Answer by Trevor Best:
Thank you for inviting me to answer this interesting question.
From my reading of your question, I think it can be summarised as: “and large, predatory Oriental Sweetlips are engaged in a symbiotic relationship and could not live without one another. I can’t imagine any way this could have come about through evolution as I understand the theory, so I find it difficult to believe in evolution.”
There are a few concepts bumping up against each other, and I hope I’ll be able to clarify them. Then I will propose some ways the Wrasse-Sweetlips relationship came about.
Bluestreak Cleaner Wrasses (Labroides dimidiatus) cleaning the body and mouth of Oriental Sweetlips (Plectorhinchus vittatus). Note that the Wrasse does not exclusively clean the teeth of Oriental Sweetlips. Images of Bluestreak Cleaner Wrasses cleaning other sorts of fish may be found at the end of this answer. Image source:
If Evolution (Macroevolution to be precise.) were true…
Evolution is “true”, in the sense that it has been observed happening. We have countless examples of direct observation of genetic change spreading through a population in response to external selection pressure, resulting in a visible change in a population’s phenotype. However, the term “macroevolution” does not really make any statement more precise; if anything, it does the opposite.
Most biologists do not use the term “macroevolution” often or at all, because the distinction between microevolution and macroevolution is not a real one. It doesn’t describe some specific, hard-and-fast difference in the real world. It’s just a label used (rarely) for convenience, to differentiate between a small amount of evolutionary change accumulated over a short period of time (“microevolution”), or a large amount of evolutionary change accumulated over a long period of time. Macroevolution just means “lots of microevolution”. I can’t see how using the term macroevolution adds any precision to the question of how sweetlips came to allow wrasses to clean their teeth.
Note that the term macroevolution is very popular with creationists, because it allows them to admit that microevolution happens, while implying that this is a different process to macroevolution (which takes a very long time, so creationists hope the evidence for it will be easier to dismiss). By combining the artificial distinction between microevolution and macroevolution with the equally artificial concept of “kinds”, creationists hope to be able to indefinitely move the goalposts so that any evidence for evolution can be dismissed as microevolution. Every time an organism of kind “x” is shown to be related to kind “y”, creationists can simply redraw the boundaries and claim that x and y are actually the same kind, so no “macroevolution” has taken place.
But if it is true that microevolution happens, and if it is true that living things have existed for a long time, then it must be true that macroevolution happens unless there is something stopping it. (That is, there has to be some reason why a gene pool would stop accumulating change as it approached the edge of one of the arbitrary boundaries of ‘kind’ that humans use to lump life into convenient categories). In my experience, creationists who accept microevolution but dismiss macroevolution present no mechanism or evidence for the claim that microevolutionary changes stop accumulating as a species approaches the boundary of a “kind”.
In any case, I don’t think the word ‘macroevolution’ really adds anything useful to your question.
Clarifying the timeline
… the fact that the Sweetlips would have to develop teeth in the exact time frame as they developed the instinct to allow the other fish to enter their mouth …
This sentence suggests gaps in your understanding of the sequence in which fish, and teeth, and specific species of modern fish, appeared.
According to the fossil record, the first fish evolved the the Cambrian period, which was 540 to 485 million years ago. These first fish were the common ancestors of all fish known today, including sharks, minnows, wrasses, sweetlips, manta rays and so on, but as yet they had not evolved into wrasses or sweetlips. They also did not yet have teeth.
Fish teeth evolved from scales in the early Devonian, approximately 420 million years ago, and have since undergone significant change. This first toothed fish was the common ancestor of both bony fish (including many modern species, such as wrasses and sweetlips) and cartilaginous fish (sharks and rays). The earliestdeveloped from this lineage soon afterwards (also around 420 million years ago), and all bony fish are descendants of these toothed fish. (In this sense, defining Oriental Sweetlips as a sort of fish with teeth is a bit like defining Welshmen as a sort of human with teeth).
The earth’s oceans constitute an enormous, diverse ecosystem, which includes many, many potential evolutionary niches. For over 400 million years, myriad species of fish have gradually adapted to fill many different niches, sometimes in startling ways. Some eat vegetation, some detritus and some other fish. Some are tiny and can hide amongst coral, others are enormous predators, and many more are of intermediate sizes, forming a vast, interlocking food chain. Among the predators, most or all are somewhat selective about their diet, eating particular species, and sizes, of fish. This is the niche they are adapted to.
Carnivorous fish don’t necessarily devour every smaller fish they see.
Of the many billions of fish in the ocean, in every generation a few deleterious mutations appear, which are winnowed out, and an occasional useful mutation appears and spreads through the population, creating the opportunity for adaptation, or change. Keep in mind, they have been doing this for over 400 million years, which is a near-unimaginable amount of time.
The relationship between Bluestreak Cleaner Wrasses and Oriental Sweetlips is not a new one – in fact, it pre-dates the advent of Oriental Sweetlips themselves. Oriental Sweetlips are a kind of. According to Wikipedia, grunts “also engage in mutualistic relationship with cleaner gobies of genus Elacatinus, allowing them to feed on on their bodies.”
Grunts that aren’t cleaned of ectoparasites can still survive and reproduce, but clearly they will be at a disadvantage to those that allow wrasses to clean them. This means that the relationship between wrasses and any given species of larger fish is not the sort of symbiosis you imply (obligate mutualism) but rather facultative mutualism.
Symbiosis and Mutualism 
Symbiosis is generally defined as a relationship between two or more species in which at least one benefits. Its definition has changed over time (see) and the definition you are using seems to be similar to the one I learned many years ago: “a relationship between two different species in which neither species could survive without the other.”
The most common system of definition today would describe an inter-species relationship in which neither species could live without the other as “obligate mutualism”. It would contrast this definition to one in which each species benefits but they could survive without the relationship (“facultative mutualism”).
Obligate mutualism typically evolves from facultative mutualism.
From my quick background reading (thank you), Bluestreak Cleaner Wrasses clean many different species of fish. In fact, these mutual relationships are so well established that populations of wrasses establish ‘cleaning stations’ and other species of fish line up to be cleaned. The other fish recognise the cleaner wrasses by their distinctive stripe. So it seems Bluestreak Cleaner Wrasses could definitely survive without Oriental Sweetlips – they would just clean other fish.
As for your claim that “Wrasses’ main food source is whatever they clean off of Sweetlips’ teeth”, do you have a source for that claim? The references I have seen state that Bluestreak Cleaner Wrasses clean the external surfaces, including the mouths, of various different types of fish.
Your question and question details imply that you think that Sweetlips, or their ancestors, could not have survived with dirty teeth. Why do you think that? And if this were true, why wouldn’t wrasses also die from dirty teeth, given that Sweetlips eat freshly killed prey, whereas Cleaner Wrasses eat the very parasites and rotting dental filth that would supposedly kill the Oriental Sweetlips?
Your question also seems to imply that Oriental Sweetlips, a large carnivore, would be expected to eat wrasses. From my limited research, this doesn’t seem to fit their diet. According to:
In the wild their diet consists mostly of small crustaceans, starfish, snails and other similar prey items.
However, it’s true that wrasses do clean the teeth of some fish that eat other fish quite regularly, and I can’t say with any certainty that Oriental Sweetlips don’t eat fish, so I will address how that might have evolved.
The evolution of mouth cleaning could have happened very gradually. The mutualist relationship might have begun with nimble wrasses’ ancestors only tentatively and sneakily nibbling parasites from the flanks of the Sweetlips’ ancestors. Occasionally the larger fish would have been able to snap up the wrasse, and would have remained diseased. But specimens that were genetically predisposed to ignore this behaviour (limiting their diets and recognising the Wrasses’ distinctive appearance) would have been cleaner and healthier, and would have wasted less energy chasing these small, fast fish. Genes that favoured ignoring wrasses would gradually have spread at the expense of genes that favoured more indiscriminate feeding behaviour. Eventually, the ancestral grunts would have recognised and tolerated the ancestral wrasses, never eating them. At this point, it is unsurprising that they would allow wrasses to clean their mouths.
Oriental Sweetlips depend on clean teeth about as much as humans do. We can survive with dirty teeth, but we are more likely to be diseased. For example, gum disease can lead to heart disease. More importantly, the wrasses remove parasites from the Oriental Sweetlips and other fish species. So by not including Bluestreak Cleaner Wrasses in their diet, Oriental Sweetlips are more likely to survive long enough to reproduce. But before the mutualist relationship with wrasses developed, the ancestors of Oriental Sweetlips could still survive, just not as competitively as they do now.
 The Wikipedia entry on
I have made some minor amendments to this answer in response to ‘s comments below.
 Thank you to (comments, below) for bringing to my attention the fact that the consensus definition of symbiosis and mutualism may have changed somewhat since I studied biology, back in the Mesozoic Era. My answer has been modified accordingly.
 Pictures of cleaner wrasses cleaning things other than Oriental Sweetlips’ mouths: