Comparative Reproductive Strategies

Given the radically different physical characteristics of the different environments on San Juan Island, organisms from different habitats might be expected to vary in their life history characteristics. One important life history characteristic is larval biology, which may profoundly influence dispersal among populations, population size and its annual variation, population density, and other ecologically significant aspects of a species' biology.

One way in which the larval biology of species may be described is by larval feeding mode. This aspect of an organism's life history may be considered shorthand for many of the abovementioned ecological variables, because it is correlated with potential dispersal distance and population size, among other ecological traits.

Larval Feeding Mode

	Total Species	Lecithotrophic Larvae	Direct Developers and Brooders	Nonfeeding Benthic Larvae	Planktotrophic Larvae
Eagle Cove	58	3	7	7	41
Snug Harbor	36	3	3	6	24
Garrison Bay	16	1	2	4	9
Cattle Point	32	2	3	3	24
False Bay	13	1	3	1	19
Argyle Creek	31	2	6	4	19
Overall	78	6	12	8	52

Number of species with given larval feeding strategies at each habitat. Data is expressed in terms of number of species with each larval feeding strategy over the total number of species with known larval biology at each site.

Some definitions:

planktotrophy means feeding on small suspended material in the water column; these larvae are also planktonic themselves
lecithotrophy means feeding on egg yolk or other materials put in the egg by the mother; these larvae do not feed on anything else until after they settle to the bottom and become nonplanktonic juveniles
direct development in this context means developing from a fertilized egg to a juvenile form of the adult without entering the plankton; often this development happens in an egg mass or capsule, or in some brood space within the parent
benthic larvae live outside the parent's body but do not get up into the water column

Interestingly, despite the different physical challenges that species face at each site, there is no significant difference in the distribution of larval feeding types among habitats. At all six sites, most species have planktotrophic larvae that feed and grow in the water column before settlement onto the bottom and metamorphosis into the juvenile or adult form. Though this result may owe something to the small number of species that were considered, it is nevertheless striking that these dramatically different life histories are distributed so evenly across the six field sites.

Below, the same data are presented graphically and as percentages of total species present in each site for which reproductive data were available. Note the similarity among each of the graphs. (Legend provided for 'Overall' graph describes the individual site graphs as well).

Shared Habitats

Many of the species found in San Juan habitats are widespread, occurring at multiple sites under very different ecological conditions. In order to discover whether species with certain larval feeding types were more likely to be widespread than species with other larval types, the average number of habitats in which a species was found was calculated for each larval type. These data are presented below.

	Lecithotrophic Larvae	Direct Developers and Brooders	Nonfeeding Benthic Larvae	Planktotrophic Larvae	Overall (known reproductive type)	Overall (including unknown reproductive type)
Average number of habitats per species (max 6)	2	2	3.125	2.385	2.367	1.971
Number of species considered	6	12	8	49	78	169

Average number of habitats per species for organisms with different larval feeding types. These were species found in at least one of the six San Juan habitats surveyed.

Here again, no significant difference was found among larval types with respect to the average number of habitats occupied. Most species, including those with long periods of larval development (planktotrophy) and those that never leave the microhabitat of the mother (brooders) are found in 2-3 of the 6 habitats surveyed. Thus, species with planktotrophic larvae are not observed to be ecological generalists to any greater degree than are species with direct development in the six field sites considered.

Reproductive biology, at least insofar as it may be approximated by larval feeding mode, does not appear to vary between habitat sites, and neither does it seem to be correlated with habitat specificity.

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