Entrance size is an important factor for males of the western Pacific blenny species Rhabdoblennius ellipes when choosing a nest cavity.
Research by Japanese scientists, published in the most recent volume of the Journal of Fish Biology, suggests that nest entrance size matters more to males of the blenny species Rhabdoblennius ellipes than nest volume or nest length.
Rhabdoblennius ellipes is a small species of blenny found throughout the intertidal rocky shores of the western Pacific Ocean. Males of this species exhibit brood care by guarding and aerating a clutch of eggs for 6-7 days within a nest cavity. During this time males actively chase would be predators away from the nest.
Small holes made by bivalve molluscs or the vacant shells of vermitid gastropods are most commonly used by males as nesting sites.
From 2003-2006, Japanese researchers investigated the nest choice preferences of male R. ellipes in the wild and under aquarium conditions by providing males with artificial nesting cavities of various diameters.
Under natural conditions in a tide pool, researchers found a strong correlation between male body length and the size of the nest entrance opening (based on 26 egg-filled nests). No relationship was found between male body length and the length or volume of the nest cavity. When bricks drilled with holes of varying diameters (labeled small, large and x-large) were introduced in to tide pools as artificial nests a similar result was discovered, with males spending more time in holes with the smallest diameter. A similar result was also discovered under aquarium conditions.
When male R. ellipes leave their nests to feed their eggs are frequently preyed upon by other small fishes such as gobies and other blennies and small invertebrates, including hermit crabs, small crabs, shrimps and snails. By choosing nests with narrow entrance holes, male R. ellipes may be providing more protection for their eggs against larger egg predators. Males may even be looking out for themselves by decreasing the chances of predation from larger predators whilst egg guarding.
For more details on the nesting preferences of R. ellipes see the paper: Takegaki, T., Matsumoto, Y., Tawa, A., Miyano, T. and Natsukari, Y. (2008). Size-assortative nest preference in a paternal brooding blenny Rhabdoblennius ellipes (Jordan & Starks). Journal of Fish Biology, 73: 93-102.