ROLE OF PHYLOGENETIC CONSTRAINTS IN DETERMINING REPRODUCTIVE PATTERNS IN DEEP-SEA INVERTEBRATES

The majority of deep-sea invertebrates appear to reproduce ''continuously'' while some species show a marked seasonal rhythm. Deep-sea habitats are not as unvarying as once believed and some areas of the deep-sea floor are subject to a variety of perturbations, including seasonal fluxes of organic matter originating from surface phytoplankton blooms. Some authors suggest that these organic pulses might activate the reproductive processes of seasonally-reproducing species while having no immediate effect on ''continuous'' breeders. In this paper, we hypothesize that the different reproductive responses to organic matter can be explained by phylogenetic constraints involving interspecific differences in gonadal morphology, nutrient storage and mobilization, the mechanisms of vitellogenesis, feeding biology, digestive processes, and selective use of nutrients. Invertebrates process nutrients in different ways with some having the capacity for ''fast egg-production'' while others engage in ''slow egg-production.'' In addition, the pace of gametogenesis and the frequency of spawning will be influenced by the presence of storage tissues, which may release energy reserves to the gonads at a predetermined rate. As a result, seasonal organic fluxes to the deep-sea floor will provoke a variety of reproductive responses. We propose three patterns that may clarify the correlation between seasonal phytodetrital pulses and seasonal reproductive patterns in some species: (1) species initiate gametogenesis immediately in response to organic input and undergo spawning soon after; (2) species spawn when seasonal pulses coincide with conditions favorable for their planktotrophic larvae; or (3) seasonal organic input initiates and synchronizes gametogenesis, producing a future spawning episode after an extended period of vitellogenesis. In those species showing seasonal reproduction, the proximate cause is the seasonal pulse of phytodetritus while the ultimate cause stems from the phylogenetic history of the organism. Reproductive periodicity of deep-sea species is generally predicted from an analysis of gamete development. Consequently, we suggest that seasonal breeders be referred to as having ''synchronous gametogenesis'' while so-called ''continuous'' breeders be referred to as having ''asynchronous gametogenesis.'' These terms better describe the condition of the developing gametes without making inferences regarding spawning patterns. Generalizations about the potential response of groups of unrelated taxa to various environmental parameters (e.g. organic pulses) fail, because phylogenetically diverse species respond differently.