Stage One:
Observation.
Determine the physical limitations of the crab people.
- What is their range of motion?
- Can they climb trees?
- What is their swimming speed and ability? [Some crabs can swim and some cannot]
- How mobile and agile are they on land?
- What is their diet? Does it consist of items a human can safely ingest?
- How strong are their claws and shells?
- Et cetera
Determine the social aspects of the crab people.
- Do they have a spoken language?
- Do they communicate verbally and/or with gestures?
- Is there a hierarchy, or do they simply defer to the eldest crab person?
- What concepts of property does their society use? Is food shared among all the crab people? Does each crab forage for their self?
- Do they seem to carry any property past simple survival?
- Et cetera
Stage Two
First Contact
- While they are aggressive, are they openly hostile?
- If they are't immediately hostile, and cannot collect coconuts themselves, are they open to trade (such as for fish or shellfish)?
- Engage an individual in a situation that leaves me several routes of escape should things go badly. Determine trade possibilities.
Stage Three
Survival
If hostile, I'll engage in a guerrilla war against the crab people. Based on their shell's strength, mobility, and agility, direct assaults on individuals may be viable; otherwise I'll rig traps and lure them in. A pit filled with water that they cannot climb or dig out of sounds nice. Light a fire under it and slowly boil the crab for dinner. Devise a simple pulley and braid vines to form a rope; raise a bolder and drop it on an unsuspecting crab person. A few individuals will be captured and maintained. I won't wipe out a species, despite any hostilities.
If trade is possible, begin negotiations and determine to what extent the crab people can be taught. I wouldn't expect them to pick up calculus right away, but maybe simple tools and machines would be possible. With some luck, both the crab people and I can prosper on the island (and beyond?).