical signal for each species it would ensure that they meet with biologically compatible partners with the least delay. 

This is speculative and there is no evidence for the idea. The alternative possibility is that the shrimps settle randomly with survival only of the those that settle on or find substrate that matches their burrowing capabilities. Our observations have shown that the distribution of shrimps and their partner shrimpgobies is determined by the characteristics of the substrate. depth and distance from the reef being relevant only inasmuch as they are predictors of the probable substrate. 

This was shown in an 18 month survey we carried out at the Low Isles off the Queensland coast. The study area was silty but medium and coarse sand could be found In small localised patches in consequence of the island’s being a silty mangrove island with a secondary sandy coral cay. These patches were home to shrimp and goby species found nowhere else on the study site and only found with any consistency near the outer barrier reef 35 miles away. These patches of atypical habitat are only a few metres in extent and it is a bit of a stretch to say that suitable species are attracted there by chemical cues not chance. 

We have seen the same thing in the Solomon Islands. We know of an apparently uniform 10,000 square metre flat coral terrace colonised by two shrimpgoby species, the Blackchest Shrimpgoby, Amblyeleotris guttata and the Flagtail Shrimpgoby, A. yanoi. 

The Flagtail Shrimpgobies, A. yanoi, are restricted to just a 5 square metre patch in the middle of this habitat, where they associate only with Randall’s Shrimp. Whereas the Blackchest Shrimpgobies, A. guttata, colonise the whole of the remaining area accompanied by five species of shrimp, but not Randall’s Shrimp. 

It seems wasteful for this sort of demarcation to depend on random planktonic spread of the unrelated embryos and random encounter of the goby with the right shrimps. Perhaps there is a combination of mechanisms. 

The fact that these shrimps are not found on areas of the sea bed where the partner goby does not occur and the association is maintained from the first stage of their benthic life (Yanagisawa 1976) seems to support the existence of chemical cues in the initiation of this obligatory relationship between shrimpgoby and partner shrimp. 

How do they get to the right part of the reef

Meeting up is only part of the problem. How do the young shrimp and shrimpgoby ‘know’ they have arrived at the right patch of reef as defined by the critical criteria of depth, sediment characteristics and distance from the reef? The shrimp can hardly dig multiple trial burrows until they arrive at a site with the proportions of sand, silt and rubble that their physiology and anatomy requires. 

Both shrimps and gobies have lived from fertilisation to hatching in an environment that reflects the ecology of the burrow. The predilection for these ecological characteristics could become implanted in their nervous system so that at the end of their planktonic stage the juvenile fish and shrimp actively seek out the similar, appropriate depth, sediment and distance from the reef. 

Other decapods rely on chemical cues to find their way to the host with which they associate. For example Periclimenes pedersoni and P. yucatanicus rely on cues released by their host anemones for enhanced late larval settlement, a matter of importance in obligatory commensal relationships. (Goy 1990, quoted by Calado 2003)

It may be that in some cases the young do not drift very far from the parental burrow. Some pairs are found only in quite specific situations such as small caves in the reef. Newly hatched young would be released into relatively current-free water and may travel quite short distances along the wall before settling into similar overhangs. Something similar may apply to young released from burrows in sheltered muddy habitats. 

How did this partnership evolve?

The relationship between shrimpgobies and their partner shrimps is remarkable and complex but it may have simple origins. Gobies of several species rely on finding a burrow to hide in when danger threatens. In the short term they will use any burrow available. If a particular shrimp burrow is used regularly the shrimp and goby may come to recognise and tolerate each other, sharing the burrow harmoniously, though not with any real system. Something like this has been described in the artificial conditions of an aquarium involving a snapping shrimp and the Ornate Sandgoby, Istigobius ornatus (which is not one of the shrimpgoby species)(Debelius, 1986). Over evolutionary time the chance association, being beneficial to both animals, could evolve into a more structured relationship, ultimately becoming the full blown partnership described in this book. 

There are several examples of this partnership in its transitional stages. In the Western Atlantic there are three gobies that have been described as associating with the shrimp Alpheus floridanus. One, the Orangespotted Goby, Nes longus, communicates by a reliable system of tail flicks. Another, the Dash Goby, Ctenogobius saepepallens, uses tail-flick communication only intermittently, while a third species, the Notchtongue Goby, Bathygobius curacao, is reported to provide no early warning system. Confronted with danger it simply flees head first into the burrow. This again seems to be a work in progress! (Karplus 1992, Randall et al. 2005, Kramer et al. 2009 Quoted by Thacker et al. 2011 Phylogeny). 

Of greater interest are those Indo-Pacific gobies that display a facultative relationship with known partner shrimps. 

Yanagisawa (1978) describes three levels of relationship in Japanese fish. 

The first is represented by the Threadfin Dartfish, Ptereleotris hanae, a fish that hovers, seeking refuge as readily in crevices as burrows and never communicates with the shrimp. Interestingly at the time of publication, the Whitecap Shrimpgoby Lotilia gracilosa was thought to belong in this category. 

The second is represented by the Striped Sandgoby, Acentrogobius pflaumi, a bottom dweller with a weak, arbitrary and facultative tactile alarm communication with the shrimp. 

In this book we discuss what appears to be a facultative relationship developing with another speciesof Acentrogobius. The photo shows the Cenderawasih Goby, Acentrogobius cenderawasih, interacting with a Pigpen Shrimp. We watched this goby and shrimp work together at the burrow entrance. The shrimp maintained contact with the goby with its antennae when outside the burrow. This is a recently described goby species from Indonesia although we recorded this in the Solomon Islands. They are not known to form partnerships with shrimps in Indonesia. 

The third level is represented by all the known shrimpgobies where there is a complex and obligatory communication with the partner shrimp.

Communication between Shrimp and Goby

Most of the communication between goby and shrimp relates to the goby’s role as lookout and guardian. It keeps watch from the entrance, characteristically perched at an angle on its joined pelvic fin. From this vantage it surveys the surrounding area for food, rivals and danger. 

In the face of seriously threatening danger the goby spins round and dives head first into the burrow. The shrimp gets no warning when this happens but it has such good reflexes that it will shoot backwards into the burrow so fast that it arrives before the fish! This is a response to sudden muscular tension and abrupt movement and is not a communication as such. This does not mean that all shrimp responses are nonspecific and coincidental. This is a finely nuanced conversation. 

While keeping watch the goby will from time to time suddenly dart forward to capture a morsel of food or to display to another goby. This temporary loss of contact does not panic the shrimp. It evidently can distinguish this behaviour from panicked retreat. If further reassurance is needed it is provided by the goby swimming backwards, using its pectoral fins, until antenna contact can be re established with the shrimp. A goby approaching the burrow tail first is reassuring. One coming head first is not!

When the nature of the danger is less certain the goby transmits with its tail a repertoire of more subtle communications. 

While it sizes up the danger the goby will not permit the shrimp to leave the burrow. This is achieved by a specific warning, a sharp flicking of its tail, and sometimes by physically holding the shrimp back. The shrimp responds to this message by staying just within the entrance, sometimes with every appearance of impatience to resume bulldozing. 

When the goby cannot quite asses the danger it makes repeated small jumps. If this goes on too long the shrimp will ignore it and resume work. 

When it transpires that this was a false alarm, for example a diver has moved off or appears to be uninterested, the goby gives an “All Clear” signal by gently waving its tail from side to side, and the shrimp will resume bulldozer duties. 

Sometimes the shrimp misses the “all clear”, perhaps because it is busy far back in the burrow. When this happens the goby backs its tail right into the burrow and waves it from side to side to encourage the shrimp to emerge. The same signal is used at any time there has been no sign of the shrimp for a while. It usually results in immediate appearance of the shrimp, as though it had been patiently waiting inside the entrance for just this message. 

When the shrimp needs to move more than antenna length away from the burrow for bulldozing or foraging, the goby accompanies it. The goby occasionally initiates the movement away from the burrow entrance by moving out first so the shrimp has to follow if it is to maintain antenna contact. This is usually a matter of following an existing excavation so it can be extended but the direction may be more random. 

There must be a period of adjustment at the beginning of the partnership. The shrimp and goby start off as strangers and they have to develop their pattern of communication. We have seen and recorded this process where the burrow is manned by a pair of large fully grown shrimps and the goby is an inexperienced juvenile. The shrimps who are much bigger than the goby push it around quite aggressively if it gets in their way. or if it is slow to accompany them out along the construction channel. This combination of large shrimps and small gobies probably arises from the loss of the original adult goby leaving them having to wait for a juvenile in search of its first burrow. 

From time to time the goby will wander off on its own, often for a considerable distance of two or three metres. It is not clear why they do this. It may be to find food or simply to check out the neighbours. They are quite territorial and spend a lot of time fin-signaling to others of the same species in nearby burrows. While it’s shrimpgoby is away the shrimp stays out of sight in the burrow. Shrimps don’t go walkabout on their own. 

We have seen a disastrous misunderstanding when a goby went walkabout while the shrimp was maintaining antenna contact. The shrimp trustingly kept walking alongside until, half a metre from the burrow, it seemed to recognise that this was wrong and stopped. The goby continued, contact was lost, the shrimp was in unfamiliar territory and, being disorientated, it fled for shelter in the wrong direction. After a three metre scramble in a strange flattened posture it found refuge in a burrow that was only just big enough for it to squeeze into. Moments later a smaller shrimp of the same species emerged from that burrow and went down another nearby burrow, possibly setting off a chain reaction. 

Sometimes there is a breakdown in communication. in the video following the Violet Shrimp has been abandoned by the Cebu Shrimpgoby, Cryptocentrus cebuanus. The Shrimp returns to the burrow and chases the Goby back out side with a couple of nips with its nippers.

The relationship between shrimpgoby and shrimp may look like that of a boss and a worker but the shrimp does not hesitate to push the goby around when its behaviour is not up to standard. 

A goby that that gets in the way of bulldozing operations is unceremoniously pushed aside or may even have a load of sand dumped over its back. 

A goby that does not accompany its shrimp as it works along the gutter will have its fins nipped until it gets going or, if it is an inexperienced juvenile, will be picked up with a load of sand and carried to where it is needed. 

Some species of goby spend time hovering above the burrow entrance displaying to neighbouring gobies. If they hover too high the shrimp cannot maintain antenna contact. When this happens we have seen the shrimp stretching up towards the fish and dragging it down by its tail. 

The distribution of Shrimp and Goby pairs

HABITAT REQUIREMENTS 

The distribution of shrimp and goby pairs is broadly related to habitat characteristics. The significant features that we observe are the substrate, the water depth and the distance from the reef. 

THE INFLUENCE OF SUBSTRATE

The most significant criterion of suitable habitat is substrate that matches the shrimp’s engineering requirements. There are shrimp that only live in silt and others that never make burrows in this material. Within each substrate, be it silt, fine or coarse sand or rubble, there is a group of shrimps that favour this zone and a corresponding shrimpgoby population. 

You might ask why the gobies don’t just associate with a wider selection of shrimps so that they can live in a broader range of habitats. The fact that they don’t suggests that habitat factors are just as important to the fish. In other words the choice of silt or sand is not entirely made by the shrimp. Some phylogenetic studies split shrimpgobies along the lines of silt dwellers and sand dwellers. Although there are obvious exceptions this division also suggests that some piscine choice is involved. 

The significance of substrate characteristics is discussed in detail in chapter 3 on the Biology of Shrimps. 

Substrate is the most obvious, but not the sole habitat characteristic determining the distribution of the shrimp and goby pairs. Water depth and distance from the coral reef seem to also be important. 

THE INFLUENCE OF WATER DEPTH ON DISTRIBUTION IN A GIVEN SUBSTRATE

At a given location on a constant substrate it quickly becomes apparent that some species are found only in the shallows, others on the slope and others only on the flat sea bed. We have studied this at lizard Island in the Great Barrier Reef lagoon:

The area we studied was a dissected fringing reef with a uniform white sand substrate. The white sand extends with little change in consistency from 3 metres down a 40 degree sand and rock slope to a mixed sand, algae and Halophila seagrass flat at 20 metres. We found shrimp goby distribution to be partitioned in a well defined vertical arrangement. It is unusual to find any of these species out of their depth range at this site although we are only talking about a horizontal distance of 40 metres. 

The distribution we observed is this:

Shallows, 0 to 5 m

Ctenogobiops feroculus, Ct. crocineus, Ct. mitodes, Ct. pomastictus, Cryptocentrus cinctus and Cr. strigilliceps. 

Slope, 5 to 15 metres

Amblyeleotris guttata, A. periophthalma, A. steinitzi and A. wheeleri. 

Deeper slope, 15 to 20 metres

Amblyeleotris ogasawarensis and A. diagonalis. 

Halophila seagrass flat, > 20 metres

Amblyeleotris stenotaeniata, Cryptocentrus fasciatus, Tomiyamichthys nudus, T. lanceolata, Vanderhorstia dorsomacula, and V. phaeosticta. 

THE INFLUENCE OF DISTANCE FROM THE REEF ON DISTRIBUTION IN A GIVEN SUBSTRATE

We recorded the horizontal species distribution in the same locality in a sand bay with uniform substrate and only slight change in depth from 3 to 6 metres. The species distribution, relative to distance from the reef showed a similar, although not identical, pattern of distribution. 

On the sand patches between the rocky extensions of the reef. 

Ctenogobiops feroculus, Ct. crocineus, Ct. mitodes, Ct. pomastictus, Cryptocentrus strigilliceps, Amblyeleotris wheeleri and A. guttata. 

From 10 metres from the reef edge 

Amblyeleotris periophthalma, A. steinitzi, A. diagonalis, Tomiyamichthys nudus and Cryptocentrus cinctus. 

More than 50 m from the reef edge. 

Amblyeleotris stenotaeniata, Tomiyamichthys lanceolata, Vanderhorstia dorsomacula and V. phaeosticta. 

Naturally these records are generalisations. The occasional individual will be found on the ‘wrong’ habitat, but always with a usual partner shrimp. We have repeated the survey at Low Isles, Fitzroy Island and Frazer Island finding some variation in species, but a similar pattern. 

Depth and distance from the reef are determinants of distribution for gobies and shrimps alike, but only if the substrate type, silt, sand or mixed rubble, remains constant. Otherwise substrate particle size is the determinant. 

Density and Stability of Shrimp and Goby populations

There is no standard density of shrimp/goby populations. Populations of mixed species are the rule and burrow separation is not constant. Broadly speaking burrows are anything from 30cm to a metre apart depending on the characteristics of the habitat, but some areas are colonised by locally dense populations of pairs and other areas looking no different have none. 

It does seem that in a specific area the population tends to stabilise and to remain stable despite immigration or loss of gobies. 

This has been studied by Thompson (2004). Marked adult gobies were introduced to demarcated study areas in which the location of every burrow and the location and size of every goby and shrimp was known. A third of the introduced gobies evicted resident gobies from their burrows, in every case a smaller one. The overflow were presumed eaten or moved a long distance away. 

The population density remained the same. The controlling factor is the number of shrimp burrows the area supports. There are more gobies available than burrows and burrows occupied by a shrimp alone are rare and probably temporary. 

If the goby vacates a burrow, for example in response to the need of the newly matured to mate, the shrimps stay inside the burrow. Being unable to forage they lose condition until a replacement goby arrives. 

If the shrimp vacates a burrow, perhaps in response to a similar imperative, the burrow soon collapses from lack of maintenance. This leaves the resident goby homeless and it will look for a new burrow which will be vigorously defended by the current occupant. In the end it will evict a slightly smaller goby, not a larger one and not a very small one unless it is partnered by a much larger shrimp. The reason for this is that an inappropriately small shrimp would only be able to construct an uncomfortably small burrow. 

In general shrimps and their partner gobies are of similar maturity. 

Predator concentration and shrimp/goby population stability

Bare areas of shrimp/goby habitat are often more free of predators than more structurally complex habitats. Thompson (2005) has studied the effect of increasing predation pressure within specially marked study sites by providing more cover for the predators. This was found to increase the density of predators and to change the composition of the population qualitatively, but not to alter the density of shrimp/goby pairs. The percentage of pairs with adult gobies, however, declined in favour of pairs with small or juvenile fish. 

The inference was that following the loss of adult gobies the burrows remained available until immigration of smaller gobies lower on the hierarchical scale filled the gaps. These were recruited from “floater” gobies that live on the sand without shelter until a burrow becomes available (Yanagisawa 1982, Thompson 2004). 

The study does not indicate that only large gobies get eaten, simply that all deaths are replaced with small fish. 

There was no effect on the shrimp population suggesting that they are much less vulnerable to predation. 

Shrimps and Shrimpgobies in Aquariums

The aquarium trade is heavily dependent on collection of marine creatures from the ocean. This obviously has the potential to endanger target species and their ecology. This is taken seriously by responsible aquarists. 

The pleasing symbiosis of the shrimpgobies and their shrimps makes them an attractive acquisition for the aquarium, and makes them worth the collector’s time. How much does this endanger them?

Calado et al (2003) recorded pretty well all the decapod shrimps traded in Europe and North America over a twelve month period in 2002. The number of partner shrimps traded was small, and Involved only three commoner species:

The Tiger Shrimp, Alpheus bellulus

The Black-sided Shrimp, Alpheus djeddensis

and Randall’s Shrimp, Alpheus randalli

Alpheus bisincisus on Calado’s list is not a partner shrimp, Ryanskiy (2016)

The impact on these species is small. Altogether only nine Alpheid shrimps were traded. Biology has more to gain from the observations made by aquarists than is lost by collection. We would welcome observations by aquarists and would be happy to comment on any you send to us FishesoftheGBR@yahoo. com. au 

We have learned so much from the observations of burrow layout and shrimp and goby reproduction reported by aquarium keepers. Some of this information is unobtainable in the field and note keeping can only increase your pleasure in your aquarium. 

Hopefully this book should guide you as to which species form partnerships in the wild and in which habitats.