Why is group size correlated with the size of the host sea anemone in the false clown anemonefish?

Abstract

When social groups monopolize discrete habitat patches, group size may be positively correlated with patch size. The correlation can be a direct consequence of limited resources. Alternatively, it can be an indirect consequence of patch-size effects on a dominant group member. We asked which of these two mechanisms was responsible for a positive correlation between the group size of false clown anemonefish (Amphiprion ocellaris Cuvier in Cuvier and Valenciennes, 1830) and that of the host sea anemone Stichodactyla gigantea (Forskål, 1775). We argue that some false clown anemonefish groups may have reached the carrying capacities of their hosts, but that the group size : patch size correlation in the population as a whole is best interpreted as an indirect consequence of a positive relationship between anemone size and the length of the dominant group member. The dominant's length in turn limits group size because dominant group members inhibit the growth of their subordinates. Thus, a correlation between group size and patch size need not imply resource limitation of subordinate group members.

Extreme habitat specialisation and population structure of two gorgonian-associated pygmy seahorses

“…The absence of statistical correlation between pygmy seahorse density and host density indicates that the association with gorgonians does not determine seahorse population dynamics. These results contrast with expectations for many of the reef's habitat specialists, such as the fishes Amphiprion and Gobiodon, whose population sizes tend to be linked with those of their hosts (Kuwamura et al 1994, Munday et al 1997, Mitchell & Dill 2005. However, in the case of pygmy seahorses, a large proportion of potential hosts is vacant.…”

“…The link between group size and host size has been studied in various species of reef fishes where a correlation between these factors has been attributed to resource limitation or as an indirect consequence of patch size effects, such as through dominance of 1 group member (Elliott & Mariscal 2001, Mitchell & Dill 2005. We found no significant effect of gorgonian host area on the number of resident Hippocampus bargibanti or H. denise.…”

“…Polygamy is predicted where one sex is able to monopolise partners, which is possible in pygmy seahorses given that groups are isolated together on a host gorgonian and at a comparatively high density (Emlen & Oring 1977, Ihara 2002, Kokko & Rankin 2006, Hernaman & Munday 2007. Other recent studies of seahorses have shown that the strict social and genetic monogamy observed in early research of seahorse reproductive biology is subject to variation (Kvarnemo et al 2000, Moreau & Vincent 2004, thus the mating systems of pygmy seahorses would be an interesting direction for future research.The link between group size and host size has been studied in various species of reef fishes where a correlation between these factors has been attributed to resource limitation or as an indirect consequence of patch size effects, such as through dominance of 1 group member (Elliott & Mariscal 2001, Mitchell & Dill 2005. We found no significant effect of gorgonian host area on the number of resident Hippocampus bargibanti or H. denise.…”

“…The populations of habitat specialists tend to be smaller than those of generalists and take longer to recover following perturbation, as a result of invasion by generalists and an inability of specialists to respond plastically to change (Vázquez & Simberloff 2002, Hastings & Galland 2010. The populations of some habitat specialists are closely linked with those of their preferred hosts, whilst others appear more loosely regulated by host abundance (Mitchell & Dill 2005). Reliance on another organism or biologically-derived habitat places specialists at further risk, particularly when this is a habitat forming structure, such as coral or a gorgonian, which is itself affected by disturbance on the reef.…”

A new perspective on size hierarchies in nature: patterns, causes, and consequences

“…We expect that adaptive regulation of growth and non-random size hierarchies will be found in other fish species. Some likely candidates include other members of the genus Amphiprion (Allen 1972;Fricke 1979;Ochi 1986;Hattori 1991;Mitchell and Dill 2005), and members of the goby genera Gobiodon and Paragobiodon (Lassig 1976(Lassig , 1977Hobbs and Munday 2004), which are sex changers that form groups composed of a dominant breeding pair and a number of subordinate non-breeders. Also, members of the damselfish genus Dascyllus (Coates 1980;Sweatman 1983;Forrester 1990;Booth 1995;Schmitt and Holbrook 1999), which are sex changers but form groups composed of a dominant male and a number of subordinate females.…”