A widespread but inconspicuous feature across the plant world is the presence of domatia on leaves or other organs. Domatia (doe-MAY-shaw, not doe-MAT-ee-ah) are tiny homes the plant provides, sometimes with food, to varied small critters, often ants, mites, or even bacteria in symbiotic exchange for services rendered. Among local native species, especially conspicuous domatia decorate the leaves of members of the Combretum Family: Buttonwood (Conocarpus erectus) and White Mangrove (Laguncularia racemosa); domatia occur also on their locally cultivated and escaped exotic cousin Tropical-Almond (Terminalia catappa).
Domatia come in varied sizes and shapes. Those on leaf blades usually sort into four different categories: pouchlike (sometimes called marsupiform), hair tufts (as in many oaks), pits, and pockets. Those of Buttonwood and of White Mangrove appear to be the pit type—tiny cavities in the underside of the leaf. Their tiny size begs the question of, “what micro-creature lives in there, and what can it do for the plants?”
Specific mite data on today’s plants are not in hand, but in general itsy bitsy leaf domatia are thought of as refuges for predatory or fungus-eating mites serving their botanical landlord by eating other mites or pathological foliar fungi.
The White Mangrove and Buttonwood domatia are useful for identification. In Buttonwood (including “Silver Buttonwood”) flip over the leaf, then look along the central vein; the domatia are in the angles where the secondary veins branch off the main vein, looking like little blackheads. In White Mangrove they are in a line along both leaf margins, about a quarter inch in from the edge.