By Fiona Gowland, University of Aberdeen, holder of The Society's Centenary Research Award for 2002

Growth rates of marine ectotherms typically reflect ambient conditions i.e. high temperatures and high food levels promote growth whilst low temperatures and low food levels retard growth.  Although positive relationships have been reported between temperatures, food levels and growth rates in juvenile and adult squid, the impact of thermal regime upon embryonic growth and muscle structure has never been tested.  Fiona Gowland (University of Aberdeen) used the Centenary Research Award to gain an insight into embryonic growth processes in squid.

In squid, increases in body mass and length occur via production of new muscle fibres (hyperplasia) and via enlargement of existing muscle fibres (hypertrophy).  The majority of mantle bulk comprises circular smooth muscle fibres organised into regular rectangular blocks by thin partitions of radial muscle fibres (Fig.1). Squid circular fibres are usually present in two structurally distinct forms: those used anaerobically during powerful jetting (niitochondria-poor) and those used aerobically during slow jetting (mitochondria-rich).

Fiona examined growth and mantle muscle structure in twenty-four hatchling Sepioteuthis australis reared under stable (16.00C ± 0.10) and fluctuating (16.87C + 3.59) thermal conditions of similar mean temperature.  She assessed embryonic growth by measuring dorsal mantle length (Fig. 1).  Fiona's study revealed that embryos reared under stable temperature were significantly smaller than those reared under fluctuating temperature.  Although muscle structure varied significantly with mantle position (anterior, mid or posterior), it did not vary significantly between individuals or thermal regimes.  Narrow muscle blocks were most frequent within anterior and posterior mantle regions, and broad muscle blocks were most common in mid-mantle regions.  Anterior mantle regions were additionally dominated by small muscle fibres.

The results of Fiona's work indicate that anterior and posterior mantle regions are the main sites of embryonic muscle growth in Sepioteuthis australis.  Her study also provides novel evidence for ontogenetic change in the location of primary growth sites.

Fiona submitted her PhD thesis in September 2002 (within three years).  She has written seven manuscripts from her PhD work: two are published, two are in press and three are in review.  Fiona writes popular science articles for the Journal of Experimental Biology, and is currently teaching on undergraduate science courses whilst applying for post-doctoral positions.

Figure 1: Schematic diagram of squid mantle muscle showing the orientation of different fibre types.  Mantle muscle mass predominately comprises circular fibres that are partitioned into rectangular blocks by thin regions of radial muscle fibres.  Circular fibres are metabolically and structurally differentiated into three distinct zones: inner, central and outer (after Ward & Wainwright, 1972).